Adenocarcinomas in situ (AIS) of the lung refer to a relatively new entity which falls under the spectrum of pre-invasive lesions of the lungs. This entity partly replaces the noninvasive end of the previous term bronchoalveolar carcinoma. Adenocarcinoma in situ is defined as a localized adenocarcinoma of <3 cm that exhibits a lepidic growth pattern, with neoplastic cells along the alveolar structures but without stromal, vascular, or pleural invasion ¹.

Pathology

AISs are localized adenocarcinomas of ≤3 cm, with no growth pattern other than lepidic, with neoplastic cells along the alveolar structures but without stromal, vascular, lymphatic, or pleural invasion, and no features of necrosis ⁶. 

There are three histopathological subtypes, the most common is non-mucinous, with mucinous or mixed subtypes being rarely found. 

Radiographic features

CT

While adenocarcinoma in situ is usually seen as a pure ground-glass nodule or a part-solid lung nodule, there can be overlap among the imaging features of atypical adenomatous hyperplasia, adenocarcinoma in situ, and invasive adenocarcinoma of the lung ¹.

Nuclear medicine

FDG PET-CT

Adenocarcinomas in situ are commonly associated with PET false-negative results. FDG PET-CT is recommended when assessing subsolid ground-glass lung lesions that have a solid component measuring more than 8 mm ⁵. 

Treatment and prognosis

Adenocarcinoma in situ carries an excellent prognosis, with reported survival rates of 100% following complete tumor resection ⁶.

For follow-up guidelines consider - Fleischner Society pulmonary nodule recommendations