Radiologist’s Guide to the Ninth Edition TNM Staging System for Quantitative and Qualitative Assessment of Pleural Mesothelioma

Pleural mesothelioma is a highly lethal, rare thoracic malignant neoplasm arising from mesothelial cells. Previous versions of the clinical TNM staging classification have relied on a qualitative radiologic assessment of tumor invasion. However, these qualitative assessments have not proved practical to implement, with poor interobserver agreement and lack of prognostic significance. The updated ninth edition of the International Association for the Study of Lung Cancer TNM staging system for pleural mesothelioma, jointly published by the American Joint Committee on Cancer and Union for International Cancer Control, proposes key evidence-based changes. The ninth edition incorporates quantitative measurements of pleural tumor and fissural thickness in the T descriptors for pleural mesothelioma and removes the qualitative descriptors that cannot be reliably detected on CT scans, such as endothoracic fascia, lung parenchymal invasion, and nontransmural diaphragm invasion or involvement. This article explains the rationale behind these changes and provides a practical guide to the ninth edition of the TNM staging system in clinical practice.