Path beyond the blind end-unravel the imaging spectrum of appendiceal pathologies

The appendix is involved in a diverse spectrum of inflammatory, infectious, benign, and malignant conditions that extend far beyond acute appendicitis. Although acute appendicitis remains the most common appendiceal emergency, numerous entities—including reactive appendiceal inflammation, inflammatory bowel disease, appendiceal endometriosis, mucinous and non-mucinous neoplasms, lymphoma, post-transplant lymphoproliferative disorder, and rare mesenchymal tumors—may present with similar clinical symptoms and overlapping imaging findings. Accurate distinction among these conditions is essential because management ranges from conservative medical therapy to appendectomy, right hemicolectomy, cytoreductive surgery or systemic oncologic treatment. Furthermore, several common mimics of appendiceal disease, including mesenteric adenitis, terminal ileitis, epiploic appendagitis, cecal diverticulitis, gynecologic disorders, and ureteric calculi, can closely resemble appendiceal pathology and may lead to inappropriate treatment if not correctly recognized. This narrative imaging review provides a comprehensive multimodality imaging approach to appendiceal diseases using ultrasound (US), computed tomography (CT), magnetic resonance imaging (MRI), and molecular imaging techniques. Emphasis is placed on imaging features that facilitate differential diagnosis, clinicopathologic and surgical correlation, recognition of disease mimics, assessment of complications, and determination of disease extent. The review highlights how radiologists contribute not only to diagnosis but also to treatment planning, surgical decision-making, staging, surveillance, and multidisciplinary patient management. Emerging applications of dual-energy CT, radiomics, artificial intelligence, and advanced molecular imaging are also discussed. By integrating imaging findings with clinical and pathologic considerations, this review aims to improve diagnostic accuracy, guide appropriate management, and strengthen the radiologist's role in the comprehensive evaluation of appendiceal pathology. Similar content being viewed by others

The vermiform appendix hosts a wide range of pathologic conditions ranging from common infections and inflammatory processes to rare neoplastic entities. Recent advances in imaging have significantly improved the detection and characterization of these diseases, many of which were historically diagnosed only at surgery or on histopathological analysis [1, 2]. Although acute appendicitis and appendicoliths remain the most frequent clinical encounters in practice, clinicians must navigate overlapping presentations of a wide array of other conditions, including inflammatory bowel disease, foreign bodies, and rare neoplasms. These entities often present with similar clinical and imaging features, posing diagnostic challenges for radiologists and clinicians alike [2, 3]. Appendiceal neoplasms, although rare, encompass a broad spectrum of tumors with distinct biological behaviors and prognostic implications. Differentiation among these entities is important, as management strategies range from simple appendectomy to extensive cytoreductive surgery and systemic therapy [2, 4]. Cross-sectional imaging has transformed the evaluation of appendiceal diseases. Advanced CT techniques provide excellent diagnostic accuracy for inflammatory and neoplastic processes, while ultrasound and MRI play important complementary roles, particularly in pediatric, pregnant, and radiation-sensitive populations. Beyond detecting the pathology, imaging is increasingly used to characterize lesion biology, identify complications, assess disease extent, guide surgical planning, and monitor treatment response. Emerging techniques, including dual-energy CT, radiomics, and molecular imaging, further expand the diagnostic and prognostic capabilities of radiology. This review provides a comprehensive multimodality imaging overview of appendiceal pathologies with emphasis on diagnostic imaging findings, clinicopathologic and surgical correlation, differential diagnosis, and emerging imaging technologies. By integrating common and uncommon appendiceal diseases, this review aims to enhance diagnostic accuracy and support optimal multidisciplinary patient care. Flowchart 1 summarizes the spectrum of appendiceal disorders described in this review, encompassing both common conditions and rare appendiceal neoplasms. Anatomy, histology, and positional variations of the appendix The appendix develops from the midgut during embryologic intestinal rotation as a diverticular extension of the cecum. Typical morphologic characteristics include blind-ending tubular configuration, diameter ranging from 6 mm to 11 mm, and a mural thickness of 1.5 mm [5]. The appendix demonstrates substantial positional variability due to differences in embryologic midgut rotation, cecal descent, and mesenteric mobility [6]. Although the appendiceal tip demonstrates considerable positional variability, its base remains constant, originating from the posteromedial cecum approximately 1.7 cm to 2.5 cm inferior to the ileocecal valve at the convergence of three taenia coli bands. The length of the appendix ranges between 1 and 25 cm, averaging about 8 cm [7, 8]. Recognition of appendiceal orientation is clinically important because symptom localization, inflammatory spread, and potential diagnostic mimics depend strongly on appendiceal position on cross-sectional imaging [2, 9]. Historical surgical observations together with contemporary radiologic series consistently demonstrate retrocecal positioning as the most common appendiceal configuration, followed by pelvic orientation, with less frequent subcecal, pre-ileal, retro-ileal, and paracecal locations (Table 1) (Fig. 1) [6, 9]. The appendix receives its primary blood supply from the appendicular artery, which arises from the ileocolic artery and travels within the mesoappendix (the appendix’s mesentery). Branches of the anterior and posterior cecal arteries provide additional blood flow to the base of the appendix. Venous drainage occurs through the ileocolic and right colic veins, while lymphatic drainage is directed to the ileocolic lymph nodes located near the superior mesenteric artery [3, 8, 10, 11]. The histologic structure of the appendix closely resembles that of the colon, consisting of layers from the epithelium through the lamina propria, muscularis mucosa, submucosa, muscularis externa, and serosa. Its epithelium is composed of simple columnar cells (including enterocytes, goblet cells, and mucous cells) forming crypts, with enteroendocrine cells. In contrast, the lamina propria between crypts contains a higher density of lymphoid nodules than the colon. Thus, the appendix is not merely a site of appendicitis but can also be involved in other conditions, including inflammatory bowel disease and neoplasms [3, 8, 11]. Imaging modalities in the evaluation of the appendix Ultrasound (US) is the preferred initial imaging modality, particularly in pediatric and pregnant patients, due to its lack of ionizing radiation [12]. Graded compression with a high-frequency linear transducer displaces bowel gas, improving visualization. Normal sonographic features include a compressible blind-ending tubular structure, diameter < 6 mm, preserved mural stratification, and absence of hyperemia [13]. Computed Tomography (CT) represents the reference standard for adult appendiceal evaluation, with diagnostic sensitivity exceeding 94% [1, 14]. CT enables reliable identification of the appendix, detection of appendicoliths, evaluation of perforation or abscess formation, and identification of alternative diagnoses [2, 14]. Magnetic resonance imaging (MRI) increasingly serves as a radiation-free alternative, particularly in pregnancy and younger patients [15]. The diagnostic performance of MRI is comparable to that of CT in the evaluation of acute appendicitis when optimized protocols are employed. In addition, MRI may help evaluate appendiceal tumors, including characterization, extent, and distant spread [16, 17]. The role of nuclear medicine is mainly the use of 18FDG PET/CT for staging and evaluation of treatment response in malignancies involving the appendix [18]. Appendicitis and appendicoliths Acute appendicitis remains one of the most common surgical emergencies, and imaging plays a crucial role in diagnosis. The basic pathogenesis involves appendiceal luminal obstruction, which can result from various causes, including appendicolith, lymphoid hyperplasia, mass, or worms [19]. Acute appendicitis commonly occurs in young patients, mainly affecting children, adolescents, and young adults. The classic clinical presentation is usually a diffuse or periumbilical abdominal pain that later localizes to the right lower quadrant, nausea, and anorexia. Though graded-compression US, CT, and MRI are all reliable modalities for imaging suspected appendicitis, CT is preferred [11, 19]. US remains the first-line imaging modality, especially in children, thin patients, and pregnant women. At US, a noncompressible blind-ending tubular structure in the right lower quadrant with a diameter > 6 mm is highly suggestive of acute appendicitis [11, 19, 20]. Additionally, US shows mural thickening (> 3 mm) and periappendiceal inflammatory changes, appearing as increased fat echogenicity. The fluid-filled hypoechoic center surrounded by inner echogenic mucosa/submucosa and outer hypoechoic muscularis appears as a target sign on US [20]. On CT, a diameter ≥ 10 mm, or 6–10 mm in association with additional inflammatory findings including wall thickening, wall hyperenhancement, and peri-appendiceal fat stranding, supports the diagnosis of acute appendicitis (Fig. 2) [11, 19]. MRI evaluation of suspected appendicitis is mainly limited to paediatric patients and pregnant women. MRI demonstrates findings similar to CT, characterized by an appendiceal diameter greater than 6 mm and associated T2W hyperintense peri-appendiceal inflammatory changes (Fig. 2) [11, 19]. T2 hyperintense inflammatory wall thickening, along with possible restricted diffusion on MRI, which is a reliable indicator of active inflammation, similar to the wall hyperenhancement on CT [19]. Associated findings may include thickening of the adjacent caecal wall, reactive mesenteric lymphadenopathy, and localized fluid [19]. Imaging interpretation may be challenging in atypical cases, including tip appendicitis, stump appendicitis, and unusual appendiceal locations such as inguinal (Amyand hernia) or femoral hernias [21]. Congenital intestinal malrotation may result in atypical appendiceal positions and potentially delay diagnosis because of non-classical symptom localization. Stump appendicitis should be considered in patients with prior appendectomy who present with right lower quadrant pain, particularly when CT demonstrates an inflamed appendiceal remnant with surrounding fat stranding [21]. In some cases, mucocele can coexist with appendicitis. On CT, when there are findings of dilatation of appendix with luminal diameter > 13 mm, cystic luminal contents and peripheral mural calcifications in a patient with acute appendicitis, possibility of coexisting mucocele should be raised because surgical management will be altered (Fig. 3) [22]. Additionally, several conditions mimic appendicitis clinically, such as mesenteric adenitis, epiploic appendagitis, terminal ileitis, Meckel’s diverticulitis, cecal diverticulitis, and typhilitis. Also, gynecological pathology such as complicated ovarian cyst, ovarian torsion, and pelvic inflammatory disease, and urological pathology such as obstructing right ureteric calculus can clinically manifest similarly to appendicitis. Recognizing mimics is crucial to avoid unnecessary surgery, underscoring the role of radiology in the management of these patients [23, 24]. Common mimics of appendicitis and their imaging features are described in Table 2. Preoperative imaging evaluation of complications of acute appendicitis is critical because it guides management. Acute severe appendiceal and peri-appendiceal inflammation, along with decreased mural enhancement, is seen with gangrenous appendicitis (Fig. 4). Perforation is suggested by extra-luminal air, focal wall discontinuity, reduced mural enhancement, decreased Doppler vascularity, or an extraluminal appendicolith. While each of these imaging features individually demonstrates limited sensitivity for detecting perforation, their combined presence substantially improves diagnostic accuracy [25]. Perforated appendicitis may rarely present with remote abscess formation involving the hepatic, perihepatic, retroperitoneal, or perinephric regions [26]. Peri-appendiceal inflammation may manifest as a phlegmon or evolve into a rim-enhancing abscess requiring drainage [19] (Fig. 5). Pylephlebitis, or septic thrombosis of the mesenteric or portal veins, is an uncommon but serious complication that may lead to hepatic abscess. Small bowel obstruction can result from inflammatory ileus, secondary ileitis, entrapment within an inflammatory mass, or late fibrous adhesions [19]. An appendicolith (fecalith) represents as an intraluminal calcified concretion. It is readily identified as an echogenic focus with posterior acoustic shadowing on US or as a high attenuation focus on CT [11, 19]. Although not pathognomonic for appendicitis, its presence in the setting of appendiceal dilatation and mural inflammation supports luminal obstruction as the inciting mechanism. Importantly, appendicoliths are associated with an increased risk of complicated appendicitis, including perforation and abscess formation [4, 19]. Extraluminal migration of an appendicolith is highly suggestive of perforation and may be accompanied by extraluminal gas, focal wall discontinuity, phlegmon, or rim-enhancing abscess on contrast-enhanced CT (Fig. 5) [19]. Careful assessment of the appendicolith’s location relative to the appendiceal wall is essential for evaluating disease severity and guiding management. Although uncommon, foreign bodies in the appendix have been reported in children. Parasites like Enterobius vermicularis have been found in surgically removed appendices [4, 11]. Management of appendicitis is guided by whether the disease is uncomplicated or complicated (e.g., perforation, abscess, or phlegmon). Uncomplicated cases may be managed non-operatively with observation, bowel rest, and antibiotics. In the presence of an abscess or phlegmon, image-guided drainage (percutaneous or transrectal) can be considered. However, evidence increasingly favours initial non-operative treatment followed by operative management for unsuccessful conservative treatment [27]. Urgent surgery is indicated for complicated appendicitis with generalized peritonitis. Laparoscopic appendectomy is preferred over the open approach due to shorter hospital stay and fewer complications. For inflammatory bowel disease/Crohn’ s-related appendicitis, the management depends on the severity of the background IBD, including conservative versus surgical approach [28]. Reactive appendiceal inflammation and other inflammatory disorders of the appendix Radiologists must also distinguish primary appendicitis from reactive or secondary appendiceal inflammation. Reactive appendiceal inflammation may arise secondary to adjacent abdominal or pelvic inflammatory or infectious conditions, making comprehensive evaluation of the entire bowel essential to exclude alternative primary causes. For example, sigmoid diverticulitis can produce apparent appendiceal wall thickening when the sigmoid colon is positioned in the right lower quadrant. Similarly, pelvic inflammatory disease involving the right fallopian tube may lead to secondary appendiceal changes. In contrast, inflammatory bowel disease, particularly Crohn’s disease, can directly involve the appendix, presenting as chronic, circumferential transmural wall thickening with preserved mural stratification and surrounding inflamed, noncompressible fat near the ileum, typically without peri-appendiceal fat stranding. Because Crohn’s disease often affects multiple segments of the gastrointestinal tract, concurrent involvement of the ileum or colon—manifesting as wall thickening, and fat proliferation —is frequently observed (Fig. 6). Therefore, chronic appendiceal inflammation should be suspected in the presence of similar chronic changes elsewhere in the bowel, especially when peri-appendiceal fat stranding is absent [19, 21]. Xanthogranulomatous appendicitis is an uncommon chronic inflammatory process of the appendix that is frequently associated with recurrent or delayed-treated appendicitis and is diagnosed primarily on histopathology. Although CT and ultrasound findings are nonspecific, recognition of this entity is important because it may be associated with marked periappendiceal fibrosis, adhesions, and phlegmon, potentially mimicking a neoplasm and complicating surgical management [29]. Appendiceal neoplasms Appendiceal neoplasms are uncommon tumors of the gastrointestinal tract that may be of epithelial or non-epithelial origin. Based on the mucin production, epithelial tumors are classified into mucinous and non-mucinous neoplasms. The majority of epithelial neoplasms are mucinous, accounting for 70% of epithelial tumors [30]. Mucinous neoplasms Appendiceal mucinous neoplasms are classified into four types: mucinous adenoma, low-grade appendiceal mucinous neoplasm (LAMN), high-grade appendiceal mucinous neoplasm (HAMN), and mucinous adenocarcinoma as per the 2016 Peritoneal Surface Oncology Group International consensus classification based on histopathologic and biologic behavior including depth of invasion, degree of cellular atypia, presence of signet cells, and if mucin is present outside the lumen [31]. According to PSOGI guidelines, the distinction between extra-appendiceal acellular mucin (pM1a) and mucin with neoplastic epithelium (pM1b) is a critical prognostic factor, with acellular cases showing near 100% survival after resection. Conversely, mucin containing viable cells indicates true pseudomyxoma peritonei (pM1b), which often necessitates aggressive cytoreductive surgery and HIPEC due to high disease progression rates [32]. Appendiceal mucinous adenomas are composed of mucin-rich epithelium with villous architecture and cause circumferential involvement of the appendix [2, 3]. LAMNs have villous epithelium with low-grade cellular atypia, submucosal fibrosis, and mucin and/or cells outside the appendix. HAMNs exhibit high-grade cellular atypia (enlarged nuclei, mitosis, stratification) and cribriform-villous growth, with all other LAMN features [3, 33]. HAMNs are an intermediate risk between LAMNs and mucinous adenocarcinomas. Mucinous adenocarcinomas have infiltrative invasion, with neoplastic cells extending beyond the muscularis mucosa, to be diagnostic [2, 3]. The mucin can dissect the appendiceal wall in both LAMN and HAMN, leading to the development of pseudomyxoma peritonei [34]. The presence of signet ring cells has been associated with poor prognosis. The characteristic radiological finding of a mucinous neoplasm is a mucocele, which is a macroscopic description of a mucus-distended appendix. The extra-appendiceal spread of mucin can be localized in the peri-appendiceal region or disseminated as pseudomyxoma peritonei [3]. They have a characteristic expansile spread rather than infiltrative invasion [3]. Mucocele Abdominal radiographs may show a soft-tissue opacity in the right lower quadrant with curvilinear mural calcification, which is highly suggestive of mucocele but is present in fewer than 50% of cases [3, 35]. US of the pelvis or right lower quadrant will reveal a cystic ovoid mass with variable internal echogenicity (Fig. 7). Acoustic shadowing may be present due to mural calcification, and concentric echogenic layers can give the appendix an onion-skin appearance. It is important in females to separately delineate the right ovary from the mass to distinguish a cystic ovarian neoplasm or tubo-ovarian abscess [2, 3, 11, 36]. Mural calcification can lead to a porcelain appearance of the appendix [11]. Cross-sectional imaging helps delineate the relationship of the appendix with the cecum in comparison to US. CT would show a dilated tubular appendix filled with homogeneous hypoattenuating material. It is sensitive to detecting mural calcifications [2, 3] (Fig. 7). The presence of intraluminal gas, air-fluid levels, wall thickening, and peri-appendiceal fat stranding is concerning for infection [34]. On MRI, the mucocele shows simple fluid signal on T2-weighted images. T1 signal intensity is variable depending on mucin content with iso-hypo intensity [3, 37] (Fig. 7). Malignant mucoceles due to adenocarcinomas are associated with mural nodularity and irregular wall thickening, with peri-appendiceal soft-tissue deposits [3, 38] (Fig. 8). Pseudomyxoma peritonei Pseudomyxoma Peritonei is essentially the presence of mucin in the peritoneum and overlying the serosal surface of organs, with one of the common causes being appendiceal primary neoplasm. Imaging manifestations include hypoattenuating soft-tissue mucinous peritoneal implants and omental nodularity/caking with loculated ascites (Fig. 9). Mucin deposition over solid organs can lead to scalloped margins. Linear calcifications can sometimes be identified within the mucinous deposits [3]. Complications such as intussusception into the colon with mucocele as a lead point and right ureteral obstruction due to mass effect can be seen. Rarely, torsion with gangrene and hemorrhage can arise [2, 34]. A rare mucocele variant, myxoglobulosis, is seen on CT as multiple intraluminal calcified spherules [3]. The management of mucinous neoplasms depends on histological grading and peritoneal involvement. Adenomas and LAMNs confined to the appendix are treated with appendectomy [39]. However, right hemicolectomy should be considered for positive tumor margins, perforation, tumor size greater than 2 cm, high-grade histology, invasion of the muscularis propria, and nodal involvement. For HAMNs, a right hemicolectomy is typically recommended due to the high-grade nature of the neoplasms. If there is extra-appendiceal spread, CRS (cytoreductive surgery) followed by HIPEC (Hyperthermic Intraperitoneal Chemotherapy) is the standard of care. Long-term surveillance is required due to the risk of recurrence [40]. Non-mucinous neoplasms of the appendix Non-mucinous adenoma and adenocarcinoma of the appendix Among appendiceal epithelial neoplasms, which account for less than 1% of all gastrointestinal malignancies, the non‑mucinous colonic type is a very small subset. As described in a large pathology series, the relative frequencies of adenomas and adenocarcinomas are 2% and 7%, respectively [2, 41]. In another study, a 2:1 ratio of mucinous to non-mucinous neoplasms was reported [42]. In a retrospective study, among epithelial tumours, 70% are mucinous, and 30% are non-mucinous type lesions [30]. On CT, adenomas may appear as focal, regular mural thickening or an intraluminal polypoid lesion within the appendix (Fig. 10), and adenocarcinoma presents as irregular wall thickening, heterogeneous enhancement, or a soft‑tissue mass [2] (Fig. 11). Absence of cystic dilatation is a key distinguishing feature from mucinous neoplasms. Advanced disease may demonstrate regional or distant lymphadenopathy and invasion of adjacent organs. The extent of disease at the time of diagnosis is a more important predictor of survival than histology [43]. MRI findings of adenoma may demonstrate mild wall thickening or a small intraluminal mass with intermediate signal intensity on T1‑ and T2‑weighted images [2]. Adenocarcinoma, on the other hand, may demonstrate heterogeneous signal intensity and diffusion restriction reflecting increased tumour cellularity [3]. Histologically, non-mucinous appendiceal adenomas and adenocarcinomas produce less than 50% of mucin. They resemble colorectal neoplasms and may appear as cuboidal or columnar epithelial cells. Appendiceal adenocarcinoma is characterized by invasive malignant glandular epithelium infiltrating beyond the muscularis mucosa [2, 41]. Management of adenoma typically involves simple appendectomy because the lesion is confined to the mucosa and malignant potential is limited [39]. Adenocarcinoma treatment depends on the stage and extent of disease. If high‑grade dysplasia or localized adenocarcinoma is present, right hemicolectomy and standard systemic chemotherapy are performed. Metastatic tumours do not respond to standard chemotherapy and are treated with regimens similar to colon adenocarcinoma, like a combination of fluorouracil, platinum, and irinotecan [39, 44]. In cases with peritoneal dissemination, cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy (HIPEC) is shown to improve survival [45, 46]. Neuroendocrine tumour of the appendix Neuroendocrine tumours (NETs) are the most common primary neoplasms of the appendix, accounting for about 0.3–0.9% of appendectomy specimens [3, 11, 30]. They have an incidence of 0.15–0.6 cases per 100,000 persons/year, with slight female predominance and maximum incidence under 40 years of age [47]. The WHO 2022 classification categorizes appendiceal neuroendocrine neoplasms by differentiation and proliferation, grading well-differentiated tumours as NET G1, G2, or G3 based on Ki-67 indices. Crucially, it separates high-grade, well-differentiated NET G3 from poorly differentiated, aggressive neuroendocrine carcinomas (NECs), a biological distinction that directly alters imaging selection and therapeutic management [41, 48]. The majority occur at the distal tip of the appendix, hence relatively indolent clinical behaviour and low metastatic potential [3]. On imaging, 75% of NETs are most commonly located at the appendiceal tip, with 25% occurring at the base. They may appear as a small nodular mass or wall thickening [3, 30, 49]. Due to their high vascularity, NETs demonstrate avid hyperenhancement on contrast CT, and they may contain calcifications, mimicking appendicoliths [2, 3] (Fig. 12). Local lymph node involvement and distant metastasis are seen in < 10% of patients. Local invasion of the adjacent mesentery, with irregular borders, calcifications, and a desmoplastic reaction similar to that seen in small-bowel neuroendocrine tumours, can be seen, which helps differentiate from other tumours [3, 50]. Larger lesions demonstrate metastatic involvement at diagnosis in one-third of the cases, usually to regional nodes and uncommonly to the liver. Approximately 95% of patients present with tumours less than 2 cm in size, which are unlikely to have metastases [51]. DOTATATE PET/CT scan is becoming standard of imaging for detection and staging of well differentiated neuroendocrine tumours [52]. In 111 octreotide scintigraphy and F18-FDG PET are useful in staging of advanced neuroendocrine tumours, with octreotide scan having superior sensitivity compared to FDG-PET in well-differentiated lesions and vice versa for poorly differentiated lesions [53]. Histologically, appendiceal NETs arise from subepithelial chromaffin cells and are composed of small cells with uniform round nuclei containing stippled chromatin and stain for serum chromogranin A (CGA) and synaptophysin, with CGA being more sensitive but less specific for metastatic detection. Tumours are well or poorly differentiated, with well-differentiated being most common [2, 3]. Management of appendiceal NETs depends primarily on tumour size, location, and the presence of high‑risk histopathologic features. Tumours larger than 2 cm carry a higher risk of lymph node metastasis and are typically treated with right hemicolectomy with regional lymph node dissection. In cases of metastatic disease, somatostatin hormone status, tumour burden, and metastatic site are considered. The treatment of metastatic tumours is complex and includes cytoreductive surgery for resectable tumours, chemo/radioembolization, somatostatin analog therapy, and targeted therapies such as mTOR signal inhibitors, VEGF receptor 1–3 inhibitors, angiogenesis inhibitors, and various combination chemotherapies for unresectable tumours [54, 55]. Lymphoma of the appendix Lymphoma of the appendix is extremely rare, although the gastrointestinal tract is a common extranodal site of lymphoma involvement. It most commonly occurs as secondary spread from adjacent bowel lymphoma or nodal mass. Clinically, appendiceal lymphoma often presents with symptoms similar to acute appendicitis, including right lower quadrant pain, fever, and leucocytosis [2, 3, 50, 56,57,58]. Imaging findings of appendiceal lymphoma are similar to small bowel lymphoma and manifest as diffuse wall thickening and dilatation of the appendix with maintenance of tubular shape (Fig. 13). Aneurysmal dilatation of the appendix, similar to small bowel lymphoma when seen, is characteristic and a key imaging finding. When the appendix is secondarily involved, regional lymphadenopathy and adjacent small-bowel changes are observed. On US, mural thickening is seen as a hypoechoic area [2, 3, 56]. F18-FDG PET-CT helps stage lymphoma, showing abnormal increased radiotracer uptake in the appendix. Additionally, response to treatment can be better evaluated with FDG PET-CT [59, 60]. Histologically, lymphomas involving the appendix are mostly non-Hodgkin lymphomas, with mantle cell and diffuse large B-cell lymphomas being the most common histologic types [2, 3, 50]. Appendiceal lymphoma demonstrates diffuse infiltration of the appendiceal mural wall by lymphocytes [56]. Management of appendiceal lymphoma depends on whether the disease is primary or part of systemic lymphoma. Following diagnosis, patients typically undergo systemic staging with CT and FDG PET-CT. Treatment is surgery with chemotherapy and radiation therapy based on the staging of the disease. Post-transplant lymphoproliferative disorder (PTLD) of the appendix PTLD involves the GI tract in 5%−33% of overall transplant recipients, with the distal small bowel being the most common site [61]. To our knowledge, PTLD involving the appendix is rare, with only a few cases reported. The overall incidence of PTLD directly parallels immunosuppression burden and endogenous lymphoid mass associated with different allograft types. Highest risk of PTLD is seen with intestinal and multi-visceral transplants (11% to 33%) followed by lung transplants (3% to 10%), Heart transplants (2% to 8%), liver transplants (1% to 5.5%), kidney transplants (0.8% to 2.5%) and allogenic hematopoietic stem cell transplants (0.6% to 1.7%) [62, 63]. PTLD is caused by EBV infection in 60% − 85% of all solid organ transplants and almost all cases of stem cell transplants. The majority of PTLD cases occur within the first year of transplant [64, 65]. Histologically, appendiceal PTLD demonstrates dense infiltration of lymphoid cells within the appendiceal wall extending into the submucosa and muscular layers, similar to lymphoma. Imaging features of PTLD are similar to those of lymphoma, can appear as a bowel wall mass or bowel wall thickening with dilatation, and are unlikely to result in obstruction (Fig. 14). Luminal ulcers are more common with GI involvement by PTLD than by lymphoma [64]. Lymphadenopathy and short-segment intussusceptions can occur, but in only a minority of patients. The US has a limited role in the evaluation of PTLD and is mainly used for guided percutaneous biopsy. CT and F18 FDG PET/CT are the mainstay imaging modalities for staging and assessing treatment response [64, 65]. Management of PTLD focuses on restoring immune surveillance while controlling lymphoid proliferation. The first step is usually a reduction of immunosuppressive therapy, which may allow partial immune recovery and regression of early PTLD lesions. Surgery with a combination of chemotherapy and radiotherapy is used in treating PTLD based on the stage of the disease. Appendiceal endometriosis Appendiceal endometriosis is a rare extra-pelvic manifestation of endometriosis, occurring in approximately 1% to 2.8% of patients with known pelvic endometriosis. Although categorised as a benign non-neoplastic process, due to its capacity for local invasion it can mimic appendiceal neoplasm. Pathologically, endometrial implants preferentially involve the appendiceal serosa and outer muscularis propria while characteristically sparing the mucosa. This deep infiltration incites smooth muscle inflammation, hyperplasia and intense subserosal fibrosis [66, 67]. On cross-sectional imaging, appendiceal endometriosis mimic appendicitis or infiltrating mass appearing as diffuse thickening of wall or mass with surrounding fat stranding. On MRI it manifests as an eccentric, solid mass or nodule at the appendiceal tip exhibiting low T2 signal intensity with characteristic punctate hyperintense T1 foci representing haemorrhagic blood products. On real-time ultrasound or dynamic MRI, radiologists may also observe fixed kinking or tethering of the appendix to adjacent pelvic structures, serving as a critical diagnostic clue to differentiate this condition from standard acute inflammation [66, 67]. Laparoscopy has both diagnostic and therapeutic value in management of appendiceal endometriosis with appendectomy being the treatment of choice. Rare tumors of the appendix Ganglioneuromas and paragangliomas of the appendix are very rare. In patients with known NF1, there are case reports of appendix ganglioneuromatosis. Neurofibromas and schwannomas are rare in the gastrointestinal system and even rarer in the appendix [2, 3, 68]. Mesenchymal tumors of the appendix are very rare, and if seen, they are mostly benign. Contrary to colon, leiomyomas are more common than leiomyosarcomas if the appendix is involved. Gastrointestinal Stromal tumours are the most common mesenchymal tumors of the appendix [69]. Kaposi sarcoma involving the appendix has been reported in patients with acquired immunodeficiency syndrome (AIDS). Kaposi sarcoma and lymphoma involving the appendix in AIDS are difficult to distinguish on imaging, with Kaposi sarcoma being more nodular and focal than lymphoma [2, 3, 61]. Evolving role of imaging Dual-energy CT (DECT) enhances diagnostic certainty in appendiceal pathology through low-kiloelectron volt (low-keV) virtual monoenergetic imaging (VMI) and iodine density mapping. Low-keV VMI (40–50 keV) maximizes the contrast-to-noise ratio of the inflamed appendiceal wall, while iodine maps quantitatively assess absolute tissue perfusion (mg/mL). This spectral framework enables accurate, early identification of transmural ischemia and micro-perforations, distinguishing simple acute appendicitis from gangrenous variants [70, 71]. Complementing this, radiomic parameters extract sub-visual, voxel-level texture signatures (such as Gray-Level Co-occurrence Matrix features) to generate an objective “Rad-score.” This advances in imaging can predict response to non-operative antibiotic management versus immediate surgery [72]. Recent research into photon-count CT has shown improved spatial resolution, energy-resolved imaging, and spectral post-processing compared to conventional CT [73]. Deep learning models and convolutional neural networks (CNNs) are actively transforming workflows through automated 3D appendiceal segmentation and advanced pathology classification. Cutting-edge frameworks deploy a clinical-radiomics-fused approach, combining automated imaging features and texture profiles with vital signs and laboratory metrics like absolute neutrophil counts. This multi-parameter network outperforms traditional clinical metrics (e.g., the Alvarado score) and optimizes diagnostic specificity [74]. MRI is superior to CT for evaluating peritoneal disease, with increased use of DWI, ADC, and subtraction post-gadolinium contrast images for early detection and differentiation of tumour components [75]. PET/CT is being widely utilized in staging of appendiceal neoplasms and assessing for response post-treatment. A recent study showed that 18 F-fibroblast-activation protein inhibitor (18 F-FAPI-42) PET/CT is superior to FDG PET/CT for detecting primary malignancies and metastases of the appendix [76]. Utilizing Somatostatin receptor PET (SSTR-PET), such as Ga-68 DOTATATE and Cu-64 DOTATATE PET/CT, has been shown to increase the accuracy of initial staging and detection of metastases in patients with neuroendocrine tumours, including appendiceal NETs [52, 77]. While clinically comparable, Cu has a shorter positron range producing superior spatial resolution for sub-centimeter lesions, alongside with a 12.7 h extended half-life provides reduced time-sensitive production and distribution challenges associated with short 68-min half-life of Ga [78].

Appendiceal pathology encompasses a wide and complex spectrum that extends far beyond acute appendicitis, including diverse inflammatory, infectious, and neoplastic conditions with significant clinical implications. Advances in multimodality imaging have transformed the ability to accurately detect, characterize, and stage these entities. A thorough understanding of appendiceal anatomy, positional variability, and key imaging patterns is essential for differentiating among pathologies that often present with overlapping features. Integrating imaging findings with clinical context and histopathologic correlation enables more precise diagnosis and tailored management strategies. Despite major advances in imaging, important diagnostic challenges remain, including differentiation of reactive versus primary appendiceal inflammation, identification of early mucinous neoplasms, detection of occult perforation, and characterization of small appendiceal tumors. To bridge these gaps, future research should focus on advanced imaging techniques such as dual-energy and photon-count CT, advanced MRI, and radiomics-based machine learning. By providing a comprehensive imaging-based overview of both common and uncommon appendiceal disorders, this review highlights the evolving role of radiologists in multidisciplinary patient care. Data availability No datasets were generated or analysed during the current study.

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