What happened, in summary

A 48-year-old woman had Stage IV small cell lung carcinoma with metastases to the brain, pancreas, and liver. At the referring hospital, she received durvalumab with platinum chemotherapy and radiation therapy, followed by durvalumab monotherapy for 1 year. Her cancer was assessed as stable, and she had no immune-related adverse events during that treatment period. One month after the last immune checkpoint inhibitor injection, she developed fever plus pain and swelling in the right sternoclavicular joint, the joint where the collarbone meets the breastbone. Arm and neck movement became restricted. Blood tests showed elevated C-reactive protein at 10.74 mg/dL, and MRI showed inflammatory changes around the joint. CT and MRI did not show fracture, abscess, crystal deposition, osteonecrosis, or metastasis. Because infection and immune checkpoint inhibitor-induced arthritis were both possible, she was treated with ampicillin/sulbactam, garenoxacin, IV methylprednisolone 500 mg for 1 day, then oral prednisolone 20 mg/day tapered over 25 days. Her CRP improved, but pain persisted at 8 out of 10 despite NSAIDs and a fentanyl patch. Rheumatology reassessment found persistent swelling and tenderness only in the right sternoclavicular joint. Rheumatoid factor, anti-CCP antibody, antinuclear antibodies, blood cultures, and urine cultures were negative. FDG-PET showed uptake in the same joint. After 24 days of antibiotics with minimal improvement, the findings supported grade 3 immune checkpoint inhibitor-induced arthritis rather than infection. She received infliximab 3 mg/kg IV at 0 and 2 weeks. Two weeks after the second dose, pain, ESR, and CRP had normalized, and she stopped pain medication. Durvalumab was suspended for 1 year because of the adverse event, but the arthritis did not recur and her cancer remained stable.