What happened, in summary

A 74-year-old woman with a history of smoking was diagnosed with lung adenocarcinoma. She received 5 cycles of pembrolizumab as first-line treatment and achieved a partial response in both the lung tumors and chest lymph nodes.

About 2 months after her last pembrolizumab infusion, she developed burning pain and progressive weakness in her legs. The weakness spread to her arms and was accompanied by changes in her voice. Blood tests showed marked muscle injury, and electrical testing supported a muscle disorder. Her doctors diagnosed probable pembrolizumab-induced polymyositis.

She was treated with intravenous corticosteroids and intravenous immunoglobulin. At one point, swallowing and voice problems were severe enough that she needed temporary tube feeding. Improvement began within 5 days, first in her arms and then in her legs. The feeding tube was removed as she recovered, and steroid treatment was gradually reduced.

She completed rehabilitation and eventually regained full mobility and independence. She continued clinical and imaging follow-up, and her lung cancer was stable on the most recent CT scan in December 2025.