Atypical Adenomatous Hyperplasia
Key Facts
Terminology
Microscopic proliferation of atypical alveolar lining cells measuring < 5 mm in greatest diameter
Clinical Issues
Incidental microscopic finding in lung tissues removed for other reasons
Microscopic Pathology
Lining cells are round, cuboidal, or low columnar
Hobnail cells or “peg” cells with apical snouts are often seen
Nuclei are round and hyperchromatic with evenly dispersed chromatin
Intranuclear inclusions may be present in some cells
Ancillary Tests
Atypical cells are strongly positive for TTF-1
MIB-1/Ki-67 proliferation index is very low (< 1%)
 Scanning magnification of atypical adenomatous hyperplasia of the lung shows a well-delineated area  of lung parenchyma that stands out from the surrounding tissue. |
 Higher magnification of atypical adenomatous hyperplasia shows alveolar septa lined by cells with enlarged, hyperchromatic nuclei with mild fibrosis of the underlying interstitium. |
TERMINOLOGY
Abbreviations
Atypical adenomatous hyperplasia (AAH)
Synonyms
Bronchioloalveolar adenoma
Definitions
Microscopic proliferation of atypical alveolar lining cells measuring < 5 mm in greatest diameter
ETIOLOGY/PATHOGENESIS
Pathogenesis
Recent studies support that these lesions may be the earliest precursors of peripheral lung adenocarcinoma
CLINICAL ISSUES
