Introduction: Stage classification provides a consistent and concise nomenclature about the anatomic extent of the cancer. This is a fundamental cornerstone in the management of patients; it enables reporting results and facilitates comparing one treatment to another and judging how closely clinical trial results apply to an individual patient. A nomenclature must be relatively static; however, periodical refinement is needed to adjust to a changing landscape of clinical relevance. Changes must be well justified and thoughtfully developed to maintain the ability to communicate clearly and facilitate comparisons across time. Methods: For thoracic malignancies (lung, pleura, thymus, and esophagus), the International Association for the Study of Lung Cancer (IASLC) has leveraged its worldwide multidisciplinary reach, permitting a sophisticated approach to this process. Refinement of stage classification for the ninth edition of TNM is underway; this article describes the approach adopted by the IASLC Staging and Prognostic Factors Committee. Results: Key guiding principles include the ability to maintain communication over time, a classification that discriminates homogeneous cohorts of tumors consistently across the world in multiple settings, treatment approaches, and patient characteristics, including clinical relevance and practical applicability. The IASLC has again assembled a large international database to permit multifaceted analysis. Providing confidence that the classification performs consistently in multiple settings, treatments, and patients requires consistent discrimination in multiple subset analyses. Although observed outcomes of patients in the 2011 to 2019 database are essential, considerations about how the classification will be used are also important to ensure clinical relevance and applicability. Conclusions: The strategy developed by the Staging and Prognostic Factors Committee is carefully designed to provide useful refinements to the stage classification of thoracic malignancies for the ninth edition of TNM classification of cancers.
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