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Bronchioloalveolar Carcinoma (BAC)

DEFINITION OF BAC (1999 WHO Classification)

An adenocarcinoma with growth along pre-existing alveolar structures (lepidic growth pattern);
No evidence of stromal, vascular or pleural invasion.

GENERAL FEATURES

BAC represents 1-9% of lung cancers;
qWomen: men = 2/1;
q86% smokers;
qRadiographically, ground glass opacities;
qGross features:
- §Solitary nodule (nonmucinous BAC), or multiple nodules
- Diffuse consolidation (pneumonia pattern, mucinous BAC)
qHistopathological subtypes:
- §Nonmucinous, 60-75%;
- Mucinous, 25%;
- §Mixed mucinous and nonmucinous, very rare
qExamine the entire tumor. Do not make a diagnosis solely based on biopsies/cytology

GROSS FINDINGS

Nonmucinous BAC
- Tumor origin
  - Clara cell: most cases, apical snouts and apical PAS+ granules
  - Type 2 pneumocyte: eosinophilic intranuclear inclusions
  - No clinical significance
- Histopathological Features
  - Cuboidal or columnar cells, eosinophilic or clear cytoplasm
  - Grow along preexisting alveolar structures in a linear fashion
  - Stratification of nuclei or free-floating tufts of tumor cells
- Immunohistochemically
  - CK7 +
  - TTF1 +
  - CK20 −
Mucinous BAC
- Histopathological Features
  - Growth along the alveoli in a continuous fashion
  - No stromal, vascular, or pleural invasion
  - Tall columnar cells; bland, basally located nuclei; abundant apical cytoplasmic mucin
- Immunohistochemically
  - CK7: +
  - CK20: +
  - TTF1: −
  - CDX2: −
- Higher-stage disease and worse prognosis than non-mucinous BAC.

DIFFERENTIAL DIAGNOSES: BAC vs Classic Adenocarcinoma

I. Histopathological Features

Histology Features	P-Value
Clean background	0.009
Absence of 3-dimensional clusters	0.0049
Predominance of mucinous cells	0.0245
Absence of nuclear overlap	0.0049
Absence of irregular nuclear membranes	0.0245
Flat sheets	0.0039
Orderly arrangement of cells with round uniform nuclei	0.0039
Fine granular chromatin	0.000054
Nuclear grooves	0.009

II. Immunohistochemistry Profiles

REFERENCES