Cytologic Findings and Ancillary Tests Results of Sclerosing Pneumocytoma: Our Institutional Experience.

Introduction: Sclerosing pneumocytoma (SP) is a rare benign tumor and a potential diagnostic pitfall. Our aim was to review the cytologic features of our surgically diagnosed SP cases including the clinical, immunohistochemical and available molecular findings.

Materials and methods: A computerized search from 2013 to 2020 for surgical cases with corresponding cytology specimens diagnosed as SP was performed. The clinical data, cytology, and surgical specimens were collated for analysis.

Results: Six cytology specimens were collected. All were female (mean age = 35). Three have incidental lung nodules and three with cough. Cytologic findings showed variable architectural pattern (papillary, solid, singly scattered, acinar/rosette-like) and cellular heterogeneity (surface, stromal, epithelioid, plasmacytoid cells). Atypia was identified in 4/6 cases. The original cytology diagnoses were negative = 1, SP = 2 and adenocarcinoma = 3. The latter diagnoses were amended to SP after review of the surgical specimens. The three false positive cases on review have cytologic features mimicking adenocarcinoma. Immunohistochemical stains showed tumor cells (surface and stromal) were positive for TTF-1, and EMA with only the surface cells positive for pancytokeratin and Napsin A. Though two cases sent for molecular testing were negative for AKT1 or CTNNB1 exon 3 mutation, our panel did not evaluate AKT1 exon 4.

Conclusions: SP is a diagnostic pitfall with 50% initially misdiagnosed as adenocarcinoma. Integrating the clinical/radiologic findings, cytologic features, and performance of immunohistochemistry on cell block are helpful in avoiding misdiagnosis. Molecular testing for recurrent mutations, if present, could be helpful for diagnosis and possible therapy options. However, routinely used molecular testing may not always capture relevant molecular markers for SP.