Desmoid-type fibromatosis in an uncommon location: A case report of shoulder involvement misdiagnosed as rhabdomyosarcoma

Introduction and importance: Desmoid-type fibromatosis is an uncommon tumor characterized by its local invasiveness, with shoulder involvement being notably infrequent. The optimal treatment strategy for this tumor remains a topic of ongoing debate.

Case presentation: A 47-year-old Tunisian woman with a history of hypothyroidism, presented with pain and swelling in her left shoulder for a year. Examination revealed a firm, painful 4 cm mass, and MRI showed a poorly defined intramuscular tumor in the deltoid muscle, initially suspected to be rhabdomyosarcoma. However, surgical biopsy confirmed desmoid-type fibromatosis of the shoulder. The patient underwent surgical wide en-bloc resection of the tumor. The patient's recovery was uneventful, and she received physical therapy. At the three-year follow-up, she reported residual shoulder pain after heavy lifting, improving with analgesics. Examination showed no neurological deficits and a Constant score of 83 out of 100.

Clinical discussion: Due to their deep-seated nature and infiltrative growth patterns into neighboring subcutaneous tissues or muscles, along with the presence of myxoid or fibrotic components, desmoid-type fibromatosis can present challenges in distinguishing them from malignant soft tissue neoplasms based on imaging characteristics.

Conclusions: While radiologic evaluations may indicate characteristics suggestive of a malignant soft tissue tumor, histological confirmation is imperative prior to initiating surgical intervention. Continued research into the optimal treatment approaches for desmoid-type fibromatosis is essential for improving future patient outcomes and quality of life.