Higher magnification showing florid mesothelial hyperplasia in the wall of an inflamed hydrocele. It shows a sparse number of simple non-branching tubules, cords, and nests which are widely spaced without back-to-back arrangement. In contrast, malignant mesotheliomas show complex tubular and diffuse arrangement of malignant mesothelial cells that form expansile masses and show destructive stromal invasion. Malignant mesotheliomas show homozygous deletion of 9p21 locus in about 80% of cases which can be detected by FISH. This molecular abnormality has not been detected in florid mesothelial hyperplasia and is a potentially useful diagnostic tool in challenging cases.Ref: Chiosea S, Krasinskas A, Cagle PT, et al. Diagnostic importance of 9p21 homozygous deletion in malignant mesotheliomas. Mod Pathol, 2008;21:742-747.