Proliferative Myositis of the Breast in a Patient with Contralateral Phyllodes Tumor: A Unique Pseudomalignant Lesion.

Aim: We report a case of proliferative myositis (PM) of the breast, which is the second reported in the English literature.

Case Presentation: A 49-year-old woman underwent surgery due to a fibroadenoma in the right and phyllodes tumor in the left breast. One month after these surgeries, a right breast mass rapidly grew at the surgical site, and biopsy did not provide a diagnosis. The lesion's rapid growth and high cellularity led to a suspicion of malignancy, and the mass was surgically removed. The immunohistochemical markers used were estrogen receptor (ER), S100, smooth muscle actin (SMA), desmin, pan-cytokeratin, Myoblast Determination Protein 1 (MyoD1), myogenin, caldesmon, calretinin, CD34, Anaplastic Lymphoma Kinase (ALK), P53, P63, and Ki67. A low Ki67 index and mitosis, lack of staining with rhabdomyosarcoma markers, and a typical checkerboard pattern ruled out malignancy. The diagnosis was PM.

Conclusions: This case emphasizes the importance of recognizing pseudomalignant breast masses to avoid catastrophic overtreatment.