## Genetic Alterations in SCLC **Key Point:** Small cell lung cancer is defined by near-universal inactivation of both p53 and RB (retinoblastoma) tumor suppressor pathways, occurring in >90% of cases. ### Mechanism of p53 and RB Loss 1. **p53 inactivation** — Loss of the "guardian of the genome"; allows uncontrolled proliferation and evasion of apoptosis 2. **RB inactivation** — Loss of G1/S checkpoint control; permits unregulated cell cycle progression 3. **Synergistic effect** — Combined loss of both pathways drives the aggressive neuroendocrine phenotype characteristic of SCLC ### Contrast with Other Lung Cancer Types | Feature | SCLC | Non-Small Cell LC | |---------|------|-------------------| | p53 loss | >90% | 50–70% | | RB loss | >90% | 15–30% | | EGFR mutation | Rare (<5%) | Common in adenocarcinoma (30–40%) | | ALK rearrangement | Rare (<5%) | Present in ~5% of adenocarcinoma | | BRAF mutation | Rare | Occasional in adenocarcinoma | **High-Yield:** The "two-hit" loss of p53 and RB is the molecular hallmark of SCLC and distinguishes it from NSCLC, which typically harbors single oncogenic driver mutations (EGFR, ALK, ROS1). **Clinical Pearl:** Because SCLC lacks targetable driver mutations like EGFR or ALK, it is treated with chemotherapy (platinum + etoposide) rather than tyrosine kinase inhibitors. Recent addition of immunotherapy (atezolizumab, durvalumab) has improved outcomes in limited-stage disease. [cite:Robbins 10e Ch 15] 
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