PSMA expression in Schwannoma: a potential clinical mimicker of metastatic prostate carcinoma

Radioimmunoscintigraphy using a radiolabeled antibody against prostate-specific membrane antigen (PSMA) is frequently used to detect prostate carcinoma (PCa) recurrence and metastasis to lymph nodes, soft tissues, and bone. PSMA expression has been shown in occasional nonprostatic neoplasms (e.g., urothelial adenocarcinoma) and in the vasculatures of other malignancies. PSMA expression has not been described in benign neoplasms. Recently, during evaluation of a prostatic carcinoma patient, we encountered a false positive PSMA radioimmunoscintigraphy scan in a pathologically confirmed Schwannoma (SCH) lesion. The current study further evaluates PSMA expression in Schwannomas.

Eleven SCH were retrieved from our surgical pathology archives. Representative sections were immunostained with monoclonal antibody for PSMA. PSMA expression was evaluated in tumor cells and lesional vessels. Extent of staining was calculated as percent of positive cells in highest areas of expression. Positive staining was considered focal, multifocal, or diffuse based on the percent of positive cells: <5%, 5% to 75%, and >75%, respectively.

All 11 SCH showed tumoral and or vascular staining; 7 (7/11) displayed both vascular and tumoral cell staining; the remaining 4 had only vascular staining (2/11) or tumor cell staining (2/11). The extent of tumoral cell and vascular staining varied widely among lesions (tumor cells: focal in 8 and diffuse in 1; vascular: focal in 7, multifocal in 1, and diffuse in 1 lesion).

This is the first report of PSMA expression in a benign neoplasm. Given our finding of frequent expression of PSMA in Schwannomas, they should be clinically considered in the differential diagnosis of a lesion that is positive on PSMA radioimmunoscintigraphy study performed during a metastatic work-up of PCa patient.