Pathologic Features

Grossly, most ductal adenocarcinomas are solid, firm, infiltrative tumors (Fig. 40.2) with ill-defined borders. The cut surface is often gritty or slightly gelatinous.^38,39 Less commonly, gross areas of necrosis may be seen. It is often difficult to distinguish carcinoma from adjacent areas of fibrosing chronic pancreatitis.

For tumors in the head of the gland, direct invasion of the common bile duct and duodenum is common. The major pancreatic ducts coursing through the carcinoma are usually narrowed or even completely obliterated. Recognition that the center of the tumor is located in the head of the pancreas helps to distinguish pancreatic ductal adenocarcinomas from primary carcinomas of the common bile duct, duodenum, or ampulla of Vater. Some ductal adenocarcinomas exhibit cystic changes due to necrosis with cystic degeneration, as a result of cystic dilation of obstructed ducts (i.e., retention cyst formation), or because of cystic dilation of the neoplastic glands (i.e., large duct pattern).⁴⁰ In some cases, invasive ductal adenocarcinomas may be associated with a preexisting benign cystic or intraductal neoplasm, such as a mucinous cystic neoplasm or an IPMN.

Microscopically, in its conventional form, ductal adenocarcinoma is characterized by a proliferation of small tubular structures lined by cuboidal mucinous cells in abundant desmoplastic stroma (Fig. 40.3).⁴¹ In well-differentiated ductal adenocarcinomas, the growth pattern and cytologic appearance of the cells may be deceptively benign, closely mimicking non-neoplastic ductules characteristic of chronic pancreatitis (Fig. 40.4). Malignant glands usually replace the normal lobular arrangement of benign ducts with haphazardly arranged tubules. The cells that line malignant glands typically form a single regular layer, but stratification and irregular papillae may be prominent in some cases.

The cytoplasm of the tumor cells may be abundant and usually contains mucin; clear cell change is also common. The nuclei may retain a basal orientation in the cells, but loss of polarity in some of the glands is typical. The nuclei vary in size, shape, and intracellular location between cells in each gland (Fig. 40.5). A variation in size of more than fourfold between adjacent nuclei suggests carcinoma.

Perineural invasion and vascular invasion are common and diagnostically useful, although these features often are not detected in core-needle biopsies. In some cases, tumor cells infiltrate adjacent normal islets. Invasion into peripancreatic adipose tissue is also common. The finding of an immediate juxtaposition of a gland with adipocytes without intervening stroma (i.e., naked glands in fat) is a strong sign of malignancy.⁴² Another helpful feature in the differential diagnosis of benign from malignant glands is the finding of glands situated adjacent to muscular blood vessels, because normal pancreatic ducts are usually separated from large blood vessels by a considerable amount of acinar parenchyma.^43,44 However, this feature is less helpful when the pancreas is profoundly atrophic.

In poorly differentiated ductal adenocarcinomas, the neoplastic glands are usually admixed with small clusters of cells with ill-formed lumina, pleomorphic nuclei, and abundant mitotic figures (Fig. 40.6). It is common for ductal adenocarcinomas to exhibit a mixture of well-formed glands along with individual cells or solid clusters.

The periphery of ductal adenocarcinomas is often indistinct, and neoplastic glands may be found well beyond the apparent gross extent of the tumor. The finding of isolated glands in otherwise normal peripancreatic adipose tissue several millimeters from the nearest edge of the carcinoma confounds the evaluation of the soft tissue margins.⁴² Invasion of preexisting epithelial structures, such as the common bile duct, duodenal mucosa, blood vessels, or native pancreatic ducts, can result in colonization of the basement membrane of the invaded structure,⁴⁵ which may simulate a preinvasive neoplasm, such as a duodenal adenoma or pancreatic intraepithelial neoplasia (PanIN). Continuity of intraductal neoplastic cells with frankly invasive elements supports an interpretation of colonization.

The current grading system for pancreatic ductal adenocarcinoma (i.e., Klöppel’s grading scheme endorsed by the WHO) entails evaluation of glandular differentiation, mucin production, mitoses, and nuclear atypia.⁴⁶ This scheme correlates well with prognosis but is cumbersome and not widely used. Another proposal advocates assigning two scores to the two most prevalent patterns, mirroring Gleason grading for prostate carcinoma. This system is more independently significant in predicting outcome.⁴⁷ The American Joint Committee on Cancer (AJCC) staging parameters for ductal adenocarcinoma are outlined in Table 40.3.⁴⁸

Chronic pancreatitis (i.e., interstitial fibrosis, atrophy, and modestly dense inflammation) is often seen in the pancreatic tissue adjacent to ductal adenocarcinomas. Differentiating chronic pancreatitis from carcinoma is discussed later (see Differential Diagnosis). The lining of native pancreatic ducts may show proliferative changes (i.e., PanINs). Squamous, transitional, intestinal, and, rarely, oncocytic⁴⁹ metaplasia may be seen in the non-neoplastic ducts.⁵⁰

Ancillary Diagnostic Tests and Molecular Properties

Mucin histochemistry and immunohistochemical markers of ductal differentiation (i.e., glycoproteins) invariably show at least focal positivity in typical ductal adenocarcinomas. Staining for CK7, CK8, CK18, and CK19 and epithelial membrane antigen (EMA) is usually positive. CK20 is detected in about 25% of cases, and its expression is usually less widespread and intense than that of CK7. The exception to this cytokeratin expression pattern is colloid carcinoma, in which CK20 expression is strong and diffuse (discussed later).^51,52 Immunohistochemical markers of glycoproteins that are often detectable in ductal adenocarcinoma include CA 19-9, CEA, TAG72, CA 125, and DUPAN2. Of these, CEA, TAG72, and CA 125 are regarded as tumor-associated glycoproteins that are not significantly expressed in normal ductal cells and are expressed only to a limited degree in low-grade PanIN.⁵³

Cell surface–associated mucin proteins are expressed in various degrees in different types of ductal neoplasms. Most conventional ductal adenocarcinomas express MUC1, MUC3, MUC4, and MUC5AC.^51,54–57 In contrast, MUC2 is not expressed in ductal adenocarcinomas other than those with intestinal differentiation, such as colloid carcinoma. MUC6, a marker of pyloric gland differentiation, is expressed in 35% of ductal adenocarcinomas.⁵⁸

Stains for chromogranin or synaptophysin may demonstrate a minor neuroendocrine cell component associated with the neoplastic glands. Some neuroendocrine cells are non-neoplastic islet cells entrapped in the tumor, because ductal adenocarcinomas have a propensity to invade islets. However, neoplastic neuroendocrine cells also occur in ductal adenocarcinomas.^59,60

The molecular alterations in ductal carcinomas have been studied extensively in the past decade. A comprehensive genomic analysis of more than 20,000 genes disclosed that most cases have primarily point mutations, which average 63 per tumor.⁶¹ Twelve core signaling pathways were altered in 67% to 100% of the tumors. The most commonly mutated genes include KRAS, CDKN2A (formerly called p16, INK4A, or CDKN), TP53, and SMAD4.^62,63 Mutations in codon 12 of the KRAS oncogene are detected in more than 90% of ductal adenocarcinomas, and TP53 mutation occurs in 50%. Most ductal adenocarcinomas also harbor abnormalities in CDKN2A because of mutation or hypermethylation of the promoter region of DNA.⁶⁴ Loss of SMAD4 expression is found in 55% of invasive ductal adenocarcinomas.^65,66 Although many other molecular abnormalities occur in ductal adenocarcinoma,⁶⁴ this constellation of genetic changes is characteristic and distinguishes ductal adenocarcinomas from pancreatic neoplasms of other cell lineages.

Other pathways commonly altered include apoptosis, regulation of the G₁/S phase transition, hedgehog signaling, DNA damage control, cell adhesion, integrin signaling, MAPK8 (JNK) signaling, control of invasion, GTPase signaling, transforming growth factor-β (TGF-β) signaling, and NOTCH signaling.⁶¹ Mutations in mismatch repair genes, BRCA2, STK11, BRAF, TGFBR2, and MAP2K4 are less common, but some of them underlie the hereditary cases of pancreatic ductal carcinoma.⁶³

Analysis of gene expression has revealed other overexpressed molecules (e.g., fascin, mesothelin, claudin 4, S100AP, S100A6, S100P) in ductal adenocarcinomas. Some have been used as potential immunohistochemical markers to help distinguish reactive (non-neoplastic) glands from carcinoma.^67–73 Strong immunoexpression of TP53 or complete loss of SMAD4 expression can be used to support a diagnosis of carcinoma, although both markers lack a high degree of sensitivity.

Differential Diagnosis

The main diagnostic consideration for conventional ductal adenocarcinoma is the distinction of a well-differentiated carcinoma from benign ductules in areas of atrophic chronic pancreatitis.^2,74 Ductal adenocarcinoma may appear deceptively benign, and ductules in chronic pancreatitis may appear infiltrative.

On low-power examination, ductules in areas of chronic pancreatitis retain the original lobular configuration of the normal pancreas, because they represent the residual intralobular ductules remaining after the loss of acinar elements during atrophy. Each lobular cluster of ductules contains a central, larger, branched, slightly dilated duct surrounded by small, round ductules (Fig. 40.7). The stroma in each lobule often is less dense than that between lobules. In contrast, invasive glands lack a lobular growth pattern and are usually distributed more haphazardly (see Fig. 40.7). The contours of individual glands also are usually irregular and angulated. Some glands may be incomplete, with epithelium lining only part of the neoplastic lumen and stroma lining the rest.

Cytologically, nuclear enlargement, nuclear contour irregularities, and loss of nuclear polarity are important clues to a diagnosis of carcinoma. Variations in size and shape and an intracellular location between adjacent nuclei in individual glands indicate malignancy. The finding of a nucleus-to-cytoplasm ratio of 4 : 1 is considered highly suspicious for carcinoma. Dense acidophilic cytoplasm is more common in carcinomas than in benign conditions.

Unlike other sites where reactive stromal cells may display significant nuclear atypia, pleomorphism and bizarre nuclei are uncommon in the stroma of inflammatory conditions of the pancreas. Individual atypical cells in the stroma of the pancreas are much more likely to be cancer cells, especially if they contain dense acidophilic cytoplasm. The finding of atypical glands in abnormal locations is also helpful. For instance, perineural or vascular invasion, invasion of the duodenal muscularis propria, invasion of stroma closely adjacent to a muscular vessel,^43,44 and immediate juxtaposition of glands to adipocytes⁴² are features considered to be almost 100% diagnostic of carcinoma (Fig. 40.8).