Pseudo-hyperprogression of Malignant Pleural Mesothelioma Treated with Nivolumab

Currently, immune checkpoint inhibitors (ICIs) are the standard treatment for malignant pleural mesothelioma (MPM). The characteristic responses to ICI treatment include pseudoprogression (PP) and hyperprogressive disease (HPD), which require attention. Reports on PP of MPM are rare. We herein report a 59-year-old woman with MPM whose pleural lesions rapidly increased in size after initiating nivolumab treatment. This was initially judged as HPD but was subsequently diagnosed as PP owing to a marked reduction in tumor size. This case highlights the importance and difficulty of differentiating between PP and true progression, including HPD, during treatment of MPM with ICIs.