What happened, in summary

A 49-year-old Japanese man was first evaluated after 14 days of chest pain, productive cough, and hemoptysis. CT showed an approximately 37 mm tumor in the right lower lobe that had merged with right hilar lymph nodes. PET/CT showed abnormal uptake in the tumor and mediastinal lymph nodes, and transbronchial biopsy confirmed lung squamous cell carcinoma of the right hilum. He was staged as cT2aN3M1a, Stage IVA. Treatment began with carboplatin, nab-paclitaxel, and pembrolizumab. From month 3 onward, he continued monthly pembrolizumab maintenance. At 12 months, the tumor and mediastinal nodes had shrunk and PET/CT no longer showed FDG uptake, consistent with a complete response. Immune-related toxicity then became the major issue. At 14 months, edema and numbness appeared in both fingers soon after pembrolizumab; the symptoms improved, then recurred after later infusions. By 22 months he had hand arthralgia, and after the 24th pembrolizumab infusion he developed high fever, diarrhea, widespread joint pain, pitting edema of both hands and lower legs, and immobility. Imaging found no cancer recurrence. Ultrasound supported synovitis and tenosynovitis, leading to a diagnosis of pembrolizumab-induced RS3PE/inflammatory arthritis, with likely immune-related colitis. Prednisolone 50 mg/day improved pain by the next day and swelling within days. Pembrolizumab was discontinued because he was in complete remission. Eight months after steroid initiation, he remained free of lung-cancer relapse and RS3PE relapse while taking prednisolone 5 mg/day. During admission, CRP was 20.8 mg/dL and white blood cell count was 13,770/uL, while rheumatoid factor, anti-CCP antibody, and antinuclear antibodies were normal, helping separate the toxicity from rheumatoid arthritis or cancer relapse.