Smooth Muscle Tumor of Uncertain Malignant Potential (STUMP) in the Inguinal Soft Tissue: A Rare Case Requiring Differentiation From Metastatic Lymphadenopathy.

We report a rare case of a smooth muscle tumor of uncertain malignant potential arising in the inguinal soft tissue, requiring differential diagnosis from metastatic lymphadenopathy. The patient was a 74-year-old male. On the initial examination, a painless, elastic-firm mass measuring approximately 25 × 15 mm was palpated slightly cephalad and medial to the left inguinal region. The mass was non-adherent to the overlying skin or underlying tissue.  Based on preoperative imaging findings, metastatic lymphadenopathy was considered likely. However, further evaluation by the urology department found no definitive evidence of prostate cancer, and cystoscopy ruled out bladder cancer. Under local anesthesia, the tumor was excised and subjected to histopathological analysis. Histopathological examination with hematoxylin and eosin staining revealed a fibrous capsule surrounding the tumor and spindle-shaped cells with atypical nuclei arranged in loose bundles and fascicles within the tumor. Immunohistochemical staining showed positive results for desmin and alpha smooth muscle actin, while S-100 protein and CD34 were negative. B cell lymphoma 2 and Signal transducer and activator of transcription 6 were weakly positive, while β-catenin was negative. Thus, the histopathological findings were assessed, and the tumor was classified as a smooth muscle tumor of uncertain malignant potential. The patient's postoperative course was uneventful, with no evidence of recurrence at two years post-surgery. Smooth muscle tumor of uncertain malignant potential is defined as a "soft tissue tumor of uncertain malignant potential that does not meet the diagnostic criteria for conventional leiomyoma, its various subtypes, or leiomyosarcoma." To our knowledge, this case represents an exceedingly rare occurrence of smooth muscle tumor of uncertain malignant potential in the field of plastic and orthopedic surgery.