Definition/General
Introduction:
BAC is a historical term for adenocarcinoma with pure lepidic growth pattern
Now classified as adenocarcinoma in situ (AIS) and minimally invasive adenocarcinoma (MIA)
Shows excellent prognosis when purely lepidic.
Origin:
Arises from alveolar epithelium and bronchiolar epithelium
Shows lepidic spread along alveolar walls
Non-invasive growth (AIS) or minimal invasion (MIA)
Preserves lung architecture.
Classification:
WHO 2021: Adenocarcinoma in situ (≤3 cm, pure lepidic)
Minimally invasive adenocarcinoma (≤3 cm, ≤5 mm invasion)
Invasive adenocarcinoma (lepidic predominant)
No longer BAC terminology.
Epidemiology:
Peak incidence 6th-7th decades
Female predominance
Asian ethnicity
Non-smokers (40%)
Peripheral location
Multifocal disease possible.
Clinical Features
Presentation:
Asymptomatic (incidental finding)
Persistent cough
Dyspnea
Bronchorrhea (rare)
Ground-glass opacity on imaging
Slow growth.
Symptoms:
Usually asymptomatic
Chronic cough
Sputum production
Shortness of breath
Chest discomfort
Bronchorrhea (mucinous type).
Risk Factors:
Female gender
Asian ethnicity
Non-smoking status
EGFR mutations
Environmental factors
Genetic susceptibility.
Screening:
High-resolution CT
Ground-glass nodules
Part-solid lesions
Multifocal disease
Slow growth rate
PET scan (low uptake).
Master BAC Pathology with RxDx
Access 100+ pathology videos and expert guidance with the RxDx app
Gross Description
Appearance:
Peripheral location
Gray-white, firm
Well-demarcated
Preserved lung architecture
No obvious invasion
Consolidation pattern.
Characteristics:
Firm consistency
Tan-gray color
Granular surface
Maintains alveolar structure
No necrosis
Focal scarring.
Size Location:
Peripheral subpleural
Small size (≤3 cm for AIS/MIA)
Lower lobe predilection
Bilateral disease possible
Multifocal lesions.
Multifocality:
Synchronous lesions common
Multifocal/multicentric
Bilateral involvement
Field cancerization
Skip lesions.
Microscopic Description
Histological Features:
Lepidic growth pattern
Growth along alveolar walls
Preserved alveolar architecture
No stromal invasion (AIS)
Minimal invasion (≤5 mm in MIA).
Cellular Characteristics:
Columnar cells
Hobnail appearance
Intranuclear inclusions
Clara cell features
Type II pneumocyte features
Minimal atypia.
Architectural Patterns:
Pure lepidic pattern (AIS)
Lepidic with focal invasion (MIA)
Non-mucinous type
Mucinous type
Mixed patterns.
Grading Criteria:
Low-grade morphology
Minimal cytological atypia
Low mitotic rate
Invasion assessment critical
Size measurement important.
Immunohistochemistry
Positive Markers:
TTF1 - positive (non-mucinous type)
Napsin A - positive
CK7 - positive
Surfactant protein - positive
EMA - positive.
Negative Markers:
CK20 - negative
CDX2 - negative (vs GI)
p63 - negative
p40 - negative
Chromogranin - negative.
Diagnostic Utility:
TTF1/Napsin A confirm pulmonary adenocarcinoma
Mucinous type: Often TTF1 negative
Ki-67 low
p53 usually wild-type
EGFR testing recommended.
Molecular Subtypes:
Non-mucinous type: TTF1+, Napsin A+
Mucinous type: TTF1-, may be CDX2+
Mixed type: Variable expression
EGFR mutations common in non-mucinous.
Molecular/Genetic
Genetic Mutations:
EGFR mutations (50% in non-mucinous)
KRAS mutations (mucinous type)
PIK3CA mutations
TP53 mutations (rare in AIS)
ALK rearrangements (rare).
Molecular Markers:
Low mutational burden
Driver mutations present
EGFR pathway activation
Growth factor signaling
Cell adhesion alterations.
Prognostic Significance:
AIS: 100% disease-free survival
MIA: Near 100% disease-free survival
Size and invasion critical
Multifocal disease: Different prognosis
Excellent outcomes with complete resection.
Therapeutic Targets:
Complete surgical resection
Wedge resection acceptable
EGFR tyrosine kinase inhibitors
Surveillance for multifocal disease
No adjuvant therapy needed.
Differential Diagnosis
Similar Entities:
Atypical adenomatous hyperplasia
Pulmonary adenocarcinoma (invasive)
Metastatic adenocarcinoma
Pneumonic-type adenocarcinoma
Organizing pneumonia.
Distinguishing Features:
AIS/MIA: Pure/minimal invasion, excellent prognosis
AAH: Smaller, <5 mm
Invasive adenocarcinoma: >5 mm invasion
Metastatic: Organ-specific markers
Organizing pneumonia: Masson bodies.
Diagnostic Challenges:
Invasion assessment
Size measurement
Multifocal vs metastatic
Sampling adequacy
Frozen section diagnosis.
Rare Variants:
Colloid adenocarcinoma
Fetal adenocarcinoma
Enteric adenocarcinoma
Clear cell pattern.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
[Wedge resection/lobectomy] with peripheral nodule
Tumor Description
Well-demarcated adenocarcinoma measuring [X] cm with lepidic growth pattern
Growth Pattern
Pure lepidic growth pattern with preservation of alveolar architecture
Invasion Assessment
Stromal invasion: [Absent (AIS)/Present, measuring [X] mm (MIA)/Present, >5 mm (invasive)]
Subtype
[Non-mucinous/Mucinous/Mixed] type
Margins
Surgical margins negative
Multifocal Assessment
[Single focus/Multiple foci noted]
Immunohistochemistry
TTF1: [Positive/Negative], Napsin A: Positive, CK7: Positive
Molecular Testing
[EGFR mutation testing recommended/Results: [mutation status]]
Final Diagnosis
[Adenocarcinoma in situ/Minimally invasive adenocarcinoma], [non-mucinous/mucinous] type