Case Report Open Access 15 Primary sternal osteosarcoma is an exceptionally rare entity with a male preponderance. We report a rare case of a 19 -year-old nulliparous female who presented with a two -year history of a progressively enlarging right breast mass. Multimodality imaging —including chest radiography, ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) revealed a large mass arising from the sternum extending into the right breast and anterior mediastinum. Biopsy confirmed high - grade osteosarcoma, characterised by multinucleated giant cells and stromal cells with mild atypia. Immunohistochemistry was positive for vimentin and negative for CK7. Surgical resection was planned; however, the patient declined treatment. This case highlights the importance of considering rare diagnoses in atypical presentations of breast masses and underscores the value of imaging and histopathological correlation in diagnosis and management

Case Report Open Access 15 Primary sternal osteosarcoma is an exceptionally rare entity with a male preponderance. We report a rare case of a 19 -year-old nulliparous female who presented with a two -year history of a progressively enlarging right breast mass. Multimodality imaging —including chest radiography, ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) revealed a large mass arising from the sternum extending into the right breast and anterior mediastinum. Biopsy confirmed high - grade osteosarcoma, characterised by multinucleated giant cells and stromal cells with mild atypia. Immunohistochemistry was positive for vimentin and negative for CK7. Surgical resection was planned; however, the patient declined treatment. This case highlights the importance of considering rare diagnoses in atypical presentations of breast masses and underscores the value of imaging and histopathological correlation in diagnosis and management