From Leiomyoma to Smooth Muscle Tumor of Uncertain Malignant Potential: A Rare Recurrent Right Retroperitoneal Mass in a Young Male.

Smooth muscle tumors of uncertain malignant potential (STUMP) are rare mesenchymal neoplasms that exhibit histopathological features intermediate between benign leiomyoma and leiomyosarcoma. STUMP is overwhelmingly a disease of the uterus in perimenopausal and postmenopausal women; extra-uterine STUMPs are exceedingly uncommon, and involvement of the retroperitoneum in a male patient is exceptionally rare, with only a handful of cases described in the literature to date. We report a unique case of a young male who initially underwent excision of a retroperitoneal leiomyoma and subsequently developed a recurrent retroperitoneal STUMP after a six-year disease-free interval. A 25-year-old male initially presented with a progressively enlarging right flank mass and underwent complete excision. Histopathology revealed a benign retroperitoneal leiomyoma. The patient was lost to follow-up and re-presented six years later with a recurrent right retroperitoneal mass. Imaging demonstrated a heterogeneously enhancing, dumbbell-shaped lesion at the previous operative site with extension toward the deep subcutaneous plane and proximity to the right iliac crest. Wide local excision with periosteal margin resection and abdominal wall reconstruction using polypropylene mesh was performed. Histopathology demonstrated a cellular spindle cell neoplasm with elevated mitotic activity (18-20 mitoses/10 high-power fields) but without significant nuclear atypia, atypical mitoses, or coagulative tumor necrosis. Immunohistochemistry was positive for smooth muscle actin (SMA) and desmin, with aberrant MyoD1 expression, confirming smooth muscle differentiation. The lesion was diagnosed as a cellular, mitotically active STUMP. Follow-up imaging seven months postoperatively demonstrated no evidence of recurrence. This case highlights the rare occurrence of recurrent retroperitoneal STUMP in a young male - a sex in which this diagnosis is only anecdotally reported - and suggests possible biological progression along the leiomyoma-STUMP spectrum.