Malignant mesothelioma: PAS-diastase positivity and inversion of polarity in intravascular tumour

Aims: Periodic acid-Schiff (PAS)-diastase-positive material was identified within pseudoglandular structures within the small intravascular component of two pleural malignant mesotheliomas. The aim of this study was to ascertain the nature of this material and to asses the polarity of the cells forming the pseudoglandular structure. Methods and results: Immunohistochemistry was performed using antibodies to laminin and type IV collagen and the antibody HBME-1. These demonstrated the material to be basement membrane rather than mucin. The apical polarity marker HBME-1 was not related to the internal pseudoglandular structure but stained the periphery of intravascular tumour clumps. Conclusions: Pseudoluminal PAS-diastase-positive material in malignant mesothelioma may easily be mistaken for epithelial mucin, leading to an erroneous diagnosis of adenocarcinoma. The presence of basement membrane material in pseudolumina, as defined by the presence of laminin and type IV collagen, surrounded by tumour cells whose external surface expresses the apical polarity marker HBME-1 implies inversion of polarity of tumour cells within vascular spaces.