The IASLC Mesothelioma Staging Project: Proposals for Revisions of the ‘T’ Descriptors in the Forthcoming 9th Edition of the TNM Classification for Pleural Mesothelioma

Background The primary tumor (T) component in the 8th edition of pleural mesothelioma (PM) staging system is based on pleural involvement and extent of invasion. Quantitative assessment of pleural tumor has been shown to be prognostic. We explored quantitative and qualitative metrics to develop recommendations for T descriptors in the upcoming 9th edition of the PM staging system. Methods The International Association for the Study of Lung Cancer (IASLC) prospectively collected data on PM patients. Sum of maximum pleural thickness (Psum) was recorded. Optimal combinations of Psum and 8th edition cT descriptors were assessed using recursive binary splitting algorithm, with bootstrap resampling to correct for the adaptive nature of the splitting algorithm and validated in the 8th edition data. Overall survival (OS) was calculated by the Kaplan-Meier method and differences in OS assessed by the log-rank test. Results Of 7,338 patients submitted, 3,598 were eligible for cT analysis and 1,790 had Psum measurements. Recursive partitioning identified optimal cutpoints of Psum at 12 and 30 mm which, in combination with extent of invasion, yielded four prognostic groups for OS. Fmax >5mm indicated poor prognosis. cT4 category (based on invasion) showed similar performance to 8th edition. Three 8th edition descriptors were eliminated based on low predictive accuracy. Eighth edition pT descriptors remained valid in 9th edition analyses. Conclusion Given reproducible prognostication by Psum, size criteria will be incorporated into cT1-T3 categories in the 9th edition. Current cT4 category and all pT descriptors will be maintained, with reclassification of fissural invasion as pT2.