Incompletely differentiated (unclassified) sex cord/gonadal stromal tumor of the testis with a "pure" spindle cell component: report of a case with diagnostic and histogenetic considerations.

The group of incompletely differentiated (unclassified) sex cord/gonadal stromal tumors includes rare cases with predominant spindle cell morphology. We report a rare case of a "pure" spindle cell tumor of the testis with morphological and immunohistochemical features consistent with the diagnosis of "incompletely differentiated sex cord/gonadal stromal tumor". Given the spindle cell morphology, the differential diagnosis with other benign and malignant spindle cell lesions is discussed. The concurrent presence of some morphological and immunohistochemical features of both Leydig and granulosa cell lines in the tumor suggests its origin from a stromal stem cell, possibly capable of dual differentiation, but with an arrest of maturation at an early phase of differentiation.