Painless posterior thigh mass as a presentation of metastatic breast cancer

Soft tissue metastasis clinically presenting as either painless subcutaneous or painful intramuscular nodules is extremely rare and may lead to an errant clinical suspicion of sarcoma. In general, most soft tissue metastasis comes from lung carcinoma; however, to date, there have been no reports of a posterior thigh mass just beneath the skin metastasizing from breast cancer. Here, we report a case of distant soft tissue metastasis presenting as a painless solitary left posterior thigh mass measuring 1.5 cm in diameter, which was later shown by positron emission tomography-computed tomography (PET-CT) to have multiple simultaneous mediastinal lymph node metastases. Eleven months ago, the patient had undergone curative breast-conserving surgery (BCS) for cancer of her left breast. At that time, her tumor showed a triple negative phenotype. Initial PET-CT right before the BCS had shown no metastasis. After histological and radiologic evaluation for the metastases, she decided to have systemic chemotherapy.