The criteria for diagnosing pulmonary adenocarcinoma have changed considerably over time. The 2011 IASLC/ATS recommendations, adopted in the 2015 WHO guidelines, use the following criteria for adenocarcinoma in<unk> : * tumor ≤3 cm * solitary tumor * pure "lepidic" growth* * No stromal, vascular, or pleural invasion * No histologic patterns of invasive adenocarcinoma * No spread through air spaces * Cell type mostly nonmucinous * Minimal/absent nuclear atypia * ± septal widening with sclerosis/elastosis * lepidic = (i.e. scaly covering) growth pattern along pre-existing airway structures A true diagnosis of AIS can only be made once the entire tumor has been surgically removed, because it must be evaluated for evidence of microscopic invasion into the adjacent normal lung. Findings of malignant invasion would mean that the tumor must be reclassified as invasive adenocarcinoma. By this standard, AIS cannot be diagnosed according to core biopsy or cytology sampling. Recommended practice is to report biopsy findings previously classified as nonmucinous BAC as adenocarinoma with lepidic pattern, and those previously classified as mucinous BAC as mucinous adenocarcinoma. In other words, these are handled under the assumption that there is invasive disease. Classification The most recent 2015 World Health Organization (WHO) and 2011 International Association for the Study of Lung Cancer (IASLC) / American Thoracic Society (ATS) guidelines refine pulmonary adenocarcinoma subtypes in order to correspond to advances in personalized cancer treatment. AIS is considered a pre-invasive malignant lesion that, after further mutation and progression, is thought to progress into an invasive adenocarcinoma. Therefore, it is considered a form of carcinoma in<unk>  (CIS). There are other classification systems that have been proposed for lung cancers. The Noguchi classification system for small adenocarcinomas has received considerable attention, particularly in Japan, but has not been nearly as widely applied and recognized as the WHO system. AIS may be further subclassified by histopathology, by which there are two major variants: * mucinous (20–25% of cases) * non mucinous (75–80% of cases)
Adenocarcinoma in situ of the lung
Epithelial-myoepithelial carcinoma of the lung
Salivary gland–like carcinoma of the lung
Minimally invasive adenocarcinoma of the lung
Sarcomatoid carcinoma of the lung
Mucinous cystadenocarcinoma of the lung
Adenocarcinoma of the lung
Pulmonary sclerosing pneumocytoma