Masson's Hemangioma of the Vastus Medialis: A Rare Case Involving the Musculoarticular Branch of the Descending Genicular Artery.

Masson's hemangioma, or intravascular papillary endothelial hyperplasia (IPEH), is a rare, benign vascular lesion that can mimic malignancies such as angiosarcoma. Although commonly found in the head, neck, and extremities, intramuscular involvement near joints is unusual. This report presents a rare case of IPEH originating within the vastus medialis muscle and involving the musculoarticular branch of the descending genicular artery in an adolescent male. An 18-year-old male presented with right knee pain and a palpable mass without prior trauma or systemic disease. MRI revealed a vascular lesion within the vastus medialis in proximity to the knee joint. Surgical excision was performed, and histopathology confirmed the diagnosis of IPEH with no malignant features. Postoperatively, the patient regained full knee function without recurrence at 10-month follow-up. Recognition of IPEH in atypical intramuscular and periarticular locations is essential to avoid misdiagnosis and overtreatment. Histopathological examination remains the cornerstone for distinguishing this benign entity from aggressive vascular malignancies.