Progression of mesothelioma in situ to invasive disease 4 years and 10 months after initial diagnosis

Sir, In 20171 we reported the first case of mesothelioma in situ(MIS) that was diagnosed using the now accepted diagnostic criteria. MIS is a preinvasive mesothelial lesion which was recognised as a concept as early as 19922 but has only now been incorporated as a diagnostic entity into the current World Health Organization (WHO) classification of thoracictumours.3 The patient at the time presented with recurrent pleural effusions over the space of several months, and a biopsy at the time showed non-invasive focally papillary mesothelial proliferation with minimal mesothelial atypia and loss of BAP1. At that time, MTAP labelling was not available, and CDKN2A by fluorescence in situ hybridisation was not readily available; also, it is important to remember that neither were recognised as a diagnostic tool at that time. The diagnosis of MIS at that time was not recognised in this context, but because of the molecular significance of the unequivocal loss of BAP1, we flagged the finding to the clinicians and a decision for close clinical follow up was made.