Atypical adenomatous hyperplasia of the lung

Last revised by Yuranga Weerakkody on 21 Feb 2022

Atypical adenomatous hyperplasia (AAH) of the lung is a putative precursor lesion of adenocarcinoma of the lung. This entity replaces part of a spectrum of the former bronchoalveolar carcinoma (BAC) and falls under the spectrum of pre-invasive lesions of the lungs.


Atypical adenomatous hyperplasia lesions are represented by localized small cell proliferation, usually measuring ≤0.5 cm (<5 mm), of mildly to moderately atypical type II pneumocytes and/or Clara cells that line along the alveolar walls, alveolar septae, or respiratory bronchiole 3,5,8. Subgrading of those lesions according to the spectrum of cellularity and atypia is not recommended 8

Radiographic features 

Their radiographic appearances can be somewhat variable but have been commonly described as ground-glass nodules (usually pure ground glass) measuring less than 5 mm 3-5, opposed to adenocarcinomas in situ, which are also pre-invasive lesions, that tend to be larger. 

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