Answer: Rhabdomyosarcoma

Histology: Beneath the urethra is a cellular infiltrate which in areas is consistent with a “small blue cell tumor” without much cytoplasmic differentiation. This infiltrate appears condensed beneath the urothelium with sparser cells elsewhere. In areas, the cells have abundant eosinophilic cytoplasm and focally cross striations are visible.

Discussion: While this tumor could be a sarcomatoid carcinoma, which is synonymous with carcinosarcoma, there is no evidence of epithelial differentiation. Stains for various keratins were also negative. The tumor was diffusely positive for myogenin and desmin. The history of radiation for a prior prostate adenocarcinoma could favor a sarcomatoid carcinoma, although prior irradiation also is a risk factor for the subsequent development of a post-radiation induced sarcoma. Primary rhabdomyosarcomas of the prostate and bladder are frequently combined as it is often difficult to determine the origin of these lesions. Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and occurrence in the genitourinary tract, particularly in the urinary bladder, is well-documented in this age group. More than 90% of genitourinary tract RMSs are embryonal, mostly the botryoid subtype, whereas alveolar RMS (ARMS) is relatively uncommon in the bladder and prostate. In adults, RMS of the prostate and urinary bladder is extremely rare. In the context of primitive round cell tumors in the adult prostate and bladder, there can be striking morphologic overlap of small cell carcinoma with RMS, which may be compounded by synaptophysin immunoreactivity in both. In RMS a component is a primitive undifferentiated morphology with cells containing scant cytoplasm, varying round to fusiform nuclei with even chromatin distribution, and frequent mitoses. The degree of morphologic overlap with small cell carcinoma of the bladder, a relatively more common round cell tumor in adults, can be striking. As small cell carcinoma in the adult bladder is relatively much more common than RMS, the differential diagnostic dilemma is further compounded by the unexplained synaptophysin immunoreactivity. The expression of markers specific for skeletal muscle differentiation and the lack of reactivity with epithelial markers support interpretation as RMS, especially as aberrant synaptophysin reactivity, has been described previously in a molecularly confirmed ARMS.

Prostate/bladder rhabdomyosarcomas can be subdivided into low, intermediate, and unfavorable risk prognostic groups. Low risk is the embryonal subtype, either completely resected or with microscopic residual disease. Intermediate risk is embryonal with
(a) gross residual disease,
(b)metastases in patients younger than 10 years old, or
(c) alveolar subtype with no metastases.

Unfavorable risk is alveolar with metastases or embryonal in patients older than 10 years of age with metastases. The 5-year survival is 90%, 65-75%, and 40-55% for low, intermediate, and unfavorable risk groups, respectively. Metastases typically go to lung and bone. Treatment is initially chemotherapy with or without radiation and, if tumors are made resectable, then surgery. A single case of prostatic botryoid subtype of embryonal rhabdomyosarcoma has also been reported and the current case has some features of botryoid with its cambium layer of condensed tumor under the urethra.