Dumbbell-type tenosynovial giant cell tumor of the buttocks: a report of two cases.

Background: Tenosynovial giant cell tumor (TSGCT) is a benign tumor derived from the synovium of the joints, bursa, and tendon sheaths, which is mainly located around the tendon sheath of hand and foot. Extra-articular TSGCT are relatively rare and are mainly found in the soft tissue around the large joint. They are rarer when located purely intramuscular or subcutaneous, and mostly in the lower extremities. Case Description: We report two rare cases of completely extra-articular TSGCT located at the buttocks. One case was a 23-year-old young male presenting with left buttock swelling and pain for 1 year. Magnetic resonance imaging (MRI) examination revealed a dumbbell-type cystic solid mass in the left buttock, growing anteriorly from the deep surface of the gluteus muscle along the medial of the lesser trochanter. The lesion showed a heterogeneous mixed signal and was well-defined. MRI presentation needs to be differentiated with neurogenic or mesenchymal tumors, and radical resection of left gluteal tumor + neurovascular exploration surgery was performed. Another case of TSGCT we present here was diagnosed in a 55-year-old male elderly patient. Computed tomography angiography (CTA) revealed an irregular soft tissue mass in the left buttock involving the sacroiliac joint. T1-weighted imaging (T1WI) on MRI showed a mixed signal with predominantly isosignal, well-defined, and seemingly enveloped. A left buttock tumor resection with the scraping of the sacroiliac joint lesion was performed. Conclusions: Based on histopathological examination, the diagnosis was diffuse-type TSGCT for both cases. Both patients were periodically monitored after surgery, and one of them showed no imaging findings of recurrence or metastases seven years after surgery; the other case showed recurrence one year after surgery, which was resected and treated with radiotherapy, and there has been no recurrence so far. TSGCT occurring completely intramuscular is rare, with atypical clinical symptoms and imaging presentation, requiring differentiation with mesenchymal and giant cell-rich tumors.