Immunohistochemical characterization of peritoneal inclusion cysts with squamous metaplasia

Peritoneal inclusion cysts (PICs) are mesothelial-lined cysts that can uncommonly develop squamous metaplasia. Here, we describe the immunophenotype of PICs with squamous metaplasia using a comprehensive immunohistochemical (IHC) panel.

We collected surgical excisions of PICs with squamous metaplasia from our institution and cases sent to our extramural consultation service. An IHC panel was used to characterize the immunophenotype, which included antibodies to claudin 4, WT-1, D2-40, calretinin, CK5/6, p40/p63, PAX8, BAP1, MTAP and merlin. 10 cases of PICs with squamous metaplasia were identified. The average age was 51 years, with 8 females and 2 males. On histology, all PICs (n = 10) showed diffuse squamous metaplasia. The cyst lining cells exhibited a squamous epithelial phenotype with diffuse expression of claudin 4 and p63/p40 in the suprabasal cells that also showed consistent co-expression of WT1 in all cases. In contrast, the basal layer displayed a mesothelial phenotype, demonstrating immunoreactivity to WT1 and D2-40 and lacking expression of claudin 4. Calretinin showed complete loss of expression in both the basal and parabasal cells. All cases showed retained expression of BAP1, MTAP and merlin.

PICs with squamous metaplasia express a unique mixed epithelial and mesothelial immunophenotype. The basal layer shows an immunophenotype consistent with mesothelial cells, and the suprabasal layers show an immunophenotype consistent with epithelial/squamous cells. Knowing the detailed immunophenotype of PIC with squamous metaplasia is helpful for correctly recognizing this lesion.