[Application of the WHO Classification of Thoracic Tumors (2021) grading system in invasive pulmonary adenocarcinoma and its correlation with the targeted genes' variations].

To investigate the applicability of the 2021 WHO classification of thoracic tumors' new grading system for invasive pulmonary adenocarcinoma (IPA) with different clinical stages and its correlation with the characteristics of targeted genes' variation. A total of 2 467 patients with surgically resected primary IPA in Shanghai Pulmonary Hospital, Shanghai, China from September to December 2020 were retrospectively analyzed. Eligible cases were graded using the new grading system of IPA of the 2021 WHO classification of thoracic tumors. The clinicopathological data and targeted-gene abnormality were collected. The utility of new grading system of IPA in different clinical stages was investigated. The correlation of clinicopathological features and targeted-gene abnormality in different grades of IPA were compared. All 2 311 cases of IPA were included. There were 2 046 cases of stage Ⅰ IPA (88.5%), 169 cases of stage Ⅱ (7.3%), and 96 cases of stage Ⅲ (4.2%). According to the new classification system of IPA, 186 cases (9.1%), 1 413 cases (69.1%) and 447 cases (21.8%) of stage-Ⅰ adenocarcinoma were classified as Grade 1, Grade 2 and Grade 3, respectively. However, there were no Grade 1 adenocarcinomas in stages Ⅱ and Ⅲ cases. Among stage-Ⅱ and Ⅲ IPA cases, there were 38 Grade 2 cases (22.5%) and 131 Grade 3 cases (77.5%), and 3 Grade 2 cases (3.1%) and 93 Grade 3 cases (96.9%), respectively. In stage-Ⅰ cases, no tumor cells spreading through airspace (STAS), vascular invasion or pleural invasion was found in Grade 1 of IPA, while the positive rates of STAS in Grade 2 and 3 IPA cases were 11.3% (159/1 413) and 73.2% (327/447), respectively. There was a significant difference among the three grades (<0.01). Similarly, the rates of vascular and pleural invasion in Grade 3 IPA cases were 21.3% (95/447) and 75.8% (339/447), respectively, which were significantly higher than those of 1.3% (19/1 413) and 3.0% (42/1 413) in Grade 2 (<0.01). EGFR mutational rates in Grades 1, 2 and 3 IPA were 65.7% (94/143), 76.4% (984/1 288) and 51.3% (216/421), respectively. The differences among the three grades were statistically significant (<0.01). No fusion genes were detected in Grade 1 IPA, while the positive rates of ROS1 and ALK fusion genes in Grade 3 were 2.4% (10/421) and 8.3% (35/421), respectively, which were significantly higher than that of 0.5% (7/1 288) and 1.6% (20/1 288) in Grade 2 (<0.01). In stage-Ⅱ cases, only EGFR mutation rate in Grade 2 adenocarcinoma (31/37, 83.8%) was higher than that in Grade 3 adenocarcinoma (71/123, 57.7%; <0.01). However, the correlation between the new grade system of IPA and the distribution characteristics of targeted-gene variation cannot be evaluated in stage Ⅲ cases. The new grading system for IPA is mainly applicable to clinical stage-Ⅰ patients. Tumor grades of IPA are strongly correlated with the high-risk factors of prognosis and the distribution features of therapeutic targets. It is of great significance and clinical value to manage postoperative patients with early-stage IPA.