B. Spindle cell rhabdomyosarcoma

Histological Description: The tumor is a polypoid spindle cell tumor covered by a simple bland cuboidal lining. In areas, there is condensation of the spindle cells beneath the lining. The spindle cells are arranged in short fascicles with scant cytoplasm. The nuclei are uniform without a lot of pleomorphism, yet have frequent mitotic figures.

Discussion: Synovial sarcoma does not occur in this site. The epithelial lining over the spindle cell tumor is the mesothelial lining of the tunica albuginea. In biphasic synovial sarcoma, the epithelial component is cytologically malignant as opposed to the benign appearing epithelium overlying the tumor in this case. The most common paratesticular soft tissue tumors in adults are well-differentiated liposarcoma, which not infrequently can have de-differentiation, followed by leiomyosarcoma. De-differentiated liposarcoma consists of typically high grade spindle cells arranged in a haphazard growth pattern associated with a well-differentiated lipoma-like liposarcoma component. Paratesticular leiomyosarcoma, as with leiomyosarcomas elsewhere, are composed of intersecting long fascicles of spindle cells with abundant eosinophilic cytoplasm, different from what is seen in this case. Another paratesticular spindle cell tumor that must be considered is rhabdomyosarcoma. Rhabdomyosarcoma is the most common paratesticular mesenchymal tumor in children, but also can rarely be seen in adults. The majority of paratesticular rhabdomyosarcomas are embryonal subtype where one can often see overt skeletal muscle differentiation with cells having eccentric eosinophilic tails and even well-developed cross striations on occasion. Paratesticular spindle cell rhabdomyosarcoma usually seen in children is less common. The finding of a paratesticular spindle cell rhabdomyosarcoma in an adult is very rare, only described in a few cases. The prognosis of spindle cell rhabdomyosarcoma is excellent, with over a cure rate over 90%. Paratesticular rhabdomyosarcomas in general also have an excellent prognosis, better than in other anatomical sites. A pitfall with the diagnosis of spindle cell rhabdomyosarcoma, as was present in the current case, is that myogenin may be only focally positive.