Diagnostic criteria for pulmonary adenocarcinoma in situ:
(1) tumor size <= 3 cm
(2) solitary mass
(3) pure lepidic growth pattern
(4) no stromal, vascular or pleural invasion
(5) no pattern of invasive adenocarcinoma (acinar, papillary, micropapillary, colloid, enteric, fetal or mucinous)
(6) no spread through air spaces
(7) nuclear atypia is absent or inconspicuous
Additional histologic features:
(1) septal widening with sclerosis/elastosis is common
(2) cell type usually is nonmucinous but rarely can be mucinous.
