Zhang YZ, Brambilla C, Molyneaux PL, et al. - Researchers sought to determine if nuclear grading systems for epithelioid malignant pleural mesothelioma (MPM) could be applied in a biopsy-heavy setting. They conducted an independent, external validation study using the proposed system among 563 consecutive cases of epithelioid MPM diagnosed between 2003 and 2017. Of these cases, 87% of patients underwent biopsies only. On average 1 site was sampled, with a median maximum tissue dimension of 17 mm (biopsy) and 150 mm (resection). The patients had a median overall survival (OS) of 14.7 months. The frequencies of grade I, II, and III tumors were 31% (132/563), 52% (292/563), and 17% (94/563). The multivariate analysis supports the possible predictive value of 3-tier nuclear grade, 2-tier nuclear grade, and mitosis-necrosis score for OS; age, procedural type, solid-predominant growth pattern, necrosis, and atypical mitosis did not affected the predictive value. None but age was independently predictive in the scenario of a single- site biopsy with tissue dimension ≤ 10 mm. Further, the data suggest that for nuclear-grade assessment, sampling 3 sites or a maximum tissue dimension of at least 20 mm from a single site is optimal.