Lung Cancer

Lung cancer is a malignant neoplasm arising from the epithelial cells of the respiratory tract, representing the leading cause of cancer-related mortality worldwide with approximately 2.2 million new cases and 1.8 million deaths annually. The disease is stratified into two major histologic categories—non-small cell lung cancer (NSCLC) accounting for 85% of cases and small cell lung cancer (SCLC) accounting for 15%—with distinct pathobiologic features, treatment approaches, and prognoses. Lung cancer incidence peaks in the sixth to eighth decades of life with a slight male predominance, though female incidence has increased dramatically in recent decades due to smoking prevalence trends. The disease carries substantial morbidity and mortality because most patients (approximately 60-70%) present with advanced-stage disease (Stage III-IV), when curative intent treatment is no longer feasible. Mastery of lung cancer recognition, staging, molecular profiling, and multimodal treatment selection is essential for board examination success and represents a cornerstone competency in internal medicine and oncology practice, as therapeutic options have expanded exponentially with immunotherapy and targeted molecular agents over the past decade.

Lung cancer develops through a multistep process of malignant transformation driven by accumulation of genetic and epigenetic alterations in respiratory epithelial cells, with distinct molecular pathways characterizing different histologic subtypes and molecular phenotypes.

Carcinogen-Induced DNA Damage and Mutational Burden — Tobacco smoke and other carcinogens contain over 70 known carcinogenic chemicals including polycyclic aromatic hydrocarbons (PAHs), N-nitrosamines, and reactive oxygen species that directly bind to and damage DNA. These carcinogens undergo metabolic activation via cytochrome P450 enzymes (particularly CYP1A1 and CYP1B1) to form DNA-reactive metabolites that form bulky adducts with guanine residues. Accumulated unrepaired DNA damage leads to point mutations, deletions, and chromosomal rearrangements. The mutational burden in lung cancer is among the highest of any malignancy (average 8-10 mutations per megabase in smokers), with characteristic "smoking signatures" demonstrating specific transition mutations at CpG dinucleotides. This high mutational burden has important implications for immunotherapy efficacy, as increased neoantigen load correlates with improved checkpoint inhibitor response.

Oncogenic Driver Mutations and Signaling Pathway Activation — NSCLC development involves sequential activation of oncogenes and loss of tumor suppressor genes. The most common driver mutations include: (1) KRAS mutations (present in 25-35% of adenocarcinomas, particularly in smokers), which constitutively activate RAS/MAPK and PI3K/AKT signaling pathways promoting cell proliferation and survival; (2) EGFR mutations (present in 10-40% of adenocarcinomas, with higher prevalence in never-smokers and East Asian populations), typically exon 19 deletions or L858R point mutations in the tyrosine kinase domain that cause ligand-independent receptor autophosphorylation; (3) ALK rearrangements (4-6% of adenocarcinomas), where fusion proteins like EML4-ALK activate aberrant kinase signaling; (4) ROS1 rearrangements (1-2% of adenocarcinomas) producing constitutively active fusion tyrosine kinases; and (5) BRAF mutations (2-4% of cases), predominantly V600E mutations that activate the MAPK cascade. These mutations are largely mutually exclusive and define specific molecular subtypes with distinct treatment vulnerabilities.

Tumor Suppressor Gene Loss and Genomic Instability — Inactivation of critical tumor suppressor genes drives malignant transformation. TP53 mutations occur in approximately 50% of lung cancers and are associated with loss of cell cycle checkpoint control, impaired apoptosis, and genomic instability. PTEN loss and STK11 (LKB1) mutations promote metabolic reprogramming and immune evasion. RB1 loss removes G1/S checkpoint control, particularly important in SCLC where RB1 and TP53 co-inactivation is near-universal. CDKN2A/p16 inactivation through deletion or methylation abrogates both RB and p53 pathways. Loss of these suppressors combines with activated oncogenes to fully transform epithelial cells, enabling unrestricted proliferation, resistance to differentiation, and escape from apoptotic signals.

Angiogenesis and Hypoxic Signaling — Growing lung tumors require neovascularization to supply nutrients and oxygen. Hypoxic tumor microenvironments induce hypoxia-inducible factor (HIF)-1α stabilization, which transcriptionally activates vascular endothelial growth factor (VEGF) and other pro-angiogenic factors. VEGF-VEGFR signaling on endothelial cells promotes vessel sprouting and normalization of the tumor vasculature. Paradoxically, angiogenesis often results in abnormal, leaky vessels that contribute to tumor microenvironment heterogeneity with regions of severe hypoxia that select for aggressive clones with enhanced metabolic flexibility and metastatic potential.

Epithelial-Mesenchymal Transition (EMT) and Invasion-Metastasis Cascade — Advanced lung cancers frequently activate EMT programs coordinated by transcription factors including SNAIL, SLUG, TWIST, and ZEB1. EMT reduces expression of the cell adhesion molecule E-cadherin while inducing mesenchymal markers (vimentin, N-cadherin, fibronectin) and matrix metalloproteinases (MMPs). These changes confer enhanced migratory and invasive capacity, resistance to anoikis, and increased circulating tumor cell production. EMT-activated cancer cells preferentially colonize distant organs through a process of organ-specific homing mediated by expressed surface receptors and chemotactic gradients. Partial EMT states (hybrid EMT/epithelial phenotypes) appear particularly associated with metastatic competence and therapeutic resistance.

Immune Evasion and Immunoediting — Developing lung tumors accumulate mutations that reduce immunogenicity and evade host immune surveillance. Key mechanisms include: (1) PD-L1 expression on tumor cells and infiltrating immune cells, which engages PD-1 on T cells to suppress anti-tumor immunity; (2) microsatellite instability (MSI) in a minority of lung cancers causing mismatch repair deficiency and very high neoantigen loads; (3) tumor microenvironment immunosuppression through recruitment of regulatory T cells and myeloid-derived suppressor cells (MDSCs) mediated by IL-10, TGF-β, and other immunosuppressive cytokines; and (4) loss of MHC expression preventing antigen presentation. These mechanisms collectively create an immunologically cold tumor microenvironment that permits tumor progression despite retained anti-tumor immunity in host lymphoid tissues.

Tobacco Smoke Exposure — Cigarette smoking remains the single most important risk factor for lung cancer, responsible for approximately 85% of NSCLC and 95% of SCLC cases. The dose-response relationship between cumulative smoking exposure (quantified as pack-years: number of packs per day × years smoked) and lung cancer risk is well-established, with relative risk increasing approximately 15- to 30-fold in heavy smokers compared to never-smokers. Former smokers retain elevated risk for many years after cessation, with risk declining but remaining significantly elevated even 10-20 years after quitting. The carcinogenic potential of mainstream smoke (inhaled by the smoker) far exceeds that of secondhand smoke, though secondhand smoke exposure in never-smokers increases risk approximately 1.5-fold. Cigar and pipe smoking carry risk similar to cigarettes, while smokeless tobacco (chewing tobacco, snuff) is associated with increased risk of oral cancers rather than lung cancer.

Environmental Exposures and Occupational Hazards — Asbestos exposure, particularly in construction workers, shipyard workers, and brake mechanics, increases lung cancer risk approximately 4- to 10-fold, with a latency period of 20-50 years. Risk is further amplified in asbestos-exposed workers who also smoke (multiplicative rather than additive interaction). Radon gas, a naturally occurring radioactive element produced by uranium decay in soil and building materials, infiltrates basements and lower levels of buildings; chronic radon exposure at elevated concentrations (>4 pCi/L) carries epidemiologic evidence for increased lung cancer risk. Crystalline silica exposure in miners, sandblasters, and foundry workers increases risk of silicosis and lung cancer. Additional occupational carcinogens include arsenic (in smelters and pesticide manufacturers), chromium and nickel (in metal production), and polycyclic aromatic hydrocarbons (in coke production). Environmental air pollution, particularly fine particulate matter (PM2.5) and ozone, demonstrates consistent epidemiologic associations with increased lung cancer risk in large cohort studies.

Prior History of Lung Disease and Inflammation — Chronic obstructive pulmonary disease (COPD) confers increased lung cancer risk independent of smoking status, possibly through chronic airway inflammation, oxidative stress, and altered epithelial repair mechanisms. Idiopathic pulmonary fibrosis (IPF) significantly increases lung cancer risk (approximately 10- to 15-fold), with adenocarcinoma the predominant histology, occurring particularly in areas of honeycombing and active fibrosis. The fibrotic microenvironment may promote malignant transformation through TGF-β signaling and chronic injury-repair cycles. Tuberculosis and prior history of pneumonia carry modestly elevated lung cancer risk.

Genetic Predisposition and Familial Lung Cancer — Family history of lung cancer in first-degree relatives increases personal risk approximately 1.5- to 2-fold even after adjusting for shared smoking exposure, suggesting hereditary susceptibility. Rare inherited mutations in EGFR, KRAS, and other oncogenes have been identified in familial lung cancer clusters. Polymorphisms in xenobiotic metabolism genes (CYP1A1, GSTM1, NAT2) modulate individual carcinogen susceptibility, though clinical utility for risk stratification remains limited. Lynch syndrome (mismatch repair gene mutations) carries modestly elevated lung cancer risk as part of a broader cancer spectrum.

Never-Smoker Characteristics and Adenocarcinoma Predominance — Approximately 15-20% of lung cancer patients are never-smokers (lifetime smoking exposure <100 cigarettes). Never-smoker lung cancers demonstrate distinct epidemiologic and molecular features: (1) predominance of adenocarcinoma histology (>90% of cases), particularly in East Asian populations; (2) higher frequency of EGFR mutations (40-50% versus 10-15% in smokers) and ALK rearrangements; (3) female predominance; and (4) improved prognosis, potentially related to better performance status and higher likelihood of actionable driver mutations.

Hormonal Factors — Post-menopausal hormone replacement therapy, particularly estrogen-only regimens, demonstrates modest associations with increased lung cancer risk in some studies, possibly through activation of estrogen receptor signaling pathways in lung epithelium. The mechanisms underlying the female predominance in never-smoker lung cancers remain incompletely understood but may involve sex hormone signaling.

The clinical presentation of lung cancer varies dramatically based on tumor stage, location, histologic type, and presence of paraneoplastic syndromes, with many patients remaining asymptomatic until advanced disease develops.

Primary Tumor-Related Symptoms — Cough represents the most common presenting symptom, occurring in 50-75% of patients with central or proximal tumors. The cough may be persistent (>2-3 weeks), non-productive, or productive of hemoptysis. The physiologic basis involves direct irritation of the airway mucosa by tumor and/or associated inflammation. Chest pain develops in 20-40% of patients, usually pleuritic in character (worse with deep breathing or coughing) when the tumor involves the pleura, or dull and aching when mediastinal structures are invaded. Hemoptysis, the coughing up of blood or blood-tinged sputum, occurs in 10-30% of cases, typically reflecting tumor erosion into pulmonary vasculature or endobronchial tumor necrosis. While hemoptysis is often alarming to patients and prompts medical evaluation, it is relatively non-specific; however, new-onset hemoptysis in a smoker should raise high suspicion for malignancy. Dyspnea develops in 25% of patients through mechanisms including airway obstruction, pleural effusion, pneumonia distal to airway obstruction, or pulmonary embolism (patients with cancer have hypercoagulability). Wheezing may indicate partial airway obstruction by endobronchial tumor. Recurrent or post-obstructive pneumonia develops when a tumor obstructs an airway, preventing clearance of secretions distal to the obstruction; this pattern—recurrent pneumonia in the same lung region—should raise suspicion for underlying malignancy.

Symptoms from Local Tumor Invasion and Spread — Superior vena cava (SVC) syndrome develops when tumors in the right upper lobe or mediastinum obstruct venous return, causing facial plethora, upper extremity edema, neck vein distention, and dyspnea; this represents a medical emergency requiring urgent intervention. Pancoast syndrome (superior sulcus tumors) involves invasion of the brachial plexus, subclavian vessels, and lower cervical sympathetic chain, classically presenting with (1) severe shoulder and arm pain (C8 and T1 distribution) due to brachial plexus invasion, (2) Horner's syndrome (miosis, ptosis, anhidrosis, facial flush) from lower cervical and upper thoracic sympathetic chain involvement, and (3) hand muscle atrophy from T1 nerve root involvement; this distinctive syndrome is highly associated with superior sulcus lung cancer. Recurrent laryngeal nerve invasion (tumors in the left mainstem bronchus or mediastinum) causes vocal cord paralysis with hoarseness and weak voice. Esophageal invasion may cause dysphagia. Cardiac and pericardial involvement can produce atrial arrhythmias, pericardial effusion, and tamponade. Spinal cord compression from vertebral body or epidural metastases presents acutely with severe back pain, progressive weakness, numbness, and bowel/bladder dysfunction, requiring emergency MRI and intervention.

Systemic Symptoms — Constitutional symptoms including fatigue, malaise, and unintentional weight loss develop in 10-68% of patients, reflecting the systemic effects of malignancy including altered metabolism, cytokine production, and nutritional depletion. Fever can occur through production of pyogenic cytokines, particularly in necrotic tumors or those with associated post-obstructive pneumonia.

Paraneoplastic Syndromes — These represent clinical manifestations arising from systemic effects of malignancy rather than direct tumor invasion or metastases. Syndrome of inappropriate antidiuretic hormone (SIADH) occurs in 10-15% of SCLC and 1-2% of NSCLC, causing hyponatremia, osmotic demyelination, seizures, and altered mental status; SCLC cells produce ectopic ADH. Lambert-Eaton myasthenic syndrome (LEMS) develops in 2-3% of SCLC through autoimmune attack on presynaptic voltage-gated calcium channels; patients present with proximal leg weakness that improves with effort ("warm-up phenomenon"), depressed tendon reflexes, and autonomic dysfunction (dry mouth, constipation, impotence). Paraneoplastic encephalomyelitis presents with progressive cognitive decline, ataxia, or myelopathy in association with anti-neuronal antibodies (anti-Hu, anti-Yo). Paraneoplastic pemphigus (autoimmune blistering disease) presents with severe stomatitis and mucocutaneous blistering. Hypertrophic osteoarthropathy presents with periosteal new bone formation causing severe arthralgia and swelling of joints, particularly wrists and ankles; patients have characteristic "clubbing" of fingernails with loss of the normal nail angle. Hypercalcemia develops through PTHrP (parathyroid hormone-related peptide) secretion by squamous cell carcinoma (most common), occurring in 5% of lung cancer patients; presents with polyuria, polydipsia, confusion, and cardiac arrhythmias. Ectopic ACTH secretion by SCLC causes severe hypokalemia and metabolic alkalosis. Thrombocytosis and hypercoagulability increase risk of venous thromboem