Young-onset epithelioid type malignant peritoneal mesothelioma with loss of BAP1 gene expression.

Malignant peritoneal mesothelioma (MPeM) is a rare malignancy involving the abdominal peritoneum typically presenting after middle age with little known of survival in the young. We present a patient in his 20's with no known asbestos exposure diagnosed with epithelioid-type MPeM and loss of BAP1 gene expression. Our patient demonstrated diffuse unresectable disease at presentation. He had a poor response to first-line treatment carboplatin-pemetrexed chemotherapy followed by ipilimumab plus nivolumab immunotherapy in the second line and carboplatin plus gemcitabine in the third line. Our patient subsequently died 10 months after diagnosis. Despite our patient having favourable prognostic markers such as BAP1 loss in epithelioid subtype mesothelioma, our patient had poor treatment response and survival, highlighting the heterogeneity within this subgroup and likely alternative underlying factors impacting outcomes.