Epidermoid cysts are a characteristic feature of intrapancreatic but not of extrapancreatic accessory spleens.

Accessory spleens (ASs), ectopic splenic tissue at intrapancreatic and extrapancreatic sites, rarely contain epidermoid cysts. Our aim was to analyze the incidence of epidermoid cysts in ASs and perform an immunohistochemical analysis of its epithelial lining. We included in the study 148 ASs from 135 patients, for which pathological data were available. Eleven were intrapancreatic ASs (IPASs) and 137 were extrapancreatic ASs (EPASs). Six of the eleven (55%) IPASs contained epidermoid cysts, but they were not detected in EPASs. Immunohistochemical analysis showed that both the superficial/luminal and basal layer of the epithelial lining of epidermoid cysts in IPASs are negative for MUC2, MUC5AC, MUC6, WT-1, calretinin, thrombomodulin, uroplakin-II, and uroplakin-III. The superficial/luminal layer was positive for MUC4, CK7, and CA19-9 in all cases (100%), for CEA and HBME-1 in three cases (50%), and for MUC1, CK5/6, and CK20 in two cases (33%). The superficial/luminal layer was negative for p63 and D2-40 in all cases. The basal layer was positive for MUC1, CK5/6, p63, and HBME-1 in all six cases (100%), for CK7 and D2-40 in two cases (33%), and for CEA in one case (17%). The basal layer was negative for MUC4, CK20, and CA19-9 in all cases. Epidermoid cysts are a characteristic feature of IPASs but not of EPASs. Immunohistochemical analysis showed that the epithelial lining of epidermoid cysts in IPASs has a mixed character of glandular, squamous, mesothelial, and urothelial epithelium.