Masson's Tumor Mimicking Mesenchymal Neoplasia: A Case Report of Intravascular Papillary Endothelial Hyperplasia in the Deltoid Muscle.

Introduction: Intravascular papillary endothelial hyperplasia (IPEH), or Masson's tumor, is a benign endothelial proliferation that typically arises in soft tissues and lacks cytological atypia. It is often identified during the exclusion of malignant vascular neoplasms, particularly angiosarcoma. Deltoid involvement is exceptionally rare and usually associated with underlying vascular malformations. Case Presentation: A 19-year-old woman presented with a painless, progressively enlarging swelling in the right shoulder, without a history of trauma. Magnetic resonance imaging revealed a well-defined intramuscular lesion suggestive of mesenchymal neoplasia. A complete surgical excision was performed. Histopathological analysis revealed a vascular lesion composed of multiple papillary structures lined with bland endothelial cells, consistent with IPEH. No atypia or malignancy was identified. The patient remained recurrence-free at 12-month follow-up. Conclusion: Masson's tumor invariably requires surgical excision to establish a definitive diagnosis and to exclude malignant vascular neoplasms, particularly angiosarcoma. This case highlights the diagnostic challenges posed by IPEH in atypical locations and underscores the importance of histopathological confirmation. A review of previously reported cases confirmed an indolent course and a low recurrence rate. Early detection of this tumor in young patients is essential to prevent unnecessary aggressive interventions.