Solitary biceps muscle metastasis from breast cancer

Although direct muscle invasion by carcinoma is well recognised, skeletal muscle metastases are rare. Breast cancer very rarely metastasises to skeletal muscles. We present a case of breast cancer that metastasised to the biceps muscle. The woman developed breast cancer in 1990 and then developed axillary subcutaneous metastasis in 2001. In 2015, she presented with pain in the left forearm extending to the hand. Initial imaging showed no abnormalities, but the positron emission tomography-CT scanning revealed a hot spot in the left biceps muscle. Additionally, the nerve conduction study showed feature of carpal tunnel syndrome. The hot spot was deemed inconclusive in the view of normal CT and MRI scans, and the patient was treated with carpal tunnel decompression. A few months later, the patient developed a lump in the left biceps muscle, which appeared to be a metastatic lesion from her primary breast cancer. The patient was treated with radiotherapy and responded satisfactorily.