Cytomorphologic observations on the sclerosing variant of well-differentiated pancreatic neuroendocrine tumors: Towards making specific diagnoses early

Background: The recently described sclerosing variant of pancreatic neuroendocrine tumor (spNET) shows prominent stromal fibrosis and decreased tumor cellularity in surgical pathology specimens. Although prognostic data are currently ambivalent, some studies have reported metastatic disease in small primary tumors, highlighting the need for early diagnosis. The aim of our study is to evaluate cytology specimens of spNET to determine its characteristic cytomorphologic features to expedite an early diagnosis.

Methods: Twenty-five cytology cases of spNET from 23 patients and 29 cytology cases of typical pancreatic neuroendocrine tumor (tpNET) from 29 patients diagnosed as such by surgical pathology evaluation were reviewed by two pathologists to assess adequacy of diagnostic material, cellularity and fibrosis. Radiographic findings and outcome data were collected.

Results: With only 13 of 25 specimens deemed as diagnostic, spNET specimens were more often non-diagnostic (p < .01) and less often hypercellular (p = .03) compared to tpNET counterparts. While at least focal fibrosis was observed in both groups, a subset of spNET cases showed small tumor cell groups entrapped in large fibrotic fragments. Importantly, spNETs tended to be metastatic at diagnosis (both regionally and distant), with a smaller average tumor size.

Conclusion: The hypocellular nature of spNET cytology samples makes this variant difficult to diagnose. However, when adequate sample is available, a subset of spNET show characteristic cytomorphology that enables us to consider this specific diagnosis at first diagnostic sampling. It is crucial to diagnose this variant early given the propensity of small tumors with regional lymph node involvement and liver metastases.