CT and MR in peritoneal malignancies: pearls and pitfalls at preoperative examination.

CT imaging is the first option among the primary imaging modalities to detect peritoneal implants due to its availability, fast image acquisition, spatial resolution, and multiplanar reconstructions [ 7 , 8 ]. According to recent guidelines, the CT should cover the thorax, abdomen and pelvis, acquired with at least 16-slice CT equipment [ 7 , 8 ]. The soft-tissue filter should be considered as a reconstruction standard, which will also include the multiplanar reconstructions in coronal and sagittal planes at 1–1.5 mm. The slice thickness should be < 0.75 mm [ 7 , 8 , 16 – 18 ]. A CT scan might be performed before and after intravenous contrast medium administration (Table  1 ); the portal venous phase is mandatory [ 7 , 8 ]. While it is well known that the portal venous phase is regarded as the phase of choice for PC assessment, the actual effectiveness of the delayed phase is still debated [ 19 ]. Up to now, unenhanced and arterial phase are not recommended [ 7 , 8 ]. CT enterography (CTE) should be considered for the study of small bowel (SB) (Table  1 ). With dedicated distension, the detectability of small implants might rise consistently [ 20 ]. Table 1 Peritoneum CT protocol Variable CT protocol Timing Required Intravenous CM dose 0.7 gI per kg of lean body weight Recommended CM administration rate 3–3.5 mL/s Recommended Oral contrast material I. Conventional CT II. CT enterography I. 500–800 mL of water II. 1000–1500 mL of PEG I. 3 min before the exam II. 30–45 min before the exam I. Optional II. Recommended Acquisition CT phases I Portal venous phase II. Delayed phase I. 70 s after CM injection II. 7–8 min after CM injection I. Mandatory II. Optional CM contrast media, PEG polyethylene glycol Peritoneum CT protocol Oral contrast material I. Conventional CT II. CT enterography I. 500–800 mL of water II. 1000–1500 mL of PEG I. 3 min before the exam II. 30–45 min before the exam I. Optional II. Recommended I Portal venous phase II. Delayed phase I. 70 s after CM injection II. 7–8 min after CM injection I. Mandatory II. Optional CM contrast media, PEG polyethylene glycol Regarding the restaging in a post-treatment setting, the main recommendations suggest following the patient with CT, with a similar protocol used for the staging setting, during the therapy or at the end, but before the debulking surgery. In doubtful cases with high suspicion of undetectable disease on CT, 18 F-FDG PET/CT may be considered [ 7 , 8 ].

Since peritoneal seeding is related to fluid stasis, promoted by capillary force on surfaces and, more importantly, by gravity, it is pretty standard for peritoneal masses to be observed in the lower quadrants of the abdomen [ 37 ]. Then, tumor cells can be directed cranially via the right paracolic gutter to the Morison pouch. The left paracolic gutter is more rarely the site of disease because it is stopped superiorly by the phrenicocolic ligaments [ 44 ]. The recto-uterine space, or sac of Douglas, represents the caudal extension of the peritoneal cavity and is a frequent site of seeded lesions [ 45 ]. It is essential to know the histologic characterization of the primary tumor when we evaluate for possible lesions in the Douglas cavity, which is also often the site of peritoneal effusion because of its declivous anatomic location [ 21 ]. Looking at paracolic gutters, the right side is the most common site of implants due to the preferential root of peritoneal fluid, the left side is rarely involved, and the features of implants may vary according to the primary neoplasm [ 44 ]. Solid implants showed a pronounced enhancement in both CT and MRI, a low-intermediate intensity in T2 sequences, with the possibility of restricted diffusion on DWI and ADC maps (Fig.  11 ) (Table  3 ). While differentiating cystic metastases (e.g., in peritoneal mesothelioma) from fluid collections might be challenging, these exhibit low attenuation in CT and high signal in T2-weighted sequences at MRI, showing no diffusivity restriction in diffusion-weighted sequences [ 46 ] (Fig.  2 ). In the pelvis, miliary distribution is characteristic of epithelial ovarian carcinoma; after exfoliation of tumoral cells from primary cancer, these passively transferred through the peritoneal fluid to the peritoneum, giving lesions that are often numerous, superficial, and small in size and, therefore, difficult to remove with cytoreductive surgery [ 47 ]. The small size also poses a diagnostic problem: lesion detection for CT scans decreases dramatically for lesions < 1 cm [ 48 ]. At the same time, by using DWI-MRI, the imaging accuracy rises to around 88% [ 6 ] due to the high intensity of DWI. Fig. 11 Ovarian cancer with peritoneal carcinomatosis in a 69-year-old woman ( A – C ). A CT delayed phase, axial plane showed a nodular implant in the left iliac fossa (arrows), confirmed in MRI T2w and DWI (b = 800 mm 2 /s) (arrows) ( B , C ). Colon adenocarcinoma with peritoneal carcinomatosis in a 64-year-old woman ( D – F ). MRI T2w ( E ) (circle) demonstrated a nodular implant in the distal ileum, confirmed in DWI (b = 800 mm 2 /s) ( F ) (circle), but not evident on CT portal-venous phase ( D ) Ovarian cancer with peritoneal carcinomatosis in a 69-year-old woman ( A – C ). A CT delayed phase, axial plane showed a nodular implant in the left iliac fossa (arrows), confirmed in MRI T2w and DWI (b = 800 mm 2 /s) (arrows) ( B , C ). Colon adenocarcinoma with peritoneal carcinomatosis in a 64-year-old woman ( D – F ). MRI T2w ( E ) (circle) demonstrated a nodular implant in the distal ileum, confirmed in DWI (b = 800 mm 2 /s) ( F ) (circle), but not evident on CT portal-venous phase ( D ) In the left lower abdominal quadrant, localizations of peritoneal carcinosis are favored by the transverse course of the sigmoid mesocolon, which causes a relative stasis of fluids. For this reason, disease localizations are more frequent here than in the contralateral quadrant [ 37 ]. PMP usually originates from the right lower quadrant of the appendix, producing mucin, with CT and MRI characteristics similar to those described above for mucinous tumor localizations: a low attenuation in CT and high signal in T2-weighted sequences to MRI without diffusivity restriction in the diffusion-weighted sequences [ 49 ].

Adequate bowel distension is crucial in the peritoneal MRI protocol as it enhances the accuracy and confidence in image interpretation. Before the exams, fasting for at least 4–6 h is required, and then administering 1–1.5 L of pineapple juice, as an oral contrast, in about 45–60 min, is highly suggested [ 7 , 8 , 21 ] (Table  2 ). The rationale behind the use of pineapple juice is related to the negative contrast material, which helps the radiologists differentiate the small bowel lumen from the ascites and peritoneal implants. Thus, the final results of using the oral contrast material are comparable to an MR enterography but with a negative contrast material. Additionally, administering spasmolytic agents before the exam can reduce bowel peristalsis. Suboptimal distension of the intestinal lumen may obscure thin peritoneal tumors or inflammation involving the intestinal serosa, mesentery, or adjacent peritoneum, potentially being misinterpreted as an abdominal mass [ 21 ]. MRI exams should be performed on 1.5- or 3-T MR scanners, with an external phased array coil positioning. Concerning the post-enhancement T1 sequences, the delayed phase at 3–5 min post-gadolinium administration is currently recommended for increasing sensitivity in detecting small peritoneal implants more effectively than CT or PET, while arterial and portal venous phases are not mandatory [ 7 , 8 , 22 , 23 ]. Currently, there is no defined role for MRI in a restaging clinical setting; the use of MRI in young patients to reduce the dose exposure should always be discussed in a multidisciplinary team [ 7 , 8 ]. Table 2 MRI protocol for the study of peritoneum Variable Planes Slice Thickness/Gap Respiration MRE Oral CM 1000–1500 mL of pineapple juice 45–60 min before the exam  T2 SSTSE/HASTE • Coronal • Axial 5/0.6 mm • Free breathing • Respiratory  STIR-DWI (TI = 160–180 ms, b = 50–1000 mm 2 /s) • Axial • Coronal (MPR) 5/0.5 mm • Free breathing  T1 mDixon 3D • Axial 3/0 mm • Breath-hold Intravenous contrast medium  T1 mDixon 3D FFE After 3–5 min from CM administration • Axial • Coronal 3/0 mm • Breath-hold • Breath-hold CM contrast media, MRE magnetic resonance enterography, DWI diffusion-weighted imaging, STIR short T1 inversion recovery, SSTSE single-shot Turbo spin-echo, HASTE half-Fourier single-shot turbo spin-echo MRI protocol for the study of peritoneum • Coronal • Axial • Free breathing • Respiratory • Axial • Coronal (MPR) • Axial • Coronal • Breath-hold • Breath-hold CM contrast media, MRE magnetic resonance enterography, DWI diffusion-weighted imaging, STIR short T1 inversion recovery, SSTSE single-shot Turbo spin-echo, HASTE half-Fourier single-shot turbo spin-echo

One of the significant clinical challenges is detecting small implants and giving an assessment of patients with a reduced disease burden. Then, it is essential to recognize the red-flag imaging findings that could indicate any peritoneal involvement. The presence of ascites must be carefully monitored, as it is typically one of the earliest indicators of carcinomatosis, even if it is not specific. It can appear diffuse, loculated, or septate, often because of adhesions. One of the primary causes of ascites is the obstruction of subphrenic lymphatic vessels, leading to reduced reabsorption of peritoneal fluid. Additionally, excessive fluid production can occur due to increased capillary permeability caused by tumor cells secreting vascular permeability factors [ 31 , 32 ]. An unusual form of peritoneal and omental tumor involvement is the presence of subcutaneous nodules in the anterior abdominal wall. Subcutaneous metastases in the periumbilical area are known as “Sister Mary Joseph’s nodules” (Fig.  9 ). PM can reach the umbilicus through direct extension from the anterior peritoneum or ligamentous communications (e.g., the sickle-cell, median umbilical, or omphalomesenteric ligaments) hematogenous spread, retrograde lymphatic flow, or by the entrapment of implants at the laparoscopic entry site [ 33 ]. Metastases at the port or trocar site are highly common, and these consist of cancer recurrence in the scar tissue or at the incision wound after laparoscopy [ 34 ]. The presence of cardiophrenic angle lymph nodes is a common finding in high-risk patients and has been linked to the presence of PC (Fig.  9 ). Similarly to the other imaging findings, the cardiophrenic angle lymph nodes are not specific but highly suggestive [ 35 ]. Fig. 9 CT porta-venous phase, axial plane, showed the two most relevant red flag findings in subphrenic nodes ( A ) (stars) and subcutaneous implants in the periumbilical area are known as “Sister Mary Joseph’s nodules” ( B ) (arrows) CT porta-venous phase, axial plane, showed the two most relevant red flag findings in subphrenic nodes ( A ) (stars) and subcutaneous implants in the periumbilical area are known as “Sister Mary Joseph’s nodules” ( B ) (arrows)

The extent of peritoneal carcinosis in the small bowel (SB) and its mesentery is critical in determining the eligibility of patients for cytoreductive surgery. SB involvement frequently precludes complete resection of PM due to the substantial risk associated with performing multiple bowel anastomoses, which can lead to significant postoperative complications and the potential for short-bowel syndrome [ 3 ]. When SB, mesentery, and intestinal serosa are involved, it is necessary to report whether the extent of involvement is less than or greater than 50%, the number and location of stenosis of the small bowel, and the involvement of the colon or gastric system [ 22 ]. The serosa of the SB is generally less susceptible to cancer metastases due to its lower density of milky spots, which are small channels linking the peritoneal cavity to submesothelial lymphatic networks [ 50 ]. Consequently, tumors that infiltrate the peritoneum of the SB are presumed to be more aggressive compared to those that do not affect this area. The SB peritoneal cancer index (SB-PCI), the sum of the peritoneal cancer index (PCI) scores for the four specific areas of the SB, ranges from 0 to 12 (Fig.  S1 ). Elias et al [ 3 ] demonstrated that an SB-PCI > 0 and the specific involvement of the lower ileum (area 12) markedly worsened the prognosis. The highest 5 years of overall survival (70%) was observed in patients without involvement of area 12 and a PCI < 15. In contrast, the 5 years of overall survival rates for patients with involvement of area 12 are significantly lower (12% for patients with PCI ≥ 15 and 15% for those with PCI < 15) [ 3 ]. The assessment of mesentery root infiltration represents a consistent radiological challenge; it might be rarely diagnosed with certainty, but it represents one of the major criteria of inoperability (Fig.  12 ) [ 51 ]. One of the major CT findings related to the mesenteric infiltration is known as “misty mesentery” due to the lymphatic obstruction by the tumor, resulting in edema alongside the vessels and mesenteric nodal enlargement. Alternatively, the hematogenous spread may occur, resulting in tumor emboli within the mesenteric arterial branches or serosal nodular lesions [ 51 ]. However, the overlap between malignant infiltration and non-neoplastic disease should always be considered, and in difficult cases, the biopsy might be essential. Fig. 12 Mesenteric root infiltration in a 54-year-old man with colon cancer. CT portal venous phase ( A ), axial plane, the “misty mesentery” pattern with edema alongside the vessels (arrows). This pattern was confirmed also on MRI T2w ( B , arrows) and on DWI (b = 800 mm 2 /s) ( C , arrows) Mesenteric root infiltration in a 54-year-old man with colon cancer. CT portal venous phase ( A ), axial plane, the “misty mesentery” pattern with edema alongside the vessels (arrows). This pattern was confirmed also on MRI T2w ( B , arrows) and on DWI (b = 800 mm 2 /s) ( C , arrows) There is a lack of consensus about the best option among CTE and MRE to assess the SB and mesentery involvement (Table  3 ), CT accuracy in detecting peritoneal lesions in SB is around 21–25% for the four small bowel regions without bowel distention [ 52 ], and thin mesenteric tumor sheets are invisible on CT, compared to the surgical finding [ 53 ]. MRE was estimated to be superior in terms of sensitivity in evaluating the intestinal tract and mesenteric involvement, especially smaller ones [ 54 ]. Despite its numerous benefits, MRI has notable limitations, such as restricted availability, high costs, extended examination durations, and extensive training for interpreting. Consequently, CTE is typically used as the primary diagnostic tool in clinical practice, while MRE is a secondary method to enhance diagnostic accuracy. When CTE results are inconclusive, the two imaging modalities are thus considered complementary. Implants on the SB wall may present as nodules, masses between loops, or masses adhering to adjacent loops, potentially causing bowel obstruction (Fig.  11 ). The “teca pattern” is characterized by numerous small nodular implants covering the SB loops appearing as wall thickening and enhancement, restricted distensibility, distortion of SB segments with wall irregularity, or intestinal stenosis (Fig.  8 ) [ 10 , 30 , 55 ]. This thickening can cause narrowing, obstruction, and proximal loop dilation, a condition known as “frozen pelvis, “ a significant contraindication for surgery [ 56 ]. In patients with known malignancy, distinguishing between a benign condition (i.e., due to postsurgical adhesions or radiation enteritis) or malignant causes of intestinal obstruction is a radiological challenge. Russell et al [ 57 ] identified three main features that indicate malignant intestinal obstruction: (1) focal ( 10 cm but less than 50% of SB) mural thickening, (2) moderate (3–10 mm) or marked (> 10 mm) peritoneal thickening, and (3) a moderate (if the degree of enhancement equals that of the liver) or marked (if the degree of enhancement equaled that of adjacent vessels) peritoneal enhancement. On the other hand, benign intestinal obstruction is indicated by the absence of an obstructive mass and a diffuse (at least 50% of SB) mural thickening [ 56 ]. Small bowel mesentery root involvement may manifest as increased attenuation or stranding of mesenteric fat, nodules, masses, or thickening with crowded vascular bundles [ 58 ]. Peritoneal malignant mesothelioma appears with a stellate infiltration of the mesentery, increased mesenteric fat attenuation, perivascular soft-tissue thickening, and vascular-bundle stiffness [ 59 ]. However, these are nonspecific signs of mesenteric involvement, which can also be present in other pathologies, such as peritoneal tuberculosis [ 58 ].

The most common sites of PM are the greater omentum, the perihepatic and perisplenic regions. In the former, it usually shows a hematogenous dissemination, which might appear as micronodular, nodular or omental cake patterns [ 36 ]. While in the perihepatic and perisplenic regions, the deposits are usually solids, and these might secondarily invade the underlying parenchyma, and this assessment is generally entrusted to an imaging approach (Table  3 ). The absence of parenchymal invasion is suggested when a tissue plane, such as fat or ascites, exists between perihepatic metastasis and the liver parenchyma or a smooth and well-defined lesion-liver interface. Conversely, the protrusion of a perihepatic metastasis into the liver without a well-defined but ill-defined, irregular liver-lesion interface may suggest parenchyma invasion [ 38 ]. The lesser omentum is rarely involved, and when it does happen it is predominantly because of the presence of gastric cancer that results in its direct invasion [ 36 ]. In small lesions (< 1 cm), conventional CT has a sensitivity of around 25–50% due to the reduced contrast resolution [ 39 ]. While the sensitivity of MRI, with delayed post-contrast sequences, for identifying peritoneal implants < 1 cm rises to 75–80% [ 40 ]. In addition, diffusion-weighted MRI might improve the detection, especially in peritoneal reflections close to the liver and pancreas [ 41 , 42 ]. However, MRI performance is low in the left hypochondrium, in which the spleen is spontaneously hyperintense on diffusion-weighted MRI, thus making perisplenic deposits less conspicuous on DWI [ 43 ]. Table 3 Head-to-head CT and MRI in different abdominal regions Study Imaging Population Endpoint Performance Upper abdomen  Coakley et al [ 36 ] CT 31 patients CT accuracy implants ≤ 1 cm Sens: 25–50% Spec: 78–96% PPV: 44–80% NPV: 78–85%  Low et al [ 37 ] CT vs MRI 24 patients Imaging performance in CT and MRI Sens: 54% vs 84% Spec: 91% vs 87% Acc: 74% vs 86%  Low et al [ 38 ] MRI 34 patients Conventional MRI vs Conventional MRI + DWI Sens: 52–73% vs 84–90% Spec: 90–92% vs 91% Acc: 72–81% vs 88–91% Lower abdomen  Choi et al [ 87 ] CT 57 patients CT performance Sens: 45% Spec: 72% PPV: 46% NPV: 72%  Low et al [ 5 ] CT and MRI 22 patients Preoperative PCI: CT vs MRI Sens: 55% vs 95% Spec: 86% vs 70% Acc: 63% vs 88%  Cianci et al [ 88 ] MRI 24 patients Conventional MRI vs Conventional MRI + DWI Sens: 55% vs 87 Spec: 64% vs 80% Small bowel and mesentery  Low et al [ 59 ] CT and MRI 164 patients CT vs MRI Mesentery and small intestine Mesentery Sens: 36% vs 57% Small intestine Sens: 63% vs 84%  Chua et al [ 57 ] CT 47 patients CT in preoperative PCI Small intestine Acc: 21–25% Sens: 56–57% Spec: 100%  Ricke et al [ 21 ] MRI 57 patients Detection of peritoneal implants in ovarian cancer Bowel and mesentery Sens: 73–77%. PCI peritoneal cancer index Head-to-head CT and MRI in different abdominal regions Sens: 25–50% Spec: 78–96% PPV: 44–80% NPV: 78–85% Sens: 54% vs 84% Spec: 91% vs 87% Acc: 74% vs 86% Conventional MRI vs Conventional MRI + DWI Sens: 52–73% vs 84–90% Spec: 90–92% vs 91% Acc: 72–81% vs 88–91% Sens: 45% Spec: 72% PPV: 46% NPV: 72% Preoperative PCI: CT vs MRI Sens: 55% vs 95% Spec: 86% vs 70% Acc: 63% vs 88% Conventional MRI vs Conventional MRI + DWI Sens: 55% vs 87 Spec: 64% vs 80% CT vs MRI Mesentery and small intestine Mesentery Sens: 36% vs 57% Small intestine Sens: 63% vs 84% Small intestine Acc: 21–25% Sens: 56–57% Spec: 100% Bowel and mesentery Sens: 73–77%. PCI peritoneal cancer index Regarding the subphrenic space, nodular or plaque-like thickening with enhancement along the diaphragmatic surface adjacent to the liver is the imaging sign of subphrenic involvement, often well recognizable on delayed gadolinium-enhanced MR images [ 24 ]. However, the motion artifact or susceptibility artifact at the lung bases should always be considered in MRI, and these limit the potential added value of DWI for detecting subtle tumors at this location [ 41 ]. The upper mesocolic space, including the gastrohepatic ligament and the hepatoduodenal ligament, should be evaluated to provide the surgeon with relevant information about the ligament’s involvement. The gastrohepatic ligament can be recognized by identifying the left gastric artery and the hepatoduodenal ligament by detecting both portal veins, common bile ducts, and hepatic arteries. Furthermore, the fluid distention of the lesser sac is a sign of tumor implants; other signs include thickening, nodules, and stranding [ 44 ]. Perihepatic ligaments and fissures are commonly involved with variable patterns, from diffuse soft-tissue stranding to nodular or plaque-like implants; these should always be detected and reported, considering this finding is a criterion of unresectability [ 44 ]. A direct sign of tumor infiltration of the hepatic hilum is the absence of a peri-portal fat plane due to tumor replacement [ 24 ]. Overall, DWI combined with conventional MRI seems to improve the detection of malignant deposits in the right subhepatic region compared with traditional contrast-enhanced CT, achieving a sensitivity of around 95% (Fig.  10 ) [ 41 ]. Fig. 10 Pancreatic cancer with peritoneal carcinomatosis in a 64-year-old woman. MRI DWI (b = 800 mm 2 /s) ( A ) demonstrated four nodular implants in the right hypochondrium (arrowheads), of which only three were evident on T2w (arrowheads) ( B ) and two on CT portal-venous phase (arrowheads) ( C ) Pancreatic cancer with peritoneal carcinomatosis in a 64-year-old woman. MRI DWI (b = 800 mm 2 /s) ( A ) demonstrated four nodular implants in the right hypochondrium (arrowheads), of which only three were evident on T2w (arrowheads) ( B ) and two on CT portal-venous phase (arrowheads) ( C )

The peritoneum is a serous membrane that covers most of the abdominal organs. It has two main layers: the parietal peritoneum and the visceral peritoneum. Between these layers, there is the peritoneal cavity that contains the serous fluid. The transverse colon divides the abdominal cavity into supra and inframesocolic space. The former knows a right and left supramesocolic space, separated by a falciform ligament. Gastrohepatic and hepatoduodenal ligaments form the lesser omentum. The oblique small bowel mesentery divides the inframesocolic space. The right paracolic gutter is continuous perihepatic space, and the left paracolic gutter is divided from the left subphrenic space by the phrenicocolic ligament. Supra and inframesocolic spaces communicate thanks to the right paracolic gutter. Regarding the pelvis, peritoneal reflections create the midline recto-vesical pouch in males, the recto-uterine pouch (pouch of Douglas) in females, and the paravesical fossae (Fig.  1 ) [ 9 ]. Fig. 1 Anatomic scheme representation of peritoneal folds, ligaments and abdominal spaces. FL, falciform ligament; GSL, gastro-splenic ligament; SRL, splenic-renal ligament; GCL, gastro-colic ligament Anatomic scheme representation of peritoneal folds, ligaments and abdominal spaces. FL, falciform ligament; GSL, gastro-splenic ligament; SRL, splenic-renal ligament; GCL, gastro-colic ligament Peritoneal implants usually occur as secondary lesions, mainly from gastrointestinal or ovarian cancer and rarely from breast or lung cancers, each one with different histological findings that translate into several imaging characteristics. Mostly, peritoneal implants are solid with reduced enhancement in almost all histology, except for mucinous histology, which could manifest as cystic or calcified lesions, and neuroendocrine tumors, which could appear as hyperattenuating lesions in the arterial and portal venous phase [ 10 ]. Moreover, a different entity, originating from the perforated epithelial neoplasm of the appendix, known as pseudomyxoma peritonei (PMP), is usually characterized by cystic lesions with mucinous patterns with a variable aggressive behavior [ 11 ]. Among the primary malignancies of the peritoneum, mesothelioma is the most common, followed by primary peritoneal papillary serous carcinoma or primary peritoneal carcinoma [ 12 , 13 ]. Peritoneal mesothelioma is known in two variants, benign and malignant, originating from the pleura’s mesothelial cells and, most rarely, from the peritoneum, pericardium, tunica vaginalis testis, and ovarium [ 12 ]. There are three most prevalent presentations of malignant peritoneal mesothelioma: dry type, wet type, and mixed type, according to the presence of ascites [ 13 ]. Primary peritoneal papillary serous carcinoma or primary peritoneal carcinoma is an epithelial tumor that predominantly affects women in their fifth and sixth decades of life [ 14 ].

Several peritoneal diseases mimic PC, such as other secondary and primary peritoneal tumors (e.g., peritoneal lymphomatosis and peritoneal mesothelioma) or inflammatory and benign conditions (e.g., peritoneal tuberculosis, endometriosis, splenosis, and fibrosis) (Figs. S2 , S3 ). Looking at secondary peritoneal malignancies, the peritoneal lymphomatosis is one of the most relevant, representing the intraperitoneal spread of lymphoma (Fig.  S4 ). Its imaging features are similar to those of PC with diffuse thickening of peritoneal surfaces, peritoneal nodules, or masses. However, bulky homogeneous masses, diffuse retroperitoneal lymphadenopathy, hepatosplenomegaly, and hepatic or splenic nodules are helpful signs of peritoneal lymphomatosis [ 60 , 61 ]. While peritoneal malignant mesothelioma is the most common primary malignant peritoneal tumor (Fig.  S5 ). It may simulate peritoneal carcinomatosis on imaging, demonstrating diffuse peritoneal thickening, multifocal peritoneal nodules, and omental cake, with or without ascites. The diagnosis should be suggested when there is a history of asbestos exposure and concomitant pleural plaque. Additionally, calcifications and lymphadenopathy are usually absent [ 62 , 63 ]. Regarding the benign conditions, peritoneal endometriosis should not be forgotten in the differential diagnosis of pelvic peritoneal lesions. Endometriosis, defined as the abnormal implantation of endometrial cells in organs other than the uterus, affects up to 45% of women of childbearing age, presenting with chronic pelvic pain and dysmenorrhea [ 64 ]. On MRI, it is easy to identify the blood component of endometriosis localizations, most frequently located in the pelvis, but in other cases also in upper abdominal regions, appear hyperintense in T1-weighted sequences and, depending on the stage of degradation, with variable intensity in T2 and has no contrast enhancement (Fig.  S6 ). In contrast, the fibro-stromal component appears isointense in T1 and hypointense in T2 and presents enhancement even in the delayed phase, thus making it more challenging to characterize. In addition, endometriosis, like peritoneal carcinomatosis, can contribute in all quadrants of the abdomen [ 65 ]. Then, MRI is more indicated in the pelvis study because of its better sensitivity on soft tissues at this level [ 66 ], and any findings should be put in differential diagnosis with multiple local benign and malignant pathologies. A rarer benign pathology that can always occur in women is the abdominal leiomyomatosis, of uncertain origin, which may be related to uterine artery embolization and laparoscopic myomectomy [ 67 ]. It is characterized by the proliferation of smooth muscle cells, with the formation of multiple nodules that may be located primarily in the lower abdomen but may also involve the upper abdomen. The nodules appear lobulated, with well-defined margins and gradual homogeneous enhancement on both CT and MRI. On MRI, the implants may be hyperintense in T2 with intermediate hyperintensity in DWI and low signal value in ADC maps [ 67 , 68 ]. However, proper differential diagnosis is difficult, both because of the rarity of abdominal leiomyomatosis and the nonspecific imaging features of the nodules. However, the patient’s remote pathological history can help in doubtful cases. Among the inflammatory diseases of the peritoneum, it is necessary to mention the peritoneal tuberculosis (Fig.  S2 ). It is an uncommon manifestation of tuberculosis, and the imaging appearance may overlap with PC, making it difficult to differentiate between the two. However, a smooth and regular thickening of the peritoneal lining is more characteristic of peritoneal tuberculosis. Moreover, associated hypoattenuating lymphadenopathy for caseous necrosis, splenic, or lymph node calcification may suggest the diagnosis [ 69 , 70 ]. Peritoneal tuberculosis has two most common patterns: the “dry/plastic” type, characterized by caseous nodules, fibrous peritoneal reactions, and dense adhesions [ 71 ], and the “fixed fibrotic” type, characterized by peritoneal masses adhering to adjacent digestive structures, which might lead to obstruction. However, specific peritoneal tuberculosis signs are the presence of mesenteric macro-nodules, enhancement and consistent thickening of the parietal peritoneum, splenomegaly and splenic calcifications, involvement of the ileocecal wall, and retroperitoneal or peri-pancreatic lymphadenopathy with a hypodense center and ring-enhancement. Furthermore, peritoneal thickening in tuberculosis tends to be smoother and more regular than PM [ 69 , 71 – 73 ]. Another sign that can be considered typical of tuberculosis is the “omental ring” sign on CT, defined by a thin or thick rim of uniform thickness, enhancing either moderately or significantly and clearly outlining the entire or part of the omentum during the venous phase [ 74 ]. This sign can identify 85% of patients with peritoneal tuberculosis and is absent in 96% of patients with PC, making it effective in ruling out PC. One more challenging differential diagnosis is between peritoneal fibrosis and PM. The former is a condition usually occurring in a postoperative setting, also in oncological patients and patients treated with peritoneal dialysis. More literature data should be present about this head-to-head comparison. Still, the general direction is that differentiating peritoneal fibrosis from PM using conventional imaging (CT, MRI, or PET/CT) is difficult and often impossible [ 75 ].

Beyond the conventional imaging methods, new modalities have been acquiring promising results in diagnosing and assessing patients with PM, including PET/CT with fibroblast activation protein inhibitor (FAPI), PET/MRI, whole-body MRI, and spectral CT. FAPI-PET might be seen as a target imaging, due to the fibroblast activation protein inhibitor (FAPI), a radiopharmaceutical labeled with either 68 Ga (gallium-68) or 18 F (Fluorine-18), targeting the fibroblast activation protein (FAP), overexpressed in cancer-associated fibroblasts (CAF) and rarely expressed in normal tissues [ 86 ]. By using FAPI-PET, it could be possible to identify the peritumoral stroma and support matrix, mainly in epithelial tumors, and in very small lesions (> 2 mm) [ 87 ]. In comparison with conventional FDG-PET, the FAPI-PET could be superior in detecting small micronodular lesions [ 88 – 90 ], also covering the gap for low FDG-avidity neoplasms (e.g., mucinous adenocarcinoma), having a higher tumor-to-background ratio than FDG-PET due to low intestinal and hepatic uptake [ 91 ]. Until now, FAPI-PET has been used for clinical research studies, but more large-scale comparative studies with long-term follow-up are required. False-positive findings are not rare in patients with inflammatory diseases (gastritis, abscesses), granulomatous diseases, or conditions where a fibrotic reaction is present (e.g., myelofibrosis and liver cirrhosis) [ 92 ]. Remaining in functional imaging, the proposal is to integrate PET and MRI, which could enhance peritoneal implant detection by synergistically providing anatomical, functional, and metabolic information while reducing radiation exposure. Furtado et al [ 93 ] found that PET/MRI might be more effective than PET alone and MRI or in combination with CT, changing the therapeutic management in 26% (5/19) of patients. New perspectives are emerging about the usefulness of whole-body MRI, mainly in the detection of extra-peritoneal lesions, such as enlarged lymph nodes, pleural lesions, and small liver metastases [ 7 ]. Having a comprehensive view of disease burden is essential in all patients who are candidates for HIPEC treatment, and then design the therapeutic scheme tailored to the patient [ 94 ]. The recommended sequences for the whole-body MRI are the axial T2 with reduced slice thickness (≤ 4 mm), DWI with b50 and b1000 s/mm 2 , and coronal/axial T1 post-contrast (3–5 min). The use of negative oral contrast medium and the antispasmodic agent is recommended but optional [ 7 ]. Finally, spectral CT scanners could be promising in PM, thanks to their technologies based on dual-energy or multi-energy technologies. The new photon-counting spectral CT (SPCCT) offers the highest spatial resolution, a high contrast-to-noise ratio (CNR), reduced radiation dose, and enables more than two types of multi-contrast imaging [ 95 ]. These data can be converted into various types of spectral imaging, such as iodine density imaging, virtual non-contrast imaging (VMI), virtual monoenergetic imaging, and Z-effective imaging. VMI allows for low-energy reconstructions (40–50 keV) that enhance the visualization of intravascular contrast, aiding in better differentiation of small and vascularized lesions with a lower dose of contrast agent and reducing the need for costly and lengthy examinations such as MRI. To date only limited data are available in the literature regarding the applicability of spectral maps to peritoneal malignancies, however, the emerging data suggest that VMI at low energy levels produced substantially higher CNR and SNR values with superior lesion detection rate [ 96 ] and SPCCT (with a dual-contrast protocol between the peritoneum and blood vessels) has higher sensitivity (69%) and specificity (100%) compared to conventional CT for small lesions (< 5 mm) [ 97 ]. Despite the promising data emerging from the new imaging methods, validation in multicenter studies is needed for their utility in clinical practice to be approved.

Overall PM spreads within the peritoneal cavity, following the circulation and reabsorption routes of peritoneal fluid. Therefore, understanding the routes of dissemination and the dynamics of peritoneal fluid circulation is crucial for comprehending the critical signs of peritoneal carcinomatosis. There are four dissemination routes: Hematogenous spread is the principal pathway for primary tumors with a high grade of malignancy. Contiguous spread refers to the local progression of carcinomatosis, which occurs when a large tumor crosses the serous membrane to invade adjacent organs. Lymphatic spread refers to the lymphatic dissemination, the lymphatic system of the greater omentum and the right side of the subphrenic lymphatic system drains into the anterior mediastinal lymphatic chain and the right lymphatic duct. Peritoneal surface spread refers to the location of peritoneal implants, which tend to form in anatomical regions where fluid accumulates the most, linked to gravity, intestinal peristaltic movements, and the pressure gradient caused by diaphragmatic movement during respiration, and in areas with the highest fluid reabsorption. When standing, the peritoneal fluid accumulates in the most sloping areas of the peritoneal cavity, such as the recto-vesical and paravesical recesses (Fig.  1 ). During exhalation, a negative pressure gradient is generated under the diaphragm, which moves the peritoneal fluid cranially through the paracolic gutters. The fluid mainly ascends along the proper paracolic gutters, which are wider than the left and communicate freely with the subdiaphragmatic and perihepatic space [ 15 ]. Hematogenous spread is the principal pathway for primary tumors with a high grade of malignancy. Contiguous spread refers to the local progression of carcinomatosis, which occurs when a large tumor crosses the serous membrane to invade adjacent organs. Lymphatic spread refers to the lymphatic dissemination, the lymphatic system of the greater omentum and the right side of the subphrenic lymphatic system drains into the anterior mediastinal lymphatic chain and the right lymphatic duct. Peritoneal surface spread refers to the location of peritoneal implants, which tend to form in anatomical regions where fluid accumulates the most, linked to gravity, intestinal peristaltic movements, and the pressure gradient caused by diaphragmatic movement during respiration, and in areas with the highest fluid reabsorption. When standing, the peritoneal fluid accumulates in the most sloping areas of the peritoneal cavity, such as the recto-vesical and paravesical recesses (Fig.  1 ). During exhalation, a negative pressure gradient is generated under the diaphragm, which moves the peritoneal fluid cranially through the paracolic gutters. The fluid mainly ascends along the proper paracolic gutters, which are wider than the left and communicate freely with the subdiaphragmatic and perihepatic space [ 15 ].

The diagnosis, preoperative assessment, and therapeutic workflow of peritoneal malignancies mainly depend on a multimodality approach. CT scan remains the first imaging method due to its availability, robustness, and panoramic evaluation. Overall, CT has several consistent limitations concerning identifying small implants in critical regions. MRI could support the radiologist’s ability to provide a more accurate preoperative assessment, especially in evaluating essential areas. Becoming aware of the mismatch between radiological and surgical PCI and having a multimodality imaging approach is helpful to reduce the standard CT and MRI pitfalls leading to over- or underestimation of peritoneal involvement.

The imaging findings of PM on CT and MRI vary depending on the specific histologic subtypes of the primary tumor. Peritoneal implants most commonly appear as solid lesions characterized by soft-tissue plaques, nodules, or masses with variable enhancement on CT images [ 1 , 24 ]. A multiparametric imaging approach could be suggested, considering the variable PC findings. In general, different patterns of lesions have been identified. In case of low burden of disease, it is more likely to have the following patterns: Cystic metastases are generally derived from primary mucinous carcinomas or primary peritoneal malignancies (Fig.  2 ) [ 25 ]. Fig. 2 Benign peritoneum mesothelioma in a 61-year-old woman. A CT porta-venous phase, axial plane showed cystic implants in the right and left iliac fossa (white arrows), confirmed in MRI T2w ( B , black arrows) and DWI (b = 800 mm 2 /s) ( C , white arrows) Micronodular pattern: indicating the presence of peritoneal implants less than 5 mm in diameter, affecting the peritoneum and the mesenteric fat (Fig.  3 ). Infiltration of the small bowel mesentery may produce characteristic stellate patterns, which consist of increased mesenteric fat attenuation and perivascular soft-tissue thickening caused by the infiltration of the perivascular bundles [ 26 ]. Fig. 3 Colon adenocarcinoma with peritoneal carcinomatosis in a 74-year-old woman. A CT delayed phase, axial plane. Relevant diffuse implants with micronodular pattern in the left hypochondrium (arrowheads). MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane, no micronodular implants are visible The nodular pattern is characterized by a nodular lesion greater than 5 mm in diameter that might be solid or cystic, depending on the primary tumor (Fig.  4 ) [ 26 ]. Fig. 4 Colon adenocarcinoma with peritoneal carcinomatosis in an 81-year-old woman. CT portal-venous phase ( A ) and MRI T2w ( B ) demonstrated a nodular pattern in the left iliac fossa (arrowheads) and diffuse ascites (stars) Calcification: manifesting as areas of hypointensity on MRI scans, both on T1 and T2-weighted sequences and regions of hyperattenuating on CT scan (Fig.  5 ). Calcified peritoneal implants may suggest a primary ovarian serous cystadenocarcinoma, a primary papillary mucinous carcinoma, or as a result of chemotherapy [ 27 ]. Fig. 5 Mucinous colon cancer with peritoneal implants in a 57-year-old man. CT delayed phase ( A ) and MRI T2w ( B ) showed calcifications (arrowheads) within the mucinous implants Cystic metastases are generally derived from primary mucinous carcinomas or primary peritoneal malignancies (Fig.  2 ) [ 25 ]. Fig. 2 Benign peritoneum mesothelioma in a 61-year-old woman. A CT porta-venous phase, axial plane showed cystic implants in the right and left iliac fossa (white arrows), confirmed in MRI T2w ( B , black arrows) and DWI (b = 800 mm 2 /s) ( C , white arrows) Benign peritoneum mesothelioma in a 61-year-old woman. A CT porta-venous phase, axial plane showed cystic implants in the right and left iliac fossa (white arrows), confirmed in MRI T2w ( B , black arrows) and DWI (b = 800 mm 2 /s) ( C , white arrows) Micronodular pattern: indicating the presence of peritoneal implants less than 5 mm in diameter, affecting the peritoneum and the mesenteric fat (Fig.  3 ). Infiltration of the small bowel mesentery may produce characteristic stellate patterns, which consist of increased mesenteric fat attenuation and perivascular soft-tissue thickening caused by the infiltration of the perivascular bundles [ 26 ]. Fig. 3 Colon adenocarcinoma with peritoneal carcinomatosis in a 74-year-old woman. A CT delayed phase, axial plane. Relevant diffuse implants with micronodular pattern in the left hypochondrium (arrowheads). MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane, no micronodular implants are visible Colon adenocarcinoma with peritoneal carcinomatosis in a 74-year-old woman. A CT delayed phase, axial plane. Relevant diffuse implants with micronodular pattern in the left hypochondrium (arrowheads). MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane, no micronodular implants are visible The nodular pattern is characterized by a nodular lesion greater than 5 mm in diameter that might be solid or cystic, depending on the primary tumor (Fig.  4 ) [ 26 ]. Fig. 4 Colon adenocarcinoma with peritoneal carcinomatosis in an 81-year-old woman. CT portal-venous phase ( A ) and MRI T2w ( B ) demonstrated a nodular pattern in the left iliac fossa (arrowheads) and diffuse ascites (stars) Colon adenocarcinoma with peritoneal carcinomatosis in an 81-year-old woman. CT portal-venous phase ( A ) and MRI T2w ( B ) demonstrated a nodular pattern in the left iliac fossa (arrowheads) and diffuse ascites (stars) Calcification: manifesting as areas of hypointensity on MRI scans, both on T1 and T2-weighted sequences and regions of hyperattenuating on CT scan (Fig.  5 ). Calcified peritoneal implants may suggest a primary ovarian serous cystadenocarcinoma, a primary papillary mucinous carcinoma, or as a result of chemotherapy [ 27 ]. Fig. 5 Mucinous colon cancer with peritoneal implants in a 57-year-old man. CT delayed phase ( A ) and MRI T2w ( B ) showed calcifications (arrowheads) within the mucinous implants Mucinous colon cancer with peritoneal implants in a 57-year-old man. CT delayed phase ( A ) and MRI T2w ( B ) showed calcifications (arrowheads) within the mucinous implants When the disease burden is high, it is more common to have the following patterns: Plaque-like pattern: Plaques are irregular soft-tissue thickenings created by the confluence of multiple nodular implants typically found in the upper abdomen (Fig.  6 ). When plaques cover the capsular edges of intraperitoneal organs, they appear as areas of lower attenuation than the parenchyma in post-contrast scans. This creates indentations on the surface, particularly of the liver and spleen, giving a “scalloping” appearance [ 28 ]. Fig. 6 Gastric cancer with peritoneal carcinomatosis in an 81-year-old man. CT delayed phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane showed a large implant known as a “Plaque-like” pattern (arrowheads). Omental cake: large plaques surrounded by reactive fibrotic tissue that results in the consolidation of omental fat, which appears stratified and causes posterior displacement of the bowel relative to the anterior abdominal wall (Fig.  7 ) [ 29 ]. Fig. 7 Ovarian cancer with peritoneal carcinomatosis in a 72-year-old woman. CT portal-venous phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) (arrowheads) ( B , C ), axial plane showed diffuse involvement of omental fat known as “Omental cake” pattern (arrowheads) Mass-like pattern: this pattern involves soft-tissue masses of several centimeters, resulting from the confluence of multiple nodular implants. Masses more than 10 cm in diameter are called “bulky tumors.” Teca pattern: This involves significant engagement of the visceral peritoneum lining the loops of the small bowel, causing thickening, distortion, and fixation of the mesentery. This culminates in intestinal obstruction, a “frozen pelvis” (Fig.  8 ) [ 30 ]. Fig. 8 Malignant pleural mesothelioma with peritoneal implants in a 67-year-old man. CT delayed phase and portal-venous phase ( A , B ), axial plane. Small intestine loops were completely covered by a thickened layer of visceral peritoneum (arrowheads) known as the “Teca aspect.” Moreover, the parietal peritoneum is thickened (arrows) Plaque-like pattern: Plaques are irregular soft-tissue thickenings created by the confluence of multiple nodular implants typically found in the upper abdomen (Fig.  6 ). When plaques cover the capsular edges of intraperitoneal organs, they appear as areas of lower attenuation than the parenchyma in post-contrast scans. This creates indentations on the surface, particularly of the liver and spleen, giving a “scalloping” appearance [ 28 ]. Fig. 6 Gastric cancer with peritoneal carcinomatosis in an 81-year-old man. CT delayed phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane showed a large implant known as a “Plaque-like” pattern (arrowheads). Gastric cancer with peritoneal carcinomatosis in an 81-year-old man. CT delayed phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) ( B , C ) axial plane showed a large implant known as a “Plaque-like” pattern (arrowheads). Omental cake: large plaques surrounded by reactive fibrotic tissue that results in the consolidation of omental fat, which appears stratified and causes posterior displacement of the bowel relative to the anterior abdominal wall (Fig.  7 ) [ 29 ]. Fig. 7 Ovarian cancer with peritoneal carcinomatosis in a 72-year-old woman. CT portal-venous phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) (arrowheads) ( B , C ), axial plane showed diffuse involvement of omental fat known as “Omental cake” pattern (arrowheads) Ovarian cancer with peritoneal carcinomatosis in a 72-year-old woman. CT portal-venous phase ( A ), MRI T2w and DWI (b = 800 mm 2 /s) (arrowheads) ( B , C ), axial plane showed diffuse involvement of omental fat known as “Omental cake” pattern (arrowheads) Mass-like pattern: this pattern involves soft-tissue masses of several centimeters, resulting from the confluence of multiple nodular implants. Masses more than 10 cm in diameter are called “bulky tumors.” Teca pattern: This involves significant engagement of the visceral peritoneum lining the loops of the small bowel, causing thickening, distortion, and fixation of the mesentery. This culminates in intestinal obstruction, a “frozen pelvis” (Fig.  8 ) [ 30 ]. Fig. 8 Malignant pleural mesothelioma with peritoneal implants in a 67-year-old man. CT delayed phase and portal-venous phase ( A , B ), axial plane. Small intestine loops were completely covered by a thickened layer of visceral peritoneum (arrowheads) known as the “Teca aspect.” Moreover, the parietal peritoneum is thickened (arrows) Malignant pleural mesothelioma with peritoneal implants in a 67-year-old man. CT delayed phase and portal-venous phase ( A , B ), axial plane. Small intestine loops were completely covered by a thickened layer of visceral peritoneum (arrowheads) known as the “Teca aspect.” Moreover, the parietal peritoneum is thickened (arrows)

Peritoneal involvement can result from primary or metastatic disease [ 1 ]. Rarely is the peritoneum affected by primary tumors originating from mesothelial cells, which commonly have a better prognosis than peritoneal carcinomatosis (PC). Secondary involvement of the peritoneum is the most common peritoneal involvement, known as PC, in which the primary tumor arises from various cancers, mainly gastrointestinal or gynecological [ 2 ]. PC is associated with an inferior prognosis, with an average survival from 6 months to 5 years, due to the aggressiveness of the primary cancer [ 3 , 4 ]. In the last few years, the PC prognosis has slightly improved due to the new proposals in which the combination of systemic chemotherapy, hyperthermic intraperitoneal chemotherapy, and cytoreductive surgery takes the lead [ 2 ]. From an imaging point of view, CT remains the first option to study the extension of malignant peritoneal malignancies (PM) due to its availability, cost-effectiveness, and spatial resolution, but it is limited in contrast resolution [ 5 ]. MRI could be seen as a problem solver to improve imaging performance in terms of sensitivity and specificity only in dedicated anatomical regions (e.g., small bowel or pelvis) [ 6 ]. New international guidelines were published by the major international societies [ 7 , 8 ], indicating CT as the mandatory imaging approach, even considering that MRI and PET/CT might be seen as two complementary methods. However, while MRI has a definite role in staging, especially in the study of dedicated anatomic regions or as an alternative to contrast-enhanced CT, PET has a role only in cases of doubtful metastatic lymph nodes, high suspicion of persistent disease undetectable at CT, and in peritoneal pseudomyxoma [ 7 , 8 ]. Thus, this review aims to provide an overview of the main cornerstones in peritoneal malignancies, focusing on anatomy, histology, CT and MRI protocol, imaging findings, pitfalls, and future perspectives.

In the last few years, the prognosis has slightly improved due to a combination of hyperthermic intraperitoneal chemotherapy (HIPEC) and cytoreductive surgery (CRS) [ 2 ]. HIPEC consists of the instillation of the chemotherapeutic agent at the temperature of 41–43 °C directly into the peritoneum, aiming to achieve a high and persistent drug concentration in the tumor while limiting systemic to maximize therapeutic efficacy and minimize side effects [ 55 ]. The treatment is mainly dependent on primary cancer; peritoneum primary tumors are usually treated with an upfront surgical approach, while in the case of PC, the role of neoadjuvant or adjuvant chemotherapy is relevant [ 76 ]. In identifying patients who could benefit from surgical resection, imaging has the leading role of detecting the two most essential criteria for unresectability and describing disease burden in terms of peritoneal cancer index (PCI) (Fig.  12 ) [ 77 ]. Historically, PCI was introduced as a quantitative surgical score providing important patient prognosis information [ 78 ]. It consists of surgical exploration of 13 abdominal regions, determining the extension of the cancer (9 superficial quadrants and 4 intestinal segments). In each of these regions is given a score from L0 to L3: L0 is the absence of tumor; L1 means lesion size under 0.5 cm; L2 is a lesion between 0.5 and 5 cm; L3 is a tumor larger than 5 cm. Minimum score is 0, while the maximum score is 39. From a predictive point of view, a PCI score lower or equal to 10 means a 5-year survival rate of 50%, PCI between 11 and 20 means a survival rate of 20%, and PCI over 20 means a survival rate under 10%. PCI over 20 contraindicates cytoreductive surgery [ 79 ]. Thus, the radiological challenge was to predict a radiological PCI to provide some relevant information regarding prognosis and resectability (Fig.  S7 ). Some comparative studies demonstrated intermediate results in terms of sensitivity and specificity of both CT- and MRI-PCI [ 5 ]; more specifically, the assessment of general CT-PCI achieved a sensitivity of 76% and specificity of 69% [ 3 ]. On one hand, MRI is more accurate in identifying small implants and investigating the status of the small intestine [ 5 , 80 , 81 ]. On the other hand, CT has several advantages, such as three-dimensional visualization, spatial resolution, and high availability, compared to MRI, which has high costs, long scanning time, reduced availability, potential artifacts due to patients’ movement, magnetic susceptibility, reduced inter-reader agreement, and low spatial resolution [ 1 ]. Despite the difficulties in accurately predicting PCI, it is recommended to report it in the radiology report to provide the surgeon with as much information as possible regarding the disease burden. To date, there are no universal criteria for inoperability; much depends on the decisions provided by the multidisciplinary meetings, and the experience of the surgical team [ 82 ]. There are a few points that should always be discussed: first, the quality of life for the patient after surgery, second, the possibility of getting to an R0, and finally, the extent of disease. All these points depend on the sites of disease; in fact, some are difficult to attack surgically, among them we have extra- and retroperitoneal lesions, involvement of the small intestine, hepatic hilum, mesentery, and pelvic structures such as the bladder [ 83 ]. Regarding the restaging assessment, while there is the completeness of cytoreduction score (CCR) a validated surgical score for reassessment of response to therapy [ 84 ], from the imaging perspective the recent guidelines suggest to re-assess the patients with similar CT protocol to staging setting. Considering the 18 F-FDG PET/CT only in doubtful cases, as a high suspicion of persistent but undetectable disease on CT scan [ 8 ]. Regarding post-treatment residual disease, only few data are available in the literature; however, the emerging finding concerns the inability of CT to differentiate fibrosis and necrosis from residual disease [ 85 ]. Recently, a simplified peritoneal tumor index (S-PCI) score was tested as a CT predictor of treatment response, but no significant results were obtained in the correlation between S-PCI and pathologic chemotherapy response score [ 85 ]. Even in the new international guidelines, there is no clear indication regarding residual disease or PM restaging [ 7 , 8 ]. However, it is essential to consider that in many malignancies the term of restaging may coincide with the preoperative term; in fact, neoadjuvant chemotherapy is almost always recommended in cases of peritoneal involvement.

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