An Autopsy Case of Pleural Mesothelioma Progressed Rapidly With Multiple Organ Metastases During Immune Checkpoint Inhibitor Therapy

Immune checkpoint inhibitors (ICIs) were approved in Japan in 2021 for the initial treatment of pleural mesothelioma (PM), offering anticipated prognostic benefits. However, reports of atypical responses exist. We report the case of an 80‐year‐old man with PM who, despite primary tumor shrinkage on ICI treatment, died of small intestinal perforation resulting from rapid, multiorgan metastasis. This case provided an opportunity to examine the histological changes in PM post‐ICI treatment. The rapid clinical course was suggestive of hyperprogressive disease (HPD), a pattern of unexpectedly accelerated progression. Atypical response patterns remain rare in PM, and it is noteworthy that in this case fulminant metastatic progression occurred concurrently with marked regression of the primary pleural lesion. We believe this is a significant case worthy of presentation.