Unexpected ossification after lumpectomy

Introduction: Heterotopic ossification in scar tissue is an exceptionally rare phenomenon, typically associated with prior trauma or surgical intervention. The bone formation occurs because of osteogenic precursors in the tissue, growth factors, and the presence of mesenchymal stem cells. There are few previously cases reported, with location of heterotopic ossification in breast scar tissue post-lumpectomy. Case Report: An 87-year-old woman presented with a palpable mass in the left breast. Imaging revealed a 26 mm dystrophic calcification (BI-RADS 2) without axillary involvement. The patient had undergone lumpectomy five years earlier for ER-negative, HER2-negative, metaplastic carcinoma at the same site. Needle biopsy demonstrated fibrous tissue interspersed with calcified bone trabeculae. Immunohistochemistry for myeloperoxidase (MPO) was negative, and no extramedullary hematopoiesis was observed, excluding malignancy. Complete excision confirmed heterotopic osseous metaplasia within scar tissue. Histology showed mature lamellar bone formation embedded in fibrous stroma without atypia. The accompanying immunohistochemical stains showed a positive reaction for SATB2 and a negative reaction for broad-spectrum cytokeratin, S-100, desmin, and MDM2. The proliferation rate assessed by Ki-67 was low in the osseous areas and reached up to 20% in the surrounding proliferative fibroblast-rich regions. Additional immunohistochemistry demonstrated a negative reaction for CK-AE1/AE3, high-molecular-weight cytokeratin, CK7/19, P63, P40, EMA, CD34, ERG, D2-40, and CD31. Furthermore, in situ hybridization for USP6 was negative. Discussion and conclusion: This case highlights a rare benign sequela of breast-conserving surgery. Differential diagnoses are metaplastic carcinoma and extra skeletal osteosarcoma, and complete excision is advised to exclude these.