Cytology-Radiology Correlation Series: Pancreatic cytopathology.

Judy Trieu : Conceptualization; methodology, validation; data curation; writing–original draft; and writing–review and editing. Andrew Gilman : Writing–original draft and writing–review and editing. Katerina Konstantinoff : Writing–original draft and writing–review and editing. Maria D. Lozano : Conceptualization; investigation; writing–original draft; validation; visualization; writing–review and editing; and data curation. Mauro Saieg : Supervision; data curation; writing–original draft; writing–review and editing; investigation; conceptualization; validation; and visualization.

The significance of cytologic and imaging correlation in diagnosing pancreatic lesions cannot be overstated. Although each diagnostic modality provides valuable insights individually, the integration of cytologic analysis with imaging findings enhances diagnostic accuracy and guides clinical decision making, with the fostered potential to obtain an early diagnosis of potential life‐threatening neoplasms and a have positive impact on patient management and prognosis.

Serous cystadenomas (SCAs) are benign cystic neoplasms, although there are rare reports of malignant transformation. 11 , 12 , 13 On cross‐sectional and EUS imaging, an SCA can either be microcystic or macrocystic (oligocystic). For microcystic SCAs, there may be a T2‐hypointense central scar, which can have associated calcifications (Figure  10A ); and, classically on EUS, these lesions are microcystic, resembling a honeycomb (Figure  10B ). Macrocystic SCAs are multilobulated with at least one cyst >2 cm (Figure  11 ). SCAs do not communicate with the pancreatic duct. Although the most common location is in the head of the pancreas, it can occur in any part of the pancreas. (A) Magnetic resonance image showing a microcystic lesion with T2 central hypointensity scarring, consistent with a serous cystadenoma, and (B) its corresponding endoscopic ultrasound image of the classic honeycomb appearance. (A) Magnetic resonance image of a macrocystic (oligocystic) serous cystadeonoma with large, lobulated cysts and (B) its corresponding endoscopic ultrasound image. If imaging diagnosis of this cystic lesion is not clear, aspiration of the cyst fluid can assist in making a diagnosis. Many times, epithelial cuboidal cells lining the cyst walls are not represented, and all are seen are hemosiderin‐laden macrophages because of the rich vascular nature of this lesion (Figure  12 ). When present, cuboidal cyst lining cells are usually sparse, and there is no evidence of mucin. Immunohistochemistry positivity for inhibin A or GLUT1 may be helpful. Fine‐needle aspiration of serous cystadenoma demonstrating (A) hemosiderin‐laden macrophages because of the rich vascular nature of this lesion and (B) cuboidal cyst lining cells with no evidence of mucin (Papanicolaou stain, original magnification ×40 in A and B). On H&E‐stained samples, a panoramic view shows (C) the thin walls lined by cuboidal cells with no mucin and (D) rich vascular stroma (original magnification ×10 in C, ×20 in D). Intraductal papillary mucinous neoplasms (IPMNs) comprise of 25% of all pancreatic cystic neoplasms. 14 They are divided into three groups based on the adjacent pancreatic duct: main‐duct IPMNs have segmental or diffuse dilation of the pancreatic duct >5 mm (Figures  13 and 14 ); side‐branch IPMNs have connections to a side‐branch of the pancreatic duct (Figures  15 and 16 ); and mixed‐type IPMNs involve both the main duct and secondary branches (Figure  17 ). MRI and EUS are the best modalities for evaluating a ductal connection, although it is not always obvious. Fluid aspiration of the cyst can confirm whether the cyst is mucinous. Although guidelines vary for the management of these cysts, a common recommendation is that main‐duct and mixed‐type IPMNs carry a higher risk of malignancy. Worrisome features on cross‐sectional imaging and EUS that have been associated with a higher risk of malignant transformation include cyst size >3 cm, dilated main pancreatic duct, thickened walls, mural nodule (Figure  18 ), and a solid component. 15 Morphologically, IPMNs should be divided into low‐grade/low‐risk (with a conservative observational approach) and high‐grade/high‐risk (surgical approach). Low‐grade IPMNs show neoplastic mucinous cells (Figure  19 ), whereas high‐grade IPMNs show smaller, enterocyte‐sized cells, with high nuclear cytoplasmic ratio and irregular nuclei 3 (Figure  20 ). Biochemical analysis is also important because high carcinoembryonic antigen levels and low glucose levels are supportive of such a diagnosis. 16 Magnetic resonance image of a main‐duct intraductal papillary mucinous neoplasm with a segmental dilated main duct. Endoscopic ultrasound image of a main‐duct intraductal papillary mucinous neoplasm showing segmental dilation of the main duct, tapering smoothly to a normal caliber main duct. Magnetic resonance image/magnetic resonance cholangiopancreatography of numerous side‐branch intraductal papillary mucinous neoplasms along the pancreatic body and tail. Endoscopic ultrasound image showing a side‐branch duct intraductal papillary mucinous neoplasm with thin internal septations; although not pictured here, this cyst had clear communication with a side branch of the main pancreatic duct. Magnetic resonance image of a mixed‐type intraductal papillary mucinous neoplasm demonstrating a dilated main‐duct cyst and additional side‐branch cysts. Endoscopic ultrasound image of a side‐branch duct intraductal papillary mucinous neoplasm with a mural nodule. Intraductal papillary mucinous neoplasms, low grade, showing thick mucus‐containing, neoplastic mucinous cells (Papanicolaou stain, original magnification ×40). Intraductal papillary mucinous neoplasms, high grade. Nuclear enlargement, pleomorphism, and irregular and more crowded groups of atypical cells (Diff‐Quik stain, original magnification ×40). Mucinous cystic neoplasms (MCNs) are macrocystic lesions typically found on the body or tail the pancreas. 15 They do not communicate with the main pancreatic duct. However, on MRI and EUS, the connection to the main duct is not always obvious; therefore, it can be difficult to differentiate a side‐branch duct IPMN from an MCN. On CT, MCNs may have peripheral calcifications or thin septations, which are better seen on MRI (Figure  21 ). MCNs are anechoic lesions on EUS without connection to the pancreatic duct; they typically have a hyperechoic wall because of the peripheral calcifications, but not always (Figure  22 ). Fine‐needle aspiration of these lesions obtains thick, sticky mucus with a honey‐like consistency. Cytology is variable and often sparse. The cytology is similar to that of IPMN and can range from bland to frankly malignant‐looking cells (Figure  23 ). Indeed, there is overlapping cytology of these two entities, and they are usually diagnosed generically as mucinous cysts on cytology alone. Magnetic resonance image of a mucinous cystic neoplasm demonstrating thin septations within the cyst and no clear communication of with the pancreatic duct. Endoscopic ultrasound image of a lobulated anechoic pancreatic lesion without clear communication with the pancreatic duct. A mucinous cystic neoplasm showing mucinous cyst lining with ovarian‐like stroma. (A) Unfortunately, stroma is rarely sampled in a fine‐needle aspiration procedure. (B) The mucinous epithelial cells are clustered together in a background of extracellular mucin (Papanicolaou stain, original magnification ×40 in A and B). Solid‐pseudopapillary neoplasms are rare, low‐grade, malignant tumors that are often mixed solid and cystic. On cross‐sectional imaging, the mass can be seen with or without a capsule. A cystic portion tends to be in the center, surrounded by solid components 17 (Figure  24 ). On EUS, the lesion would demonstrate mixed cystic and solid features, with heterogeneity that may represent hemorrhagic or necrotic debris (Figure  25 ). In cytology, branching papillary‐like fronds with delicate vascular cores lined by one or more layer(s) of tumor cells (Figure  26 ). Pseudopapillae are capillaries surrounded by tumor cells. In some cases, metachromatic, myxoid material can be found. Tumor cells are usually bland and uniform and typically have cytoplasmic processes and nuclear grooves (Figure  27 ). Intracytoplasmic hyaline globules (positive for periodic acid–Schiff stain) are highly characteristic of this tumor. They also can be found extracellularly (Figure  28 ). Magnetic resonance image of a solid‐pseudopapillary neoplasm with subtle solid and cystic features. Endoscopic ultrasound image of a solid‐pseudopapillary neoplasm showing a heterogenous lesion consistent with mixed solid and cystic components. Branching, papillary‐like fronds with delicate vascular cores are classic of solid‐pseudopapillary neoplasms (Papanicolaou stain, ×10). Tumor cells are usually bland and uniform, with cytoplasmic processes and nuclear grooves (Papanicolaou stain, original magnification ×20). Intracytoplasmic hyaline globules (positive for periodic acid–Schiff stain) are highly characteristic of solid‐pseudopapillary neoplasms. They can also be found extracellularly (Papanicolaou stain, original magnification ×40). Neuroendocrine tumors of the pancreas (PanNETs) are malignant tumors that may cause symptoms caused by hormone production or mass effect. On CT, they are hyperenhancing, well demarcated masses (Figure  29 ). CT has higher sensitivity for PanNETs, but MRI is more sensitive for evaluating for liver metastasis. 18 On EUS, they are round, hypoechoic lesions and tend to be more vascular (Figure  30 ). Somatostatin receptor imaging is helpful for staging and surveillance. 18 Morphologically, PanNETs usually show a dispersed cell population, with mild atypia and plasmacytoid‐looking cells. Nuclei show the classic salt‐and‐pepper chromatin and tend to go ou t of the cell, beyond cytoplasmic borders (Figures  31 and 32 ). Distinction must be made with acinar cell carcinoma (ACC) and solid‐pseudopapillary tumor, and immunohistochemical markers play an important role in this distinction. Usually, PanNETs are positive for synaptophysin, chromogranin, and INSM‐1; whereas solid‐pseudopapillary tumors are typically positive for nuclear beta‐catenin and have the characteristic perinuclear dot‐like pattern of CD99. The most useful markers for the diagnosis of ACC are trypsin and BCL‐10 19 Computed tomography scan of a hyperenhancing neuroendocrine tumor in the head of the pancreas. Endoscopic ultrasound image of a hypoechoic lesion with well defined borders in the pancreas; biopsy‐confirmed neuroendocrine tumor. (A) A neuroendocrine tumor of the pancreas with a dispersed cell population, mild atypia, and plasmacytoid‐looking cells (Diff‐Quik stain, original magnification ×20). (B) Nuclei show the classic salt‐and‐pepper chromatin (Papanicolaou stain, original magnification ×40). Fine‐needle biopsy sample of a neuroendocrine tumor of the pancreas showing the same plasmacytoid cells forming an organoid‐appearing block. Fine‐needle biopsy samples are ideal for immunohistochemistry, and, when rich in cells, grading of these tumors may be quite comparable to the final surgical report (H&E stain, original magnification ×20). Pancreatic ductal adenocarcinomas (PDAC) comprise of 90% of all pancreatic cancers. 20 On CT and postcontrast, T1‐weighted MRI, PDAC is a hypoenhancing, ill‐defined mass causing upstream pancreatic ductal dilation and parenchymal atrophy (Figure  33 ). If the mass is in the head of the pancreas, obstructing the common bile duct as well, MRI demonstrate a classic double‐duct sign with dilation of both the pancreatic duct and the common bile duct. On EUS, the mass is hypoechoic with upstream pancreatic atrophy and ductal dilation (Figure  34 ). PDAC in cytology can be well, moderately, or poorly differentiated. In well differentiated PDAC, there are abnormal architectural arrangements of pancreatic atypical ductal cells: too crowded ( tight wads : i.e., crowded, disorderly sheets and clusters of cells with some nuclear overlap and loss of polarity; Figures  35 , 36 , 37 , 38 ). Hypochromatic nuclei with parachromatin clearing also can be observed. Nuclear membrane irregularities (folds, clefts, grooves) are important diagnostic features of ductal adenocarcinoma, including well differentiates types. In poorly differentiated adenocarcinomas, atypia is severe and easily recognizable on cytology. Pancreatic adenosquamous carcinoma requires the presence of both atypical glandular cells and a substantial component (>30%) of malignant squamous cells (Figure  39 ). (A) Magnetic resonance image showing an ill‐defined mass in the head of the pancreas. (B) Magnetic resonance cholangiopancreatography demonstrates the classic double‐duct sign with a biopsy‐proven pancreatic ductal adenocarcinoma identified in the head of the pancreas. Endoscopic ultrasound image of an ill‐defined pancreatic mass; fine‐needle biopsy confirmed pancreatic ductal adenocarcinoma. Well differentiated pancreatic ductal adenocarcinoma with slight atypical nuclei but already with loss of architectural arrangement (Papanicolaou stain, original magnification ×20). (A) Pancreatic ductal adenocarcinoma can demonstrate a drunken honeycomb pattern and (B) mild pleomorphism with loss of polarization (Papanicolaou stain, original magnification ×40). Pancreatic ductal adenocarcinoma can be associated with dense fibrosis. (A) Papanicolaou‐stained smear and (B) a cell block. Pancreatic ductal adenocarcinoma can show highly anaplastic cells with (A) atypical mitosis or (B) signet ring cells (Papanicolaou stain, original magnification ×40). Pancreatic adenosquamous carcinoma has the presence of both (A) atypical glandular cells and (B) a substantial component (>30%) of malignant squamous cells (Papanicolaou stain, original magnification ×20 in A, ×60 in B). ACCs make up only 1%–2% of all exocrine pancreatic neoplasms. ACCs are solid, well circumscribed lesions with heterogenous enhancement on contrasted CT. MRI of ACCs reveals a nonspecific, hypoenhancing solid lesion (Figure  40 ). On EUS, ACCs appear uniformly hypoechoic (Figure  41 ). The tumor cells are usually reasonably well differentiated, forming irregular, disorderly sheets; crowded clusters; or characteristic microacinar structures (not the classic three‐dimensional, grape‐like clusters of normal acini; Figure  42 ). Additional immunohistochemistry staining helps with differentiating ACC from PanNET and solid‐pseudopapillary neoplasm (Figure  43 ). Acinar cell cystadenocarcinoma is a rare variant of this, with only a handful of cases reported. Acinar cell cystadenocarcinomas are mixed cystic and solid lesions with a pseudocapsule. 21 Magnetic resonance image of a nonspecific, hypoenhancing, solid mass in the head of the pancreas; biopsy confirmed acinar cell adenocarcinoma. Endoscopic ultrasound image of a hypoechoic mass in the tail of the pancreas; biopsy confirmed acinar cell carcinoma. Acinar cell carcinoma demonstrated by tumor cells that form irregular, disorderly sheets; crowded clusters; or characteristic microacinar structures. (A) Cell block of acinar cell adenocarcinoma with (A) immunoreactivity for trypsin showing (B) an acytoplasmic and granular staining pattern. Immunoreactivity for BCL‐10 (not shown) helps differentiate acinar cell adenocarcinoma from pancreatic ductal adenocarcinoma and solid‐pseudopapillary neoplasm. Although metastasis to the pancreas is rare, common metastasizing malignancies include renal cell carcinoma, melanomas, colorectal cancer, breast cancer, and sarcomas. 22 Clinical history best informs a suspicion of metastasis to the pancreas if a lesion is found on the pancreas. Cross‐sectional imaging may note concomitant lesions in the kidney or colon as well as the pancreas; however, metastatic lesions to the pancreas are typically nonspecific and ill‐defined. Multifocal pancreatic lesions are also suggestive of metastasis to the pancreas. EUS findings of metastatic lesions to the pancreas are also nonspecific and variable. Examples of common neoplasms that metastasize to the pancreas include kidney (which can occur as a single solitary mass decades after nephrectomy; Figures  44 , 45 , 46 ), breast (Figure  47 ), and colon (Figure  48 ), each with specific morphologic parameters, as described in the examples shown. Computed tomography scan of two pancreatic lesions in the setting of prior right nephrectomy for renal cell carcinoma. Endoscopic ultrasound image with a heterogenous lesion on the tail of the pancreas; biopsy confirmed metastatic renal cell carcinoma. Metastatic renal cell carcinomas to the pancreas present as groups of bland‐looking clear cells in a rich, vascularized stroma. (A) Immunohistochemistry plays a pivotal role in these cases, which are positive for carbonic anhydrase IX and PAX‐8. It is important to note that metastatic renal cell carcinoma may show up many years after the initial diagnosis, and a detailed history search is mandatory in these patients (Diff‐Quik stain, original magnification ×40x). (B) A cell block shows positivity for carbonic anhydrase IX. Another common example of metastatic carcinoma to the pancreas is metastasis from breast cancer. Morphologically very similar to a well or moderately differentiated pancreatic carcinoma, metastatic breast ductal carcinomas are very difficult to diagnose in a fine‐needle aspiration from a pancreatic lesion. Cells may lose coherence, and a more dispersed pattern may be noted. Immunohistochemistry also plays a pivotal role in these cases, along with the clinical history (Diff‐Quik stain, original magnification ×20). Colonic adenocarcinomas may also metastasize to the pancreas. More columnar‐shaped cells with acinar formation and a necrotic background provide clues to this diagnosis (Diff‐Quik stain, original magnification ×40).

Pancreatic lesions are rising in prevalence, likely because of increasing access to cross‐sectional imaging. They can be subtyped into neoplastic and non‐neoplastic lesions. Some lesions are characteristic on computed tomography (CT), magnetic resonance imaging (MRI), and/or endoscopic ultrasound (EUS), whereas others are nonspecific. A diagnosis of pancreatic lesions is typically made by clinical history, cross‐sectional imaging, and fluid and/or tissue sampling by EUS with fine‐needle aspiration or biopsy sent for cytologic examination. 1 , 2 As a matter of fact, there may be no system that emphasizes the importance of morphologic and clinical correlation more than the pancreaticobiliary tract. According to the newly published World Health Organization classification, 2 analysis of the imaging results should guide the proper evaluation of morphologic findings, deciding whether that sample is satisfactory, diagnostic, and truly representative of the lesion targeted. 2 In cystic lesions, imaging also plays a pivotal role, indicating a probable malignancy and guiding the radiologist, endoscopist, and pathologist to target the most suspicious area for a definite diagnosis and possible early detection of a premalignant or malignant lesion. 3 In the current article, part of the Cancer Cytopathology Cytoimaging Correlation Series, we present several examples of neoplastic and non‐neoplastic entities, encompassing the most common entities found in everyday practice, with detailed descriptions of their imaging and cytomorphologic findings. This atlas‐style article should be of aid for all those professionals involved in the multidisciplinary approach to pancreatic lesions, hopefully helping to achieve a more accurate and detailed diagnosis. Pancreatic pseudocysts and walled‐off pancreatic necrosis (WOPN) are types of benign pancreatic fluid collections. Pseudocysts form from either an acute episode of pancreatitis or chronic ductal obstruction, leading to ductal injury and resultant pancreatic fluid extravasation. 4 WOPN occurs after an acute episode of pancreatitis that results in necrosis of surrounding tissue. The pancreatic fluid collection becomes walled off an average of 4 weeks after initial ductal injury. 5 Cross‐sectional imaging demonstrates an encapsulated pancreatic collection (Figure  1 ); MRI has higher sensitivity in detecting necrotic debris and in evaluating ductal injury. On EUS, pseudocysts are anechoic lesions with thin walls (Figure  2A ), whereas WOPN tends to have thick walls with internal debris (Figure  2B ). In cytology, pancreatic fluid collections usually show scattered macrophages and neutrophils and no epithelial cells are present, whereas WOPN would distinctly demonstrate a necrotic background (Figure  3 ). Amylase levels of the cystic fluid are usually in the thousands, which helps achieve final diagnosis. Computed tomography image with contrast showing a large, well encapsulated pancreatic fluid collection. Endoscopic ultrasound of a two types of pancreatic fluid collections: (A) pseudocyst and (B) walled‐off pancreatic necrosis. Scattered, hemosiderin‐laden macrophages in a dirty background (Papanicolaou stain, original magnification ×40). Autoimmune pancreatitis (AIP) is a chronic, inflammatory disease of the pancreas. There are two types of AIP as classified by international consensus diagnostic criteria 6 : lymphoplasmacytic sclerosing pancreatitis (type 1 AIP) and idiopathic duct centric pancreatitis (type 2 AIP). Type 1 AIP is typically associated with immunoglobulin subclass 4 (IgG4)‐related diseases, with elevated serum IgG4 in the majority of patients, whereas type 2 AIP is associated with ulcerative colitis. 7 Classically on CT, there is diffuse enlargement of the pancreatic parenchyma, creating a sausage shape with a hypoenhancing halo or rind of soft tissue. On MRI, precontrast T1 shows decreased signal intensity throughout the pancreas as well as diffusion restriction with increased signal on diffusion‐weighted imaging (Figure  4 ). The pancreas can also be focally enlarged as well, which can easily be mistaken for malignancy. On EUS, the pancreas also looks enlarged, with reduced echogenicity, diffuse lobularity, hyperechoic stranding, and thickened pancreatic borders (Figure  5 ). Magnetic resonance images showing (A) decreased signal intensity of a sausage‐like pancreas on precontrast T1 and (B) hyperintensity on diffusion‐weighted imaging. Endoscopic ultrasound image showing an enlarged, hypoechoic pancreas with hyperechoic stranding, subtle lobularity, and areas of thickened pancreatic borders. The diagnosis of AIP can be suggested clinically with history, imaging, and serology for type 1 AIP; and biopsy confirmation is usually not performed. However, it can be helpful in unclear cases (especially in cases for which a tumor must be excluded). Typical cytology shows normal acinar cells in a background of lymphocytes and plasma cells (Figure  6 ). Histopathologic evaluation is needed with immunostaining for IgG4‐positive plasma cells (>40 IgG4‐positive cells/mm 2 or an IgG4:IgG ratio >0.4). (A) Dirty background with inflammatory cells, including plasma cells, polymorphonuclear leukocytes, and fragments of ductal cells with minimal reactive atypia . (B) The presence of plasma cells may be related to autoimmune causes, and clinical correlation is required. Chronic pancreatitis (CP) is a fibroinflammatory disease of the pancreas that may result in parenchymal fibrosis, parenchymal and ductal calcifications, exocrine and endocrine insufficiency, and/or chronic pain. 8 CP is a clinical diagnosis based on clinical history, laboratory tests, and imaging and is assessed on a spectrum as mild, moderate, and severe. On cross‐sectional imaging, parenchymal atrophy can be seen with or without calcifications in the parenchyma and/or ducts. On EUS, features of CP may include a variation of parenchymal changes, such as hyperechoic foci with shadowing, lobularity, honeycombing, cysts, and stranding, as well as ductal changes, such as ductal stones, irregular and/or dilated main duct, dilated side‐branch ducts, and hyperechoic main‐duct margins. The Rosemont criteria 9 was developed to standardize EUS diagnosis of CP based on the aforementioned features, but this is fairly subjective and may carry a high false‐positive rate. 10 Histologic evaluation is not necessary to make a diagnosis; however, when CP affects a part of the pancreas more severely—most commonly the head of the pancreas—this may resemble a malignancy on cross‐sectional (Figure  7 ) and EUS (Figure  8 ) images, thus biopsy may be performed. In cytology, ductal cells may show reactive/degenerative changes in a background of blood, mucus, degenerated cells, debris, macrophages, stromal fragments, and calcification (Figure  9 ). In some instances, reactive changes can be severe, and distinction from a well differentiated adenocarcinoma may be an issue. In those cases, correlation with history and imaging is essential in achieving a proper definite diagnosis. Computed tomography scan showing pancreatic atrophy with ductal dilation and calcifications within the ducts and parenchyma. Endoscopic ultrasound image of chronic pancreatitis showing parenchymal atrophy with hyperechoic foci and dilated pancreatic duct because of an obstructing intraductal calcification. Background of blood and degenerated cells in an image showing saponified fat necrosis and inflammation typical of active chronic pancreatitis (Papanicolaou stain, original magnification ×40).

Judy Trieu reports personal/consulting fees from Cook Medical outside the submitted work. The remaining authors declared no conflicts of interest.