Solitary Gluteus Maximus Muscle Metastasis in a Breast Cancer Patient

Tumor metastasis into skeletal muscle is uncommon, and is rarely noticeable clinically. Lung cancer and gastrointestinal tumors are the most frequent primary malignancies that metastasize to the skeletal musculature, but some reports have described muscle metastases from breast cancer [1]. The authors present a case of gluteal muscle metastasis in a patient with a known primary breast cancer. A 53-year-old woman presented with a mass on her right buttock with pain that was aggravated by walking. She had previously undergone modified radical mastectomy due to left breast cancer. A histopathological examination at the time had shown invasive ductal carcinoma without axillary lymph node metastasis. She had received five cycles of adjunctive chemoradiotherapy with cyclophosphamide, methotrexate, and 5-fluorouracil, followed by tamoxifen. Seven years after surgery, a slowly growing mass developed on her right buttock with mild throbbing pain. On physical examination, tenderness was observed, but no palpable mass was found in the right buttock. A computed tomography (CT) examination of the pelvis found a poorly defined, mildly enhanced intramuscular mass measuring 3 cm in the patient’s right gluteal maximus muscle (Fig. 1), and a subsequent needle biopsy of the lesion confirmed a metastatic, poorly differentiated invasive ductal carcinoma. In order to address suspicion of systemic metastasis, the patient underwent CT scans of the breast, positron emission tomography-CT, and bone scans. No other metastatic sites were found (Fig. 2). A wide excision and local flap coverage were performed to remove the metastatic mass along with tissue from the gluteus muscle (Fig. 3). Histological findings showed metastatic invasive ductal carcinoma (Fig. 4). She received adjunctive chemotherapy with capecitabine and no local recurrence was observed 12 months after surgery. Skeletal muscle metastasis of a primary malignancy is relatively rare and can be misdiagnosed as a primary soft tissue sarcoma; however, distinguishing between the two diagnostic possibilities is critical, because the treatment and prognosis may differ markedly [1]. Metastases of breast cancer may appear in any organ, and although different relapse sites may be observed depending on the subtype of breast cancer, the most frequent metastasis sites are bone, the liver, the lungs, and the brain [2]. Skeletal muscle metastasis of breast cancer is very rare. The following