• Smears were paucicellular  
• Rare epithelial aggregates in close association with fibrous stromal tissue fragments (see picture below) 
  
  • Small round uniform nuclei
  • Fine and evenly distributed chromatin
  • Clear cytoplasm with distinct cell borders
  • No nuclear atypia or mitoses

DDx for cystic pancreatic lesions on imaging:

(Here are the more commonly seen cystic pancreatic lesions)

• Pseudocyst (usually no epithelium; some inflammatory cells; history of pancreatitis)
• Serous microcystic adenoma (scant watery fluid aspirated; usually hypocellular smears)
• Neoplastic mucinous cyst (e.g. Intraductal papillary mucinous neoplasm; Mucinous cystic neoplasm – thick viscous mucin; IPMN connected to pancreatic ductal system)
• Neuroendocrine tumour with cystic change 

A concurrent core biopsy was also performed.

• Cystic spaces of vacious sizes separated by relatively hypocellular fibrovascular septa
• Lined by a single layer of cuboidal epithelium with clear cytoplasm
• No significant nuclear atypia

The presence of intracytoplasmic glycogen was confirmed by a positive PAS stain and negative D-PAS stain:

The lesional cells were positive for inhibin-A:

Diagnosis: Serous microcystic adenoma

• A benign cystic epithelial neoplasm, presumed to originate from a centroacinar cell / intercalated duct system
• It has a female predominance, often an incidental finding and can be situated anywhere in the pancreas
• An associated somatic VHL gene mutation is often detected in sporadic cases; Von Hippel-Lindau syndrome (germline VHL gene mutation) is associated with multifocality
• On EUS imaging, there may be a 'honeycomb' appearance, and sometimes a central scar +/- calcifications. There is no connection with the pancreatic ductal system. 
• The diagnosis of serous microcystic adenoma is tricky on fine needle aspiration cytology; the presence of a paucicellular, non-mucinous background should trigger a search for lesional epithelium with associated fibrous tissue fragments.
• The epithelial cells are arranged in honeycombed sheets and contain bland, round, central nuclei with distinct cell borders and clear cytoplasm.
  • The lesional cells may be confused with
    • Benign gastrointestinal epithelium
      • Large fragments of epithelium with associated mucin
    • Mucinous epithelium
      • More abundant, mucin-containing cytoplasm
      • Variable nuclear atypia
• Cyst fluid analysis shows low CEA levels (typically <5ng/ml); and wild-type KRAS