Mesothelioma in situ with regressive malignant pleural effusion and an unexpected evolution: A case report.

Malignant pleural mesothelioma (MPM) is an aggressive neoplasm that originates from hyperplasia and metaplasia of the mesothelial cells that cover the pleural cavity. Previous exposure to asbestos is the main risk factor. Since MPM is often diagnosed at an advanced stage with rapid evolution and resistance to treatment, it is associated with an unfavorable outcome. Mesothelioma in situ (MIS) has been postulated as a preinvasive phase of MPM; however, its diagnostic criteria have been defined only recently. Diagnosis of MIS may represent an opportunity for early therapies with better results, but the optimal approach has not been defined thus far. Here, we report on a case of a 74-year-old man with right-sided pleural effusion and a previous history of occupational exposure to asbestos for 9 years who was diagnosed with MIS after a latency of 36 years. During follow-up, spontaneous disease regression was observed 5 months after the initial diagnosis; however, it recurred in the form of invasive epithelioid MPM. There is a paucity of literature on MIS and its evolution; however, our case provides relevant knowledge of this unusual behavior, which is important to define follow-up and therapeutic strategies for future cases.