Background: Skin disorders and neuropathy often occur as side effects of chemotherapy. We encountered a patient who was treated for drug-induced skin symptoms, but the symptoms did not improve, and he was eventually diagnosed as having dermatomyositis. Case presentation: A 71-year-old man underwent chemotherapy with regorafenib in February 2020 for the postoperative recurrence of sigmoid colon cancer, but treatment was discontinued after about 2 months owing to the appearance of skin symptoms, which were thought to be side effects of regorafenib. Subsequently, his symptoms further worsened, and he was hospitalized 3 weeks after the appearance of the initial skin symptoms, and a palliative care team was asked to relieve his back pain caused by the drug-induced skin symptoms. Erythema was widely observed on the lower back and limbs, and he experienced needle stick-like pain. Furthermore, the patient demonstrated difficulty in lifting both upper limbs. As acetaminophen was effective for his pain, the dose was slowly increased with careful observation. The cause of the patient's muscle weakness was unclear, and after careful discussion of the possible causes among specialists in dermatology, neurology, and rheumatoid arthritis, a diagnosis of dermatomyositis associated with the malignant tumor was made about 10 days after his admission. The patient's symptoms gradually improved with steroid pulse treatment (methylprednisolone 1 g/day for 3 days) followed by high-dose gamma globulin treatment (2.5 g/day for 5 days), and the patient was discharged 48 days after admission. Discussion: Because this patient was referred to a palliative care team for the purpose of relieving pain caused by skin symptoms associated with chemotherapy, a crucial point is the symptoms were treated as side effects of the chemotherapy from the beginning. As neuropathy can occur as a result of chemotherapy, the pain and muscle weakness could be explained at the time; however, the symptoms continued to worsen even after the chemotherapy was stopped. Because the symptoms were not typical of polymyositis/dermatomyositis, diagnosis of the patient was delayed, even though he was treated in each specialized department. Our present case indicates that paraneoplastic syndrome should always be kept in mind when treating cancer patients.
Keywords: chemotherapy; dermatomyositis; pain management; palliative care.