Radiation-Associated Primary Osteosarcoma of the Breast.

Introduction: Non-epithelial primary mammary osteosarcomas are extremely rare. The differentials include metaplastic carcinoma and malignant phyllodes tumour. This is the first published case of primary breast osteosarcoma arising after local radiotherapy.

Case Presentation: A 73-year-old female presented with a right-sided breast lump. The same breast had been irradiated 11 years previously for invasive ductal carcinoma. Diagnostic excision revealed a highly cellular, malignant spindle-cell lesion merged with an osteoid matrix and foci of calcification and bone formation. Immunohistochemistry and molecular studies showed no lines of differentiation. Due to the lack of epithelial/glandular differentiation, in situ carcinoma or leaf-like pattern, the diagnosis of post-irradiation osteosarcoma was made. She underwent mastectomy and is disease-free at 8 months of follow-up.

Conclusion: Post-irradiation osteosarcoma should be considered in the differential diagnosis of breast lesions showing malignant osteoid. Extensive sampling and careful search for epithelial differentiation is required to guide management. Complete surgical excision is recommended.