The 2015 edition of the WHO histological classification of lung adenocarcinoma divided lung adenocarcinoma into adherent subtype, acinar type, papillary type, solid type and micropapillary type according to the main histological subtypes. There is indeed a certain correlation between this classification and the prognosis of patients, but due to the large heterogeneity of adenocarcinoma, most lung adenocarcinoma is a mixture of the above subtypes, and there is no unified grading standard. In 2020, the International Association for the Study of Lung Cancer (IASLC) Pathology Committee proposed a new grading system that classifies lung adenocarcinoma into 3 grades, corresponding to well-differentiated adenocarcinoma, moderately differentiated adenocarcinoma, and poorly differentiated adenocarcinoma.
Grade 1: Well-differentiated adenocarcinoma = predominantly mural tumour with no or less than 20% high-grade structures (solid, micropapillary, cribriform, or complex glandular structures).
Grade 2: Moderately differentiated adenocarcinoma = acinar or papillary-predominant tumour with no or less than 20% high-grade structure.
Grade 3: Poorly differentiated adenocarcinoma = tumors with high-grade structures accounting for 20% or more.
This grading system takes into account both the major histological subtypes and the mixed complexities of poor differentiation. Evidence suggests that the new grading system of the International Lung Cancer Society improves patient outcomes, and that the new grading system is easier to operate and more reproducible across pathologists.
Intra-airspace dissemination (STAS) was written into the 2015 WHO classification of lung adenocarcinoma, and has received extensive attention at home and abroad because it is related to the prognosis of lung adenocarcinoma patients. STAS is defined as a small cluster of nests of tumor cells in the parenchymal alveolar cavity beyond the edge of the tumor body, which needs to be differentiated from the tumor cells artificially caused during specimen processing. Local lobectomy in patients with STAS may be at increased risk.