Progression of Peritoneal Mesothelioma In Situ to Invasive Mesothelioma Arising in the Setting of Endometriosis With Germline BAP1 Mutation: A Case Report

Mesothelioma in situ has been proposed as a precursor to malignant mesothelioma arising in the pleura or peritoneum. We report a case of malignant peritoneal mesothelioma which progressed from mesothelioma in situ over a 10-mo period in a 24-yr-old woman with stage IV endometriosis. Initial surgery showed deeply infiltrative endometriosis with progestin effect. Postoperatively the patient had intractable pelvic pain and vaginal discharge. Imaging studies were negative. Repeat laparoscopy 10 mo later revealed vesicular lesions on the omentum and pinpoint white lesions studding the small bowel, appendix, and pelvic peritoneum. A diagnosis of epithelioid mesothelioma was established on biopsy of the omentum and confirmed by immunohistochemistry showing complete loss of BRCA1-associated protein-1 (BAP1) nuclear staining. Retrospectively, BAP1 loss was identified in the cytologically bland, single-layer surface mesothelium of the prior resection specimen, consistent with mesothelioma in situ. The patient underwent genetic testing and was found to have a pathogenic germline mutation in BAP1.