The 2015 WHO Classification had a major impact on the new 8th Edition TNM Classification. Compared to the 2004 Classification, major changes include: 1) use of immunohistochemistry throughout; 2) New emphasis on genetic studies and personalized therapeutic strategies; 3) A new classification of lung cancer in small biopsy and cytology samples; 4) adoption of the 2011 IASLC/ATS/ERS lung adenocarcinoma classification; 5) reclassification of large cell carcinoma based upon immunohistochemistry and genetics. Since publication of the 2015 WHO Classification new advances include recognition of ciliated muconodular papillary tumors, SMARCA4 and SMARCB1 deficient neoplasms and digital image analysis as a novel way to assess lung cancer morphology. This presentation will primarily focus on the impact of the new WHO Classification on the 8th Edition TNM classification. First the new TNM classification incorporates the introduction of the concepts of adenocarcinoma in situ (AIS) which should be staged as Tis (AIS) and minimally invasive adenocarcinoma (MIA) which should be staged as T1mi. AIS is defined as a lung adenocarcinoma with pure lepidic growth measuring ≤3 cm. MIA is defined as a ≤3 cm lepidic predominant adenocarcinoma with an invasive component measuring 0.5 cm or less. Both AIS and MIA should lack stromal, vascular, or pleural invasion and spread through alveolar space invasion (STAS). Lepidic predominant adenocarcinomas are lung adenocarcinomas with a predominant lepidic component that measure > 3 cm in total size or that have an invasive component measuring >0.5 cm. It is recommended to use invasive size for T-descriptor size in nonmucinous adenocarcinomas with a lepidic component. This is in keeping with a recommendation made in three editions of the UICC TNM Supplement since 2003. It is also supported by a growing amount of evidence showing that invasive size is a better predictor of survival than total size in nonmucinous adenocarcinomas with a lepidic component. Both radiologists and pathologists should report the greatest dimension for tumor size for both clinical and pathologic staging. In addition for nonmucinous lung adenocarcinomas, both the total size and invasive size should be reported with invasive size used for T-factor size determination. By computed tomography (CT) in nonmucinous lung adenocarcinomas, the presence of ground glass versus solid opacities generally correspond to lepidic versus invasive patterns respectively seen pathologically. Since, this is not an absolute correlation, when CT features suggest nonmucinous AIS, MIA and LPA, reporting of the suspected diagnosis and clinical staging, should be made as a preliminary assessment that may need to be revised after evaluation of resected specimens pathologically. Since the mucinous variants of AIS, MIA and invasive mucinous adenocarcinomas usually present by CT as a solid or consolidated nodule, and due to the lack of proven correlation between ground glass/solid CT appearance with lepidic/invasive growth pathologically it is not recommended to apply the total vs solid size assessment by CT in suspected invasive mucinous adenocarcinomas. Furthermore there is insufficient data in invasive mucinous adenocarcinomas that invasive size is a better predictor of survival than total size. Pathologic assessment of total vs invasive tumor size in resected nonmucinous lung adenocarcinomas with a lepidic component can be improved by reviewing CT scans because the lepidic component is often poorly appreciated pathologically on gross exam and size is underestimated. In addition, tumor size can be more accurately assessed after radiologic pathologic correlation in the following settings: 1) Lepidic nonmucinous adenocarcinomas that do not fit onto a single slide, 2) Sausage or bilobed shaped tumors where the maximum single diameter may be better assessed using all three CT views (axial, coronal and sagittal) rather than just axial alone, 3) Tumors removed in multiple parts, 4) Intraoperative defects in tumors, 5) Marked non-neoplastic reactions, 6) Mistaken pathologic assessment. In neoadjuvant tumors, it can be difficult to measure tumor size because tumors that show considerable treatment effect often do not have a uniform response allowing a single focus of viable tumor to be measured. It has been shown that 90% or more treatment effect is the most important prognostic finding instead of tumor size in surgically resected nonsmall cell lung cancer patients following induction therapy. One way to estimate viable tumor size is to multiply the percent of viable tumor cells times the size of the total tumor bed. This can be utilized in the setting of a single focus or multiple foci of viable tumor. Recording the percentage of treatment effect is important in addition to estimating tumor size for T-factor determination. 1. Travis WD, et al The New IASLC/ATS/ERS international multidisciplinary lung adenocarcinoma classification. JThoracic Oncol 2011;6:244-85. 2. Travis WD, et al WHO Classification of Tumours of the Lung, Pleura, Thymus and Heart. Lyon: International Agency for Research on Cancer; 2015. 3. Travis WD, et al The IASLC Lung Cancer Staging Project: Proposals for Coding T Categories for Subsolid Nodules and Assessment of Tumor Size in Part-Solid Tumors in the Forthcoming Eighth Edition of the TNM Classification of Lung Cancer. J Thorac Oncol 2016;11:1204-23. 4. 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MacMahon H, et al Guidelines for Management of Incidental Pulmonary Nodules Detected on CT Images: From the Fleischner Society 2017. Radiology 2017;284:228-43. 10. Kamata T, et al. Frequent BRAF or EGFR Mutations in Ciliated Muconodular Papillary Tumors of the Lung. J Thorac Oncol 2016;11:261-5. Pathology, WHO Classification, TNM classification