Atypical adenomatous hyperplasia of the lung
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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.
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.