Solitary Bone Plasmacytoma of the Scapula: A Rare Localization With Remarkable Radiotherapy Outcome.

Solitary bone plasmacytoma (SBP) is a rare plasma cell neoplasm characterized by localized proliferation of monoclonal plasma cells in bone without systemic involvement, which distinguishes it from multiple myeloma. Although the axial skeleton is most frequently affected, involvement of the shoulder girdle is exceptionally uncommon. We present the case of a 48-year-old woman who experienced progressive left shoulder pain and noted an enlarging mass over six months. Imaging revealed a large osteolytic lesion in the scapula with extension into surrounding soft tissue. Histopathology and immunohistochemistry confirmed the diagnosis of solitary plasmacytoma. Comprehensive staging, including bone marrow biopsy and 18F-FDG PET/CT (Fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography), excluded systemic disease. The patient underwent definitive radiotherapy, receiving 50 Gy (Grays) in 25 fractions via a 3D (three-dimensional) conformal technique with careful target delineation. Treatment was well tolerated, with only grade I radiodermatitis reported. Follow-up imaging two months after therapy demonstrated significant lesion regression, partial re-ossification, and reduction of soft-tissue involvement. SBP of the scapula represents a rare clinical entity, and radiotherapy continues to be the treatment of choice, offering excellent local control. Despite favorable local outcomes, long-term monitoring is crucial due to the potential risk of progression to multiple myeloma. Documenting rare localizations contributes to a better understanding of disease behavior and supports tailored management approaches.