There is a relatively long list of intramedullary spinal cord lesions but most are glial in origins such as ependymoma (60%) and astrocytomas (30%). Other non-glial tumors include hemangioblastoma as well as lymphoma, metastasis and other rarer tumors. 

Based on the imaging appearances with vivid enhancement and no other distinctive imaging features the main differential is between ependymoma and metastasis. The key information is that this patient has a history of breast cancer with intracranial metastases (with similar MRI characteristics) and was being treated with steroids. This perhaps accounts for the lack of edema that normally surrounds intramedullary metastases.