A 53-year-old man complaining of general fatigue, weight loss, and muscle weakness was diagnosed with polymyositis associated with anti-SRP antibody based on elevated serum levels of creatinine phosphokinase (CK), electromyogram findings, femoral muscle MRI examination, and histologic findings of muscle biopsy. Cancer screening tests detected a pedunculate polyp (type Ip) with the type V1 pit-pattern in the sigmoid colon, and a polypectomy was performed. Because histological examination revealed adenocarcinoma with submucosal invasion of 6,000μm deep (T1b) and severe lymphatic invasion (ly3), the patient underwent a laparoscopic sigmoidectomy with D3 lymph node dissection. The elevations in CK values and muscle weakness were continued after surgery, and the patient required a combination therapy with steroids, tacrolimus, and an intravenous high-dose administration of immunoglobulin, which gradually improved patient’s symptoms and decreased serum CK levels.
Anti-SRP antibody positive dermatomyositis associated with early colon cancer is extremely rare. Although anti-SRP antibody positive polymyositis is often refractory to treatment, our patient was successfully managed with carefully planned treatment combined with surgery and medication.

53 歳男性。全身倦怠感と体重減少、筋力低下を主訴に来院し、精査にて抗SRP抗体陽性多発性筋炎と診断された。スクリーニングの大腸内視鏡検査でS状結腸にIpポリープを認め、ポリペクトミーが施行されたが、病理検査にて中分化管状腺癌（深達度sm, 6,000μm, ly3）であったため、追加治療として腹腔鏡下S状結腸切除術を施行した。術後経過良好であったが、腫瘍切除後も筋炎の改善はみられず、高用量ステロイドと免疫抑制剤および免疫グロブリン大量療法を行ったところ症状の改善を認めた。多発性筋炎は悪性腫瘍を併存例が多く、腫瘍切除のみで改善を認める症例も存在するが、抗SRP抗体陽性多発性筋炎は治療抵抗性なために強力な薬剤治療を要すことが多い。本例は、外科的・内科的治療を連携して行うことで良好な治療経過を得ることができた。
皮膚筋炎に悪性腫瘍が合併することは周知であるが、多発性筋炎、特に抗SRP抗体陽性多発性筋炎に合併した大腸癌の報告例は少なく、貴重な１例と思われた。