Benign osseous metaplasia of the breast infiltrated by invasive pleomorphic lobular carcinoma: A case report

Introduction and importance: Benign osseous metaplasia (BOM) is a rare entity, with only few cases reported in the breast. Here we present an unusual case of pleomorphic lobular carcinoma of the breast infiltrating BOM, discuss potential mimics and review the literature.Case presentation: An 86 year-old female presented with right breast lump for two weeks. Clinical examination revealed a palpable mass, associated with skin tethering and nipple inversion. Mammography and ultrasound showed a densely calcified lesion associated with parenchymal distortion. Core biopsy confirmed malignancy and the patient underwent mastectomy and sentinel lymph node biopsy. Histological assessment showed a 45 mm mass of benign bone trabecula infiltrated by invasive grade 2 lobular carcinoma of classic and pleomorphic types with nodal positivity (2/2). The patient received adjuvant radiotherapy to chest wall and axilla for 3 months. She remains well on aromatase inhibitors after 9 months of follow up.Clinical discussion: Few cases of breast BOM have been reported in the literature commonly in association with benign lesions such as fibroadenomas. So far, only two cases associated with invasive classic lobular carcinoma have been reported in the literature. The main differential is metaplastic (mesenchymal/ matrix producing) carcinoma, in which the osseous component is malignant and the cancer if often of a high grade, basal phenotype.Conclusion: We present the first case of BOM of the breast associated with invasive pleomorphic lobular carcinoma. Awareness of the entity and distinction from metaplastic carcinoma and malignant phyllodes with heterologous element are important to ensure appropriate patient management.