Incidence and Characterization of Extracellular Mucin on Pancreatic Fine Needle Aspiration Biopsies

Summary: Introduction: Thick abundant mucin in pancreatic fine needle aspiration (FNA) biopsy is considered diagnostic of a mucinous neoplasm (e.g., intraductal papillary mucinous neoplasm (IPMN) and mucinous cystic neoplasm (MCN)). However, neoplastic mucin can be challenging to distinguish from cystic debris, fibrin, and gastrointestinal contaminants. In this study, we sought to characterize extracellular mucin in pancreatic FNAs of neoplastic and non-neoplastic lesions. Materials and Methods: 783 cases of pancreatic FNA were identified between 2014-2023. Text-based search for 'mucin*' was performed on extracted radiology and cytopathology reports and 88 cases were identified with sufficient clinical-radiographic data. Four types of cytologic preparation were evaluated: (1) alcohol-fixed, modified Papanicolau stained, (2) air-dried, May-Grunwald-Giemsa (MGG) stained, (3) ThinPrep and (4) cell block. The presence and quality of mucin was recorded by two reviewers, blinded to the final cytopathology diagnosis. Clinical, pathologic, and radiographic data was used to establish an overall diagnosis for each case. Results: All 88 cases had at least one smear preparation and almost all cases (87/88, 99%) had a cell block. For all diagnostic categories, extracellular mucin was seen on at least one preparation in 37/88 cases (42%) (Table 1), including 48% (16/33) of the IPMN and MCN cases. 'Thick' mucin was seen in a few cases of IPMN (3/29, 10%) and MCN (1/4, 25%), but was also identified in adenocarcinomas (6/33, 18%). When present, thick mucin was consistently identified on alcohol-fixed smears (10/10, 100%) and cell blocks (8/10, 80%). Mucin was more frequently identified on alcohol-fixed smears (32/37, 86%) (Figure 1). Conclusions: Extracellular mucin was observed proportionally more frequently in cases of mucinous neoplasms, specifically on alcohol-fixed preparations. Thick mucin was noted in IPMN and MCN, but also in cases of adenocarcinoma. Sampling and specimen triage techniques may impact identification of mucin on specific cytologic preparations.