Utilization of NKX3.1, P501S, Prostate-Specific Antigen, and Steroidogenic Factor 1 to Distinguish Malignant Leydig Cell Tumor From Metastatic Prostatic Adenocarcinoma to the Testis.

Context.—: A recent study demonstrated that NKX3.1-positive staining can uncommonly be seen in testicular Sertoli cell tumors (1 of 4 cases). Also, it was reported that 2 of 3 Leydig cell tumors of the testis showed diffuse cytoplasmic staining for P501S, although it was unclear whether it was specific granular staining that defines true positivity. However, Sertoli cell tumors do not typically pose a diagnostic dilemma with metastatic prostate carcinoma to the testis. In contrast, malignant Leydig cell tumors, which are exceedingly rare, can closely resemble Gleason score 5 + 5 = 10 prostatic adenocarcinoma metastatic to the testis.

Objective.—: To evaluate the expression of prostate markers in malignant Leydig cell tumors and steroidogenic factor 1 (SF-1) in high-grade prostate adenocarcinoma, as no data are currently published on these topics.

Design.—: Fifteen cases of malignant Leydig cell tumor were collected from 2 large genitourinary pathology consult services in the United States from 1991 to 2019.

Results.—: All 15 cases were negative immunohistochemically for NKX3.1, and all 9 with available additional material were negative for prostate-specific antigen and P501S and positive for SF-1. SF-1 was negative immunohistochemically in a tissue microarray with cases of high-grade prostatic adenocarcinoma.

Conclusions.—: The diagnosis of malignant Leydig cell tumor and its distinction from metastatic adenocarcinoma to the testis can be made immunohistochemically on the basis of SF-1 positivity and negativity for NKX3.1.