AIMS:Spermatocytic tumour (ST) is a rare testicular germ cell neoplasm that can be challenging to diagnose and for which there are few confirmatory biomarkers. Like normal spermatogonia, STs are known to express SSX proteins. Recently, a novel SSX antibody directed against the C-terminus (SSX_CT) of SSX1, SSX2 and SSX4, has emerged as a reliable biomarker for these SSX proteins and synovial sarcoma. However, SSX_CT immunostaining has not been demonstrated in ST. This study assessed the diagnostic utility of SSX_CT immunohistochemistry in ST and other tumors in the differential diagnosis with ST.
METHODS AND RESULTS:SSX_CT, OCT3/4 and C-KIT immunohistochemistry was performed on 15 STs, 38 seminomas, 13 embryonal carcinomas, 12 yolk sac tumours, 6 choriocarcinomas, 4 teratomas, 7 Sertoli cell tumors, and 6 lymphomas. Staining was scored as negative, rare, focal or diffuse. SSX_CT was positive in all (15/15) STs; diffusely in 14/15. SSX_CT was positive in 22/38 (58%) seminomas; however, only 2 cases showed diffuse expression. All other tumours were SSX_CT negative. OCT3/4 was negative in all STs, but positive in all seminomas and embryonal carcinomas. C-KIT was frequently positive in both ST (12/15; 80%) and seminoma (33/38; 87%). All other tumours were negative for OCT3/4 and C-KIT.
CONCLUSIONS:SSX_CT is a valuable and highly sensitive biomarker that supports the diagnosis of ST. Diffuse expression, observed in 93% of STs (compared to 6% of seminomas), is also highly specific for ST. Nevertheless, SSX_CT is best used in combination with OCT3/4 when ST is in the differential diagnosis.