Metastasis to the gluteus maximus muscle from renal cell carcinoma with special emphasis on MRI features

BACKGROUND The skeletal muscle is an unusual site for metastasis from renal cell carcinoma (RCC). Metastatic RCC must be differentiated from benign primary soft-tissue tumors because aggressive surgical resection is necessary. CASE PRESENTATION We present the case of a 65-year-old man with metastatic RCC in the gluteus maximus muscle (3.8 cm in diameter) found on enhanced computed tomography (CT) 6 years after nephrectomy. Retrospectively, the small mass (1 cm in diameter) was overlooked 5 years earlier on enhanced CT. Because the growth of the lesion was slow, benign tumor was a differential diagnosis. However, magnetic resonance imaging (MRI) showed that the mass had high-signal intensity on T1- and T2-weighted images (WIs) compared to that of skeletal muscle, with mild enhancement by Gadolinium. The MRI features were unusual for most soft-tissue tumors having low-signal intensity on T1-WI and high-signal intensity on T2-WI. Therefore, under a diagnosis of metastatic RCC, the lesion was resected together with the surrounding skeletal muscle. The histology was confirmed to be metastatic RCC. CONCLUSION MRI features of metastatic RCC may be beneficial in differentiating it from primary soft-tissue tumor.