A 58-year-old male smoker presents with a peripheral lung nodule. Histology shows mucin-producing glandular structures with bronchioloalveolar growth pattern. Which feature best distinguishes this adenocarcinoma subtype from invasive adenocarcinoma?

## Adenocarcinoma Subtypes: Lepidic vs. Invasive Pattern ### Histological Pattern Distinction | Feature | Lepidic (Bronchioloalveolar) | Invasive Adenocarcinoma | |---------|---|---| | **Growth pattern** | Along alveolar walls (non-invasive) | Destructive invasion of stroma | | **Stromal invasion** | Absent or minimal | Present (defining feature) | | **Pleural invasion** | Absent | May be present | | **Angiolymphatic invasion** | Absent | May be present | | **Prognosis** | Better (Stage IA even if large) | Worse (depends on invasion depth) | | **Spread** | Intrapulmonary ("pneumonic" spread) | Systemic metastases | **Key Point:** The 2015 WHO classification defines adenocarcinoma subtypes by growth pattern, not size. Lepidic (bronchioloalveolar) pattern is characterized by growth along pre-existing alveolar walls WITHOUT stromal invasion — this is the critical discriminator. **High-Yield:** Lepidic adenocarcinoma has a significantly better prognosis than invasive adenocarcinoma because the absence of stromal invasion indicates lower biological aggressiveness and reduced metastatic potential. **Mnemonic:** **LEPIDIC = Lining Epithelium Pattern In Distal lung, Invasion Cancelled** — growth follows the alveolar architecture without breaking through the basement membrane. **Clinical Pearl:** A large lepidic adenocarcinoma (even >4 cm) is classified as Stage IA if there is no stromal invasion. Conversely, a small invasive adenocarcinoma with stromal invasion is Stage IB or higher. Size alone does not determine stage in adenocarcinoma — invasion pattern does. ### Pathological Mechanism Lepidic pattern represents the least aggressive form of adenocarcinoma because: 1. Tumor cells grow along the surface of alveoli without destroying underlying stroma 2. Lack of stromal desmoplasia indicates low angiogenic potential 3. Absence of invasion limits access to lymphatics and blood vessels 4. Intrapulmonary spread occurs via aerogenous route (slow), not hematogenous (fast)