Background

The most controversial category in this proposal is the “Neoplastic: Other” category . Admittedly, the term “other” is not very satisfying, but given the wide spectrum of lesions in this category ranging from premalignant neoplasms to low-grade malignant neoplasms , it is the simplest term that is accurate. According to the National Cancer Institute for cancer terms, the word “neoplasm” is defined as “an abnormal mass of tissue that results when cells divide more than they should or do not die when they should…which may be benign…or malignant…” (http://www.cancer.gov/dictionary). These “Other” neoplasms of the pancreas are either preinvasive, premalignant neoplasms such as intraductal papillary mucinous neoplasm (IPMN) and mucinous cystic neoplasm (MCN) with low-, intermediate-, or high-grade dysplasia or neoplasms of low-grade malignant behavior such as pancreatic neuroendocrine tumor (PanNET) and solid-pseudopapillary neoplasm (SPN) . These “Other” neoplasms are much less aggressive than the high-grade and metastatic malignancies of the pancreas, a category which is dominated by pancreatic ductal adenocarcinoma (PDAC). Almost 90 % of malignancies in the pancreas are PDAC or one of the variants, so a cytological interpretation of “Positive for Malignancy” infers to most clinicians this deadly diagnosis. In addition, surgical management is highly variable and increasingly conservative since many “Other” neoplasms are detected early and at a small size due to the increased use of cross-sectional imaging. The rationale for this classification is explained in more detail below for each neoplasm, but, in general, the rational relates to the desire to standardize the cytological nomenclature and terminology with the 2010 WHO classification and terminology. The standard cytological categories of “Atypical” and “Suspicious for Malignancy” are categories that imply an indeterminate interpretation, which could lead to inappropriate patient management and possibly an unnecessary repeat diagnostic procedure.

Definition

“Neoplastic: Other” defines a neoplasm that is either premalignant such as IPMN or MCN with low-, intermediate-, or high-grade dysplasia , or a solid-cellular neoplasm such as well-differentiated PanNET or SPN. While mucinous epithelium in biliary brushing specimens may indeed represent a neoplastic change, given the lack of evidence-based literature on the cytology , histology, and management of these lesions, low-grade mucinous change of biliary epithelium will remain in the “Atypical” category (see Chap. 3) [1].

Pancreatic Neuroendocrine Tumor

To be consistent with the 2010 WHO classification, the current preferred ­nomenclature is pancreatic neuroendocrine tumor (PanNET) [2]. Synonyms include pancreatic endocrine tumor (PET), pancreatic endocrine neoplasm (PEN), and ­well-differentiated endocrine carcinoma [3]. Unless specifically classified as a high-grade neuroendocrine carcinoma (grade 3), the diagnosis of PanNET infers a well-differentiated neoplasm (grade 1 or 2) greater than 0.5 cm. Tumor size, production of hormones, and cytological atypia do not correlate with biological behavior, so despite the malignant potential of tumors > 0.5 cm, these well-differentiated tumors are classified as neoplasms and not carcinomas histologically or cytologically.

Cytological Criteria [4–6]: Pancreatic Neuroendocrine Tumor (Figs. 6.1, 6.2, 6.3, 6.4, 6.5)

  • Moderately to highly cellular smears; scant cellularity is common with cystic degeneration

  • Mostly non-cohesive single cells with or without small- to medium-sized groups or rosettes and clusters of cells

  • +/− vascular network and/or bloody background

  • Monotonous population of small- to medium-sized polygonal epithelial cells

  • Nuclei are usually bland and uniform, but can be quite atypical and may display significant pleomorphism (“endocrine atypia”)

  • Cells have round nuclei and generally visibly coarse, stippled, evenly distributed (“salt and pepper”) chromatin pattern

  • Nucleoli are usually inconspicuous but occasionally quite prominent

  • Stripped naked nuclei are not uncommon

  • Cytoplasm is relatively scant, and usually dense and eccentric, yielding a plasmacytoid appearance (this feature is best appreciated in single cells)

  • Rare variants produce cells with clear, vacuolated cytoplasm or oncocytic cytoplasm

  • Mitotic figures and necrosis are absent to rare

Fig. 6.1
figure 1

Well-differentiated neuroendocrine tumor. These parenchymal-rich, stroma-poor tumors produce cellular smears composed of a uniform population of mostly dyscohesive epithelial cells thus yielding a “solid-cellular” smear pattern. This contrasts with the glandular smear pattern of scattered three-dimensional cell clusters of adenocarcinoma. (Direct smear, Diff-Quik)

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Fig. 6.2
figure 2

Well-differentiated neuroendocrine tumor. Tumor cells are usually singly dispersed and classically plasmacytoid due to the eccentric, round nuclei. The characteristic endocrine chromatin pattern of finely stippled (“salt and pepper”) chromatin is often the key diagnostic feature allowing for a diagnosis on morphology alone. (direct smear, Papanicolaou)

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Fig. 6.3
figure 3

Well-differentiated neuroendocrine tumor. Tumor cells in a cellblock enhance a morphological diagnosis and provide readily accessible tissue for ancillary studies. (Cellblock, hematoxylin, and eosin)

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Fig. 6.4
figure 4

Well-differentiated neuroendocrine tumor. Synaptophysin is a sensitive marker for neuroendocrine differentiation and produces a diffusely strong positive staining pattern. (Cellblock, peroxidase-anti-peroxidase)

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Fig. 6.5
figure 5

Well-differentiated neuroendocrine tumor. Chromogranin A is a specific marker for neuroendocrine differentiation but may produce a patchier positive staining pattern than synaptophysin. (Cellblock, peroxidase-anti-peroxidase

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Explanatory Notes

The diagnosis of a PanNET can be made on morphology alone when the cytological features are classically endocrine as described above. Confirmatory immunohistochemical stains include endocrine markers, most commonly synaptophysin and chromogranin A [7]. Synaptophysin is more sensitive and generally produces strong diffuse staining whereas chromogranin A is more specific but is more likely to give patchy, focal staining [8]. If a fine-needle aspiration (FNA) is only suggestive of a PanNET, and a specific diagnosis is not possible, then the appropriate category is “Atypical” rather than “Suspicious (for Malignancy).”

Grading PanNETs on FNA specimens is controversial. Accurate grading is dependent on analyzing the mitotic count and/or Ki-67 labeling of tumor cells in the most active area (hot spots) of the resected specimen covering 50 high-powered fields [7, 9]. Studies have shown a strong correlation between grading on cellblocks and histology [6]; however, if a PanNET is to be resected, preoperative grading on a cellblock does not affect management and is a waste of resources since accuracy in grading would likely need to be established on histology. Grading on a cellblock is justified, however, when conservative management is desired. Although the accuracy in the distinction between G1 and G2 tumors is debatable, the detection of, or more importantly, the absence of diffuse nuclear staining of the tumor cells for Ki-67 indicating a G3 tumor is valuable information [6].

Management

Conservative management is becoming an increasingly common option for patients with small PanNETs (<  2 cm), especially in patients with advanced age and comorbid conditions [10, 11]. In addition, observation is common for patients with MEN 1 syndrome who often have small (< 2.5 cm) and not uncommonly multiple PanNETs [11]. Classifying a PanNET as “Positive for Malignancy” makes conservative patient management unnecessarily complicated and challenging. With patient access to their medical records, notes, and pathology reports, convincing a patient not to have surgery with the diagnosis of a malignant tumor is virtually impossible [12].

Discovery of the molecular mechanisms of PanNETs leading to progression and malignancy has offered management options for patients with unresectable disease [7]. Elaboration on these mechanisms is beyond the scope of this atlas, but FNA will likely play a significant role in obtaining tissue for such molecular studies in the future.

Solid-Pseudopapillary Neoplasm

Solid-pseudopapillary neoplasm (SPN) is a solid, secondarily cystic low-grade malignant epithelial neoplasm with potential for regional recurrence and metastasis. Although called a “neoplasm,” it is classified as a malignant tumor in the 2010 WHO classification [13]. The tumors occur in young females with rare exception. Imaging demonstrates a complex cyst with a solid and cystic appearance, which is variable in proportion due to the secondary nature of the cystic change. This neoplasm is included in the category “Neoplastic: Other” because, like PanNET, it has unpredictable malignant behavior with only ~ 15 % recurrence or metastasis rate, and it is not an aggressive malignancy like the malignancies in the “Positive” category [13, 14]. Classification as “Positive (for Malignancy)” is certainly an accurate category however .

Cytologic Criteria [15, 16]: Solid-Pseudopapillary Neoplasm (Figs. 6.6, 6.7, 6.8, 6.9, 6.10)

  • Cellular smear pattern composed of small, uniform cells in cohesive, often branching, and papillary cell clusters

  • Background may be clean or filled with hemorrhagic debris foamy histiocytes and multinucleated giant cells

  • Small clusters and single neoplastic cells in the background

  • Individual cells are homogeneous in appearance with little anisonucleosis

  • Nuclei are round to oval with smooth to slightly indented or grooved nuclear membranes

  • Delicate fibrovascular cores with myxoid stroma (Romanowsky stains: magenta colored, metachromatic material; periodic acid-Schiff (PAS) positive, diastase resistant)

  • Zone of cytoplasm often separates the nuclei of the neoplastic cells from the fibrovascular cores

  • Even and finely granular chromatin

  • Inconspicuous nucleoli

  • Cytoplasm is scant to moderate, nongranular to finely granular, and may contain a small perinuclear vacuole or intracytoplasmic hyaline globule

  • No to rare mitotic activity

Fig. 6.6
figure 6

Solid-pseudopapillary neoplasm. These secondarily cystic neoplasms are very vascular producing a rich, pseudopapillary meshwork of blood vessels surrounded by a myxoid stroma. (Direct smear, Diff-Quik)

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Fig. 6.7
figure 7

Solid-pseudopapillary neoplasm. Individual tumor cells are relatively uniform in size and shape with bland oval to bean-shaped nuclei with grooves and scant cytoplasm, which may contain a perinuclear vacuole or hyaline globule. (Direct smear, Diff-Quik)

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Fig. 6.8
figure 8

Solid-pseudopapillary neoplasm. Alcohol-fixed specimens highlight the bland nuclear features and absence of stippled chromatin and prominent nucleoli, but the myxoid stroma and cytoplasmic contents are not as readily identified as on air-dried stains. (Direct smear, hematoxylin, and eosin)

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Fig. 6.9
figure 9

Solid-pseudopapillary neoplasm. Cellblock preparations may provide diagnostic morphology as shown here with this small fragment of bland papillary epithelium. (Cellblock, hematoxylin, and eosin)

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Fig. 6.10
figure 10

Solid-pseudopapillary neoplasm. Beta-catenin is the diagnostic immunohistochemical stain, which shows strong nuclear positivity in virtually 100 % of these neoplasms. (Cellblock, peroxidase-anti-peroxidase)

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Explanatory Notes

Although the cytomorphology can be diagnostic, samples without the characteristic papillary architecture may lead to a differential diagnosis with the other parenchymal-rich, stromal-poor tumors of the pancreas. PanNETs may also occur in relatively young women and have round, occasionally solid, and cystic imaging features [4]. To confirm the diagnosis of SPN, immunohistochemical staining for beta-catenin is usually sufficient to highlight the strong nuclear positivity seen in virtually 100 % of tumors [17]. If the FNA is suggestive but not diagnostic of SPN, the preferred category is “Atypical” rather than “Suspicious (for Malignancy)”; however, “Suspicious (for Malignancy)” is not inaccurate.

Management

Surgical resection is the treatment of choice.

Neoplastic Mucinous Cysts (Intraductal Papillary Mucinous Neoplasm and Mucinous Cystic Neoplasm)

The two primary neoplastic mucinous cysts of the pancreas, IPMN and MCN, are premalignant cysts until invasive carcinoma occurs. Once invasion occurs, prognosis declines by up to almost 50 %, so the ideal time to resect these neoplasms is prior to invasion [3]. Surgical intervention is determined by the specific tumor type and the risk of malignancy, which is primarily based on cross-sectional imaging characteristics [18].

Both MCN and IPMNs are mucin-producing epithelial cystic neoplasms stratified by the degree of cytological and architectural atypia into low-grade, intermediate-grade, and high-grade dysplastic, premalignant (noninvasive neoplasms) and invasive carcinomas (tubular or colloid type in most cases) [3]. If the epithelial component of the cyst aspirate demonstrates the cytological criteria of malignancy, then the appropriate interpretation category is “Positive (for Malignancy).” If, however, the interpretation of a mucinous cyst is established by cytology and/or ancillary testing such as with carcinoembryonic antigen (CEA) [19, 20] or KRAS and GNAS mutational analysis [21–23], and the epithelial component is not cytologically malignant, then the appropriate category is “Neoplastic: Other” rather than “Atypical” or “Suspicious for Malignancy.” The “Neoplastic: Other” category establishes that a neoplasm exists and provides a broad categorization of the cyst that provides maximum flexibility for patient management.

Accurate grading of premalignant, dysplastic cyst lining epithelium is challenging. Confounding factors include degeneration, gastrointestinal contamination, and cyst lining heterogeneity. Simplifying the grading of the epithelial component using a two-tiered system of low-grade atypia (low-grade and intermediate-grade dysplasia) and high-grade atypia (high-grade dysplasia and adenocarcinoma) has been proposed [24]. This approach attempts to separate cysts that can be monitored conservatively (low-grade atypia) from those that should be resected (high-grade atypia). Intermediate-grade dysplasia is a challenge as the cytological features overlap with those of high-grade atypia significantly. The criteria for high-grade atypia (HGA) in the more common IPMN include small cell size (< 12 micron duodenal enterocyte) and thus elevated nuclear to cytoplasmic ratio, abnormal chromatin (hypo or hyperchromatic), and background cellular necrosis [25].

Cytological Criteria: Mucinous Cysts [24–27] (Figs. 6.11, 6.12, 6.13, 6.14, 6.15, 6.16, 6.17, 6.18)

Mucin production established:

  • Thick, colloid-like extracellular mucin

  • Cellular or inflammatory debris within the mucin

  • Thin mucin covering the slide confirmed with special stains for mucin (mucicarmine or Alcian blue pH 2.5)

  • OR

  • Elevated CEA (192 ng/ml is ~ 80 % accurate)

  • OR

  • KRAS/GNAS mutation

  • AND/OR

  • Neoplastic epithelial cells identified:

    • Low-grade mucinous epithelium, e.g., low-grade atypia (low-grade to intermediate-grade dysplasia)

      • Scant cellularity

      • Single cells, small clusters, and flat sheets of bland glandular epithelial cells (≥ 12 micron duodenal enterocyte)

      • Cytoplasmic mucin visible on routine light microscopy

      • Nuclei round and regular with even chromatin and inconspicuous to occasionally prominent nucleoli

      • Honeycomb sheet or on edge with basally located nuclei and apical cytoplasmic compartment

      • The cells may be indistinguishable from gastric contamination

      • Muciphages (foamy histocytes)

    • High-grade epithelium, e.g., high-grade atypia (at least high-grade dysplasia/carcinoma in-situ, but quality and quantity of atypia is insufficient for diagnosis of adenocarcinoma)

      • Scant to high cellularity

      • Epithelial cells that have lost the benign features of low-grade dysplasia

      • Small to large clusters and small single cells (< 12 micron duodenal enterocyte)

      • 2–4 cell-tight buds of cells

      • Three-dimensional architecture

      • Papillary arrangement (supports IPMN)

      • Single cells

      • High nuclear to cytoplasmic ratio

      • Variable amounts of cytoplasm with or without visible mucin or vacuoles

      • Nuclear membrane irregularity mild to moderate

      • Hypo or hyperchromasia

      • Variable nucleoli

      • Cellular necrosis scant to moderate

Fig. 6.11
figure 11

Mucinous cystic neoplasm. Thick, colloid-like extracellular mucin with cyst debris and no epithelial cells is a common finding on FNA since most of these neoplasms are low grade and do not shed cells into the cyst fluid. (Direct smear, Papanicolaou)

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Fig. 6.12
figure 12

Intraductal papillary mucinous neoplasm with low-grade dysplasia. This cluster of mucinous epithelium shows large (> 12 micron) columnar glandular cells with mucinous cytoplasm and small round nuclei. (Cytospin, Papanicolaou)

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Fig. 6.13
figure 13

Intraductal papillary mucinous neoplasm with low-grade dysplasia. Liquid-based processing may attenuate extracellular mucin. The thick, colloid-like mucin from this cyst is aggregated into an amorphous blob; a few tall, mucinous columnar epithelial cells are noted at the edge. (ThinPrep®, Papanicolaou)

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Fig. 6.14
figure 14

Intraductal papillary mucinous neoplasm with intermediate-grade dysplasia. Increased cellularity and stratification of cells correlates with intermediate-grade dysplasia. (Cytospin, Papanicolaou)

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Fig. 6.15
figure 15

Intraductal papillary mucinous neoplasm with intermediate-grade dysplasia. This hyperchromatic crowded group is composed of cohesive glandular cells with relatively round regular nuclei and visible mucinous cytoplasm at the edge. Single bland mucinous cells are noted at the edges of the group. The group of cells is not consistent with duodenal or gastric epithelium, is too cellular and crowded for low-grade dysplasia, and not atypical enough for high-grade dysplasia. (Cytospin, Papanicolaou)

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Fig. 6.16
figure 16

Intraductal papillary mucinous neoplasm with high-grade dysplasia. Small cells (< 12 micron) with high nuclear to cytoplasmic ratio and abnormal chromatin slough off the tufted cyst lining and villi of high-grade IPMNs. Necrosis is suggestive of but sufficiently specific for a diagnosis of malignancy. (Cytospin, Papanicolaou)

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Fig. 6.17
figure 17

Intraductal papillary mucinous neoplasm with high-grade dysplasia. High-grade cells are surrounded by thick, colloid-like mucin and necrotic cellular debris. (Direct smear, Papanicolaou)

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Fig. 6.18
figure 18

Intraductal papillary mucinous neoplasm with high-grade atypia (suspicious for invasive carcinoma). It can be difficult to distinguish high-grade dysplasia from invasive carcinoma. In this case, the cells are markedly atypical, and the cyst was associated with a mural nodule. This case could be classified as “Neoplastic: Other” with a note indicating the suspicion for invasive carcinoma or “Suspicious for Malignancy”

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Explanatory Notes

Ancillary tests are often critical in accurate classification of pancreatic cysts. Biochemical and molecular testing currently are only helpful in establishing whether a cyst is mucinous or nonmucinous; they do not accurately stratify a mucinous cyst by grade [20–22]. CEA is the best test for determining whether a cyst is mucinous, and cytology is the best test for determining grade [20, 26]. Although false positive results can occur with CEA as the test for mucinous etiology, this is rare, having been reported in an occasional pseudocyst and lymphoepithelial cyst [28, 29]. CEA analysis should be evaluated and reported when the mucinous characteristics of the cyst are not evident grossly. When an aspirate yields “thick,” “white,” or “viscous” cyst fluid, ancillary testing to determine if the cyst is mucinous is not necessary. Such a gross description should correlate with thick, colloid-like mucin on cytology. Gastrointestinal contamination does not typically produce such thick mucin. Gastric mucous may be somewhat thick and is recognized by its association with a relatively large amount of benign appearing gastric-type epithelium and/or stripped naked nuclei (which is not a typical feature of a mucinous cyst with low-grade dysplasia). Neither a low CEA level nor an absent mutation in KRAS or GNAS excludes a mucinous cyst. The category of “Neoplastic: Other” should be used when the cyst has been classified as a neoplasm, which may be based solely on ancillary tests. See example reports.

Once a cyst has been classified as mucinous, then it is the epithelial component that will establish grade based on that particular sample. The presence of HGA in a mucinous cyst fluid has a specificity of 85–88 % and sensitivity of 67–72 % for the detection of malignancy [30]. If the cells meet the criteria for invasive adenocarcinoma, then the “Positive (for Malignancy)” category is used (see Chap. 8). Cellblock preparations provide tissue for immunohistochemical staining. If the cells are suspicious for an invasive adenocarcinoma , the “Suspicious (for Malignancy)” category may also be used (see Chap. 7). Loss of SMAD4 immunostaining supports an invasive carcinoma [31].

The increasing use of next generation sequencing (NGS) may soon provide a cost-effective platform that will be able to analyze genetic mutations known to occur with progression to high-grade dysplasia and/or invasive carcinoma such as TP53, SMAD4, and CDKN2A [31–33]. Mucinous cysts are heterogeneous neoplasms, however, and the cytology sample may well underestimate the grade of dysplasia [34].

See Appendix B for a diagnostic grid helpful in the interpretation of pancreatic cysts.

Management

The analysis of the cyst fluid answering the clinical questions of mucinous or nonmucinous cyst, and low-grade (low-intermediate-grade dysplasia) or high-grade (high-grade dysplasia or invasive carcinoma) cyst coupled with the imaging characteristics determines the need for surgery. Currently all MCN are resected regardless of grade due to the anxiety and expense of life-long follow-up for progression to invasive carcinoma [35]. Main-duct and combined-type IPMNs are resected due to the inherent high risk of malignancy [3]. In contrast, branch-duct IPMNs are more often than not low-grade neoplasms identified in the pancreatic head of the elderly with comorbid conditions making pancreaticoduodenectomy a procedure with a risk greater than the risk of the cyst progressing to malignancy [18].

As such, conservative observation is frequently the management option of choice, so it is critical that the pathologist use the multimodal approach to interpretation providing a report that defines the cyst as mucinous or non-mucinous on one hand, and low-grade or high-grade on the other.

Miscellaneous Neoplasms

There are other neoplasms that rarely involve the pancreas that could be categorized in the “Neoplastic: Other” category. These include extra-gastrointestinal stromal tumor, teratoma, and paraganglioma.

Sample Reports

  • Example 1:

    • Satisfactory for evaluation

    • Neoplastic: Other

    • Mucinous cyst fluid with low-grade atypia (see note)

      Note: Benign-appearing mucinous epithelium is present from this transgastric FNA in a background of abundant extracellular mucin. The morphology of the cells is nonspecific and may represent low-grade dysplasia or gastric contamination. No high-grade atypia is identified and no necrosis is present. (If available, add CEA is elevated > 192 ng/ml (give value) supporting the diagnosis).

  • Example 2:

    • Satisfactory for evaluation

    • Neoplastic: Other

    • Abundant thick colloid-like extracellular mucin and low-grade dysplastic mucinous epithelium consistent with a neoplastic mucinous cyst. No high-grade epithelial atypia is identified and no necrosis is present. (If available, add: CEA is elevated > 192 ng/ml (give value) supporting the diagnosis).

  • Example 3:

    • Evaluation limited by scant cellularity

    • Neoplastic: Other

    • Intraductal papillary mucinous neoplasm with intermediate-grade dysplasia (see note)

      Note: No thick extracellular mucin is present, but cyst fluid CEA is 3277 ng/ml supporting the diagnosis of a mucinous cyst. Molecular analysis demonstrates a KRAS and GNAS mutation, which supports the diagnosis of an IPMN. The epithelial cells are atypical but not high grade and best classified as intermediate-grade dysplasia. No background necrosis is present.

  • Example 4:

    • Satisfactory for evaluation

    • Neoplastic: Other

    • Neoplastic mucinous cyst with high-grade atypia (see note)

    • Note: The mucinous etiology is established by the presence of extracellular mucin and elevated CEA > 192 ng/ml (give value). The epithelium is markedly atypical and represents at least high-grade dysplasia; invasion cannot be excluded. A moderate amount of cellular necrosis is present, which increases the concern for invasion.

  • Example 5:

    • Satisfactory for evaluation

    • Neoplastic: Other

    • Well-differentiated neuroendocrine tumor (see note)

    • Note: Immunohistochemical stains on the corresponding cellblock confirm endocrine differentiation showing diffuse positivity for synaptophysin and focal positivity for chromogranin A.

  • Example 6:

    • Satisfactory for evaluation

    • Neoplastic: Other

    • Solid-pseudopapillary neoplasm (see note)

    • Note: Immunohistochemical stain for beta-catenin shows strong, diffuse nuclear positivity supporting the diagnosis.