Abstract
Mesothelioma in situ is a recently described precursor to invasive mesothelioma. Thus far, all cases in the literature have involved one pleural cavity or the peritoneal cavity. We describe a patient with biopsy-proven mesothelioma in situ involving both pleural cavities and the peritoneal cavity. Genetic analysis results revealed that the patient had a BAP1 germline mutation. This is the first report of mesothelioma in situ involving multiple body cavities and raises a question of whether such patients will all have BAP1 germline mutations.