A 55-year-old Indian male smoker presents with a lung mass. Molecular analysis reveals KRAS mutation at codon 12. Which of the following is the most common type of lung cancer associated with KRAS mutations?

Explanation

## KRAS Mutations in Lung Cancer **Key Point:** KRAS mutations occur most frequently in lung adenocarcinoma (~30–40% of cases), particularly in non-smokers and smokers with peripheral lung tumors. ### KRAS Mutation Epidemiology in Lung Cancer **High-Yield:** KRAS mutations show distinct associations with histologic subtypes: | Lung Cancer Histotype | KRAS Mutation Frequency | Clinical Association | | --- | --- | --- | | Adenocarcinoma | 30–40% | Most common; peripheral location | | Squamous cell carcinoma | 5–10% | Central airway; smoking-related | | Small cell lung cancer (SCLC) | <5% | Rare; aggressive neuroendocrine | | Large cell carcinoma | 10–15% | Uncommon histotype overall | **Clinical Pearl:** KRAS-mutant adenocarcinomas tend to be more aggressive, with poorer prognosis and reduced responsiveness to EGFR inhibitors compared to EGFR-mutant tumors. ### Pathogenesis of KRAS-Driven Adenocarcinoma 1. KRAS mutation (codon 12 most common) occurs in peripheral lung epithelium 2. Constitutive RAS/MAPK pathway activation 3. Uncontrolled proliferation of mucin-producing cells 4. Development of adenocarcinoma phenotype 5. Progression to invasive and metastatic disease **Mnemonic:** **KRAS in Adenocarcinoma = Kinase-driven Rapid Advancement in Smokers and non-smokers** — most common RAS isoform in lung adenocarcinoma. ### Why Adenocarcinoma is Most Common for KRAS - Adenocarcinoma is the most common lung cancer subtype overall (~40% of lung cancers) - KRAS mutations occur early in adenocarcinoma development - Peripheral lung location favors adenocarcinoma histology - KRAS activation drives mucin production and glandular differentiation ### Distinction from Other Molecular Drivers | Molecular Driver | Associated Histotype | Frequency | | --- | --- | --- | | EGFR mutations | Adenocarcinoma | 10–15% (higher in non-smokers, Asians) | | ALK translocation | Adenocarcinoma | 3–5% | | KRAS mutations | Adenocarcinoma | 30–40% | | TP53 mutations | All types (especially SCLC) | 50–90% | | RB loss | SCLC | 90% | **Warning:** ~~KRAS mutations are common in squamous cell carcinoma~~ — KRAS is much less frequent in squamous histology; squamous tumors more often have TP53 and SOX2 mutations. [cite:Robbins 10e Ch 7]