[Skin Metastasis to the Buttocks of MSI-High Sigmoid Colon Cancer-A Case Report].

Skin metastasis of colorectal cancer is a rare condition. Herein, we report a case of skin metastasis of MSI-high sigmoid colon cancer to the buttocks. A 41-year-old male who was referred to our hospital for anemia, elevated inflammatory response, weight loss, and hematochezia during his general health examination. Computed tomography(CT)revealed a thickening of the sigmoid colon wall, fistula formation between other parts of the sigmoid colon, and enlarged mesenteric lymph nodes. The patient was suspected of having skin metastasis due to a mass being found in the upper abdominal subcutaneous region and the skin of the buttock. The buttock and abdominal wall tumors were subjected to pathological examination, which revealed adenocarcinoma. The patient was diagnosed with sigmoid colon cancer with skin metastases. The patient developed an intestinal obstruction during the examination and underwent a transverse colostomy. Subsequently, pembrolizumab was initiated as the first-line treatment because of the presence of MSI-high and RAS gene mutations (G12S mutation). CT taken after the fifth course of treatment indicated that the tumor had shrunk.