An Evaluation Into the Robustness of Grading of Pleural Mesothelioma Outside of Specialist Centres

The 2021 WHO classification of thoracic tumours recommends grading pleural mesothelioma to aid prognostication. Robustness of grading and morphological characterisation is key to its clinical utility, though validation of this grading system has largely been conducted by expert thoracic pathologists. We conducted a survey inviting pathologists across a range of practices and expertise to grade digitised images of 50 epithelioid pleural mesotheliomas that had been graded by an expert in thoracic pathology. We included slides that were considered potentially problematic such as small biopsies, focal necrosis, and rare subtypes that may affect grading (small cell and deciduoid features). Using the Sectra Uniview web viewer, participants were asked to score atypia, mitotic count, and necrosis and choose from a list of cytological and architectural features. Seventy-four pathologists anonymously participated. There was 90% agreement of consensus scores with expert opinion using the WHO 2-tier grade and 72% for the 3-tier nuclear grade but only 70% for nuclear atypia, 56% for mitoses, and 84% for necrosis. Both 3-tier nuclear grade and WHO 2-tier grading systems were significantly associated with survival. Our study affirms the overall robustness and utility of grading for pleural mesothelioma, reveals variances, and suggests the need for dedicated training.