Putative Malignant Pleural Mesothelioma in situ (MPMIS) with Sequential Acquisition of Genomic Alterations on Fluorescence in situ Hybridization (FISH) Examination.

Malignant pleural mesothelioma (MPM) is a rare and deadly disease. A precursor in situ lesion, malignant pleural mesothelioma in situ (MPMIS), has recently been proposed. On cytological examination, the distinction between reactive and malignant mesothelial cells is often challenging, and sometimes even impossible without ancillary methods. Fluorescence in situ hybridization (FISH) for detection of 9p21 deletion is a powerful diagnostic tool in this context, both in histological and in cytological specimens. Here, we present a case of MPM with initial presentation as a putative MPMIS with disomic chromosomal pattern and homozygous 9p21 deletion with subsequent development of an aneuploid pattern after whole genome duplication during tumor progression.