The WHO Classification of Thoracic Tumors (5 th edition) mainly has the following changes in the chapter of pleural malignant mesothelioma. (1) The concept of mesothelioma in situ and its diagnostic method have been established for the first time; (2) The tumour grading of pleural malignant mesothelioma was added, it was divided into low grade and high grade according to the cellular atypia, mitotic activity and presence of necrosis. (3) The morphological features of pleural malignant mesothelioma was classified into architectural pattern, cellular and stromal features, the correlation between histological features and prognosis was refined, and some of the controversial cellular types have been reclassified. In this review, we introduced the changes of related pathologic diagnosis, in the WHO Classification of Thoracic Tumors (5 th edition) and discussed its clinical significance.

The WHO Classification of Thoracic Tumors (5 th edition) mainly has the following changes in the chapter of pleural malignant mesothelioma. (1) The concept of mesothelioma in situ and its diagnostic method have been established for the first time; (2) The tumour grading of pleural malignant mesothelioma was added, it was divided into low grade and high grade according to the cellular atypia, mitotic activity and presence of necrosis. (3) The morphological features of pleural malignant mesothelioma was classified into architectural pattern, cellular and stromal features, the correlation between histological features and prognosis was refined, and some of the controversial cellular types have been reclassified. In this review, we introduced the changes of related pathologic diagnosis, in the WHO Classification of Thoracic Tumors (5 th edition) and discussed its clinical significance.