Pelvic Mesothelioma Shrinks 83% With Immunotherapy

A case report published in Frontiers in Oncology describes a notable response to immunotherapy in a rare presentation of mesothelioma. A 72-year-old woman with pelvic mesothelioma, a peritoneal variant, experienced an 83% reduction in tumor size after treatment with the PD-1 inhibitor toripalimab combined with pemetrexed and cisplatin chemotherapy.

The tumor shrank from 150 by 160 mm at the start of treatment in May 2024 to 41 by 25 mm by May 2025. The patient’s CA15-3 tumor marker fell from 41.8 to 13.8 U/mL over the same period. As of the last follow-up in August 2025, the response was sustained and the patient remained in good condition.

Two Cases, Two Very Different Outcomes

The report presents two contrasting cases that illustrate how the site of mesothelioma may influence treatment response.

Case 1 (pelvic/peritoneal): The 72-year-old woman received six cycles of toripalimab combined with pemetrexed and cisplatin, followed by toripalimab maintenance therapy. She achieved a significant partial response that continued through at least 15 months of follow-up. Her quality of life improved alongside the measurable tumor shrinkage.

Case 2 (pleural): A 66-year-old man with pleural mesothelioma showed primary resistance to platinum-based chemotherapy (pemetrexed plus lobaplatin). His disease progressed with a growing chest wall mass and pleural effusion despite treatment. The authors note that PD-L1 staining and next-generation sequencing were not performed at the time, and the tissue block is no longer available for testing. They point to factors often linked to this kind of resistance, such as BAP1 mutations, low tumor mutational burden, and an immunosuppressive tumor microenvironment, as possible explanations rather than confirmed findings in this patient.

The divergent outcomes suggest that the anatomical site of mesothelioma, and its underlying molecular features, could shape which treatments work.

Why Tumor Location May Matter

Pleural mesothelioma (in the lung lining) accounts for roughly 80% of all mesothelioma cases. Peritoneal mesothelioma (in the abdominal lining) makes up most of the remainder. Pelvic mesothelioma, as in the first case, is rarer still.

The authors propose that peritoneal mesothelioma may have a different immunological profile than pleural disease, potentially making it more susceptible to immune checkpoint inhibitors like toripalimab. Pleural mesothelioma, by contrast, frequently harbors BAP1 mutations and immunosuppressive features that blunt the immune response.

This hypothesis aligns with emerging research suggesting that treatment strategies for mesothelioma should be tailored not just to cell type (epithelioid, sarcomatoid, or biphasic) but also to the anatomical site and molecular characteristics of each person’s tumor.

Limitations and Next Steps

The authors acknowledge several limitations. Monitoring relied on CT imaging and the CA15-3 marker, which is not specific to mesothelioma. More advanced tools, such as circulating tumor DNA (ctDNA) analysis or repeat biopsies, could provide earlier detection of resistance.

The report calls for larger prospective clinical trials to validate an anatomy-based approach to mesothelioma treatment. Specifically, the authors recommend testing immunotherapy-chemotherapy combinations in peritoneal mesothelioma cohorts and evaluating targeted therapies (such as BAP1-directed treatments) for pleural cases with identified molecular drivers.

Toripalimab is a PD-1 immune checkpoint inhibitor approved in China for several cancer types, including melanoma and nasopharyngeal carcinoma. It received FDA approval in the United States in 2023 for nasopharyngeal carcinoma. Its use in mesothelioma remains investigational.