Lung Cancer — NSCLC and SCLC

• Carcinogenic exposure and initial DNA damage: Cigarette smoke exposure (or in never-smokers, environmental factors such as radon, asbestos, or secondhand smoke) introduces over 70 known carcinogens including polycyclic aromatic hydrocarbons (PAHs), nitrosamines, and free radicals. These agents undergo metabolic activation by cytochrome P450 enzymes (particularly CYP1A1) in bronchial epithelial cells, generating DNA-reactive electrophilic intermediates. Direct covalent binding to DNA bases creates bulky DNA adducts that, if not repaired by nucleotide excision repair mechanisms, result in permanent mutations. The primary lesion occurs in basal stem cells of the bronchial epithelium, which have high replicative capacity and reduced apoptotic sensitivity—a critical first "hit" in Knudson's two-hit hypothesis. Cumulative dose-response correlation (quantified in "pack-years" of smoking) demonstrates the linear relationship between carcinogen exposure and cancer incidence.

• Activation of oncogenic pathways (NSCLC molecular drivers): NSCLC tumorigenesis involves constitutive activation of growth-promoting pathways through several mechanisms. EGFR mutations (exon 19 deletions and L858R point mutations in exon 21) occur in 10-40% of adenocarcinomas (higher in never-smokers and Asian populations) and result in ligand-independent autophosphorylation of the receptor tyrosine kinase, driving continuous proliferation through PI3K/AKT/mTOR and MAPK/ERK pathways. KRAS mutations (primarily G12C, G12V, G12D) activate downstream effectors including RAF-MEK-ERK signaling and represent the most common mutation in smoking-associated adenocarcinomas (25-30% of cases). ALK rearrangements (echinoderm microtubule-associated protein-like 4 [EML4]-ALK fusion) result in aberrant tyrosine kinase activity and predominate in young, never-smoking females with adenocarcinoma histology. ROS1 rearrangements, MET exon 14 skipping mutations, and BRAF mutations activate parallel oncogenic cascades. These mutations are "driver mutations" conferring both growth advantage and—critically—sensitivity to specific targeted therapies, explaining superior outcomes with matched kinase inhibitors compared to chemotherapy.

• Inactivation of tumor suppressor pathways: Loss of function of negative regulators permits uncontrolled proliferation. TP53 mutations (the most common genetic alteration in lung cancer, present in 50-70% of cases) eliminate p53-mediated G1/S checkpoint control and apoptosis, allowing cells with severe DNA damage to continue dividing. PTEN loss removes inhibition of PI3K/AKT signaling, promoting survival. RB pathway inactivation (through RB1 mutations or CDK4/6 overexpression) permits uncontrolled G1/S transition. LKB1 mutations (STK11) are strongly associated with KRAS-mutant adenocarcinomas and confer immune-cold phenotype with reduced T-cell infiltration and poor immunotherapy response. The loss of p16 expression through CDKN2A deletions further disrupts cell cycle control. Importantly, SCLC is characterized by near-universal inactivation of both TP53 and RB (>90% of cases), explaining the particularly aggressive biology and early propensity for widespread dissemination.

• Immune evasion and microenvironment remodeling: Successful lung cancers evolve mechanisms to evade immune destruction. PD-L1 expression on tumor cells and infiltrating immune cells causes T-cell exhaustion through PD-1 engagement, a central mechanism preventing anti-tumor immune responses. Distinct immune phenotypes exist: "inflamed" tumors with high PD-L1, CD8+ T-cell infiltration, and high tumor mutational burden (TMB) respond well to checkpoint inhibition, while "cold" tumors with low immune infiltration show limited immunotherapy benefit. TGF-β signaling within the tumor microenvironment promotes fibroblast activation and immune suppression. Tumor-associated macrophages (TAMs) are recruited through CSF-1/IL-34 signaling and provide pro-growth and immunosuppressive signals. Angiogenesis is stimulated by VEGF signaling, creating hypoxic niches that further promote immune evasion and stemness. The stromal compartment (cancer-associated fibroblasts, immune cells, vasculature, and extracellular matrix) can comprise up to 50% of tumor volume and actively supports tumor growth—this is particularly relevant for understanding the limited single-agent activity of certain therapies and the rationale for combination approaches.

• Epithelial-mesenchymal transition (EMT) and metastatic dissemination: Acquisition of mesenchymal characteristics through EMT enables invasion and metastasis. Transcription factors including Snail, Slug, Twist, and Zeb1 suppress epithelial markers (E-cadherin, claudins) and induce mesenchymal markers (N-cadherin, vimentin, fibronectin). TGF-β signaling and Wnt/β-catenin pathways drive EMT. Loss of E-cadherin-mediated cell-cell adhesion permits detachment from the primary tumor. Migratory cells invade through basement membrane (assisted by matrix metalloproteinases) into lymphatic and blood vessels. Circulating tumor cells (CTCs) survive through activation of anti-apoptotic pathways and interactions with platelets that provide a protective cloak from immune destruction. Once arrested in distant organs, cells undergo mesenchymal-epithelial transition (MET) to reestablish epithelial characteristics at metastatic sites. SCLC exhibits a particularly high propensity for hematogenous dissemination, with 60-70% of patients having distant metastases at presentation—a reflection of early invasion through the visceral pleura (which is commonly involved) and rapid progression through the blood-brain barrier.

• Neuroendocrine differentiation in SCLC: SCLC arises from pulmonary neuroendocrine cells (Kulchitsky cells) in the basal layer of bronchial epithelium. The malignant transformation maintains neuroendocrine features, including dense-core neurosecretory granules (visible on electron microscopy), expression of chromogranin A, synaptophysin, and CD56, and production of neuropeptides including calcitonin, gastrin-releasing peptide (GRP), and others. This neuroendocrine phenotype contributes to SCLC's aggressive behavior (high proliferation rate with Ki-67 >70%), early lymph node and distant involvement, and predisposition to paraneoplastic syndromes including syndrome of inappropriate ADH (SIADH), Cushing syndrome, and Lambert-Eaton myasthenic syndrome. The rapid doubling time (30-40 days compared to 90+ days for NSCLC) and chemosensitivity reflect the primitive neuroendocrine biology. Recent genomic studies have identified SCLC subtypes based on transcriptional profiles (SCLC-A, SCLC-N, SCLC-P, SCLC-I) with varying degrees of neuroendocrine differentiation and immune infiltration, though these classifications are not yet standard for clinical decision-making.

• Cigarette smoking: Responsible for 85-90% of lung cancer cases, with risk increasing with duration, intensity (number of cigarettes per day), and degree of inhalation. The relationship is dose-dependent, with risk increasing 15-20 fold in heavy smokers compared to never-smokers. Carcinogenic compounds include PAHs, nitrosamines (particularly tobacco-specific nitrosamines like NNK), and free radicals generated during combustion. Even after smoking cessation, risk remains elevated for 10+ years, though the slope of risk decline accelerates over time. Former smokers account for a growing proportion of lung cancers as cessation rates increase but the legacy burden of prior exposures persists. Secondhand smoke exposure (environmental tobacco smoke) increases lung cancer risk by 20-30% in never-smokers, with a dose-response relationship to cumulative exposure.

• Genetic predisposition and never-smoker adenocarcinomas: A subset of never-smokers develop lung cancer through constitutional genetic factors or acquiredoncogenic mutations in epithelial cells without smoking exposure. EGFR mutations are enriched in this population (40-50% of adenocarcinomas in never-smokers vs. 10-15% in smokers), as are ALK rearrangements. Familial lung cancer occurs in 5-10% of cases, with genetic studies identifying germline TP53 mutations (Li-Fraumeni syndrome), BRCA1/2 mutations, STK11 mutations (Peutz-Jeghers syndrome), and others conveying familial predisposition. Genome-wide association studies (GWAS) have identified common variants in FAM36A, TERT, TP63, and other loci associated with modest increased risk even in never-smokers. Metabolic factors including polymorphisms in CYP1A1, CYP2D6, GSTM1 (glutathione S-transferase M1), and NAT2 (N-acetyltransferase 2) affecting carcinogen metabolism modulate individual susceptibility, though clinical utility is limited.

• Occupational and environmental exposures: Asbestos exposure (particularly in shipyard workers, insulators, miners, and construction workers) increases lung cancer risk 4-5 fold, with a 15-40 year latency period; the combination of asbestos with smoking has supra-additive (synergistic) effects. Radon gas (a naturally occurring radioactive decay product of uranium in soil) is the second leading cause of lung cancer after smoking, with risk increasing with prolonged basement/underground exposure; radon-related lung cancers occur predominantly in former or current smokers. Silica dust, beryllium, cadmium, and polycyclic aromatic hydrocarbons in occupational settings increase risk. Air pollution exposure, particularly fine particulate matter (PM2.5) and diesel exhaust, increases risk through chronic inflammation and direct genotoxic effects. Cooking oil fumes (particularly in Asian populations with wok cooking) may increase risk through oxidative stress.

• Pre-existing lung disease: Chronic obstructive pulmonary disease (COPD) increases lung cancer risk 2-4 fold independent of smoking status, likely through chronic inflammation, impaired epithelial repair, and oxidative stress. Idiopathic pulmonary fibrosis (IPF) confers a 5-10 fold increased risk, with the greatest risk in those with active fibrosis; the underlying mechanism involves epithelial injury, repetitive reactivation of fibrogenic pathways, and altered immune responses. Prior tuberculosis (particularly with cavitary disease) increases risk through chronic inflammation and scarring. Prior lung cancer in one site increases risk of second primary lung cancers through field cancerization (exposure of the entire bronchial tree to carcinogens) and shared genetic predispositions.

• Hormonal factors: Estrogen exposure may increase lung cancer risk in women, with observational studies showing association with postmenopausal hormone replacement therapy and oral contraceptive use; putative mechanisms include direct estrogenic signaling in lung adenocarcinoma cells (which express estrogen receptors at higher rates than in men) and immunosuppressive effects. This finding is particularly relevant for never-smoking women who develop adenocarcinoma.

• Cough: The most common presenting symptom, occurring in 50-75% of patients. A persistent cough lasting >3 weeks warrants chest imaging in smokers and former smokers. The cough typically arises from irritation of large airways by endobronchial lesions or from parenchymal involvement causing pleural irritation. A change in the character of a chronic smoker's baseline cough (deeper, more productive, or with hemoptysis) is a classic red flag. The mechanism reflects direct mucosal irritation, activation of cough receptors, and inflammatory mediators produced by the tumor.

• Hemoptysis: Occurs in 10-30% of patients and results from erosion of endobronchial tumors into small blood vessels. Hemoptysis is often blood-streaked sputum rather than massive hemorrhage, though massive hemoptysis (>30 mL per episode) can occur with cavitating or necrotic tumors. The presence of hemoptysis significantly increases the likelihood of lung cancer in a smoker with cough and should prompt urgent evaluation.

• Dyspnea: Present in 25-40% of patients, arising from multiple mechanisms including airway obstruction, parenchymal involvement with impaired gas exchange, pleural effusion, pericardial effusion, or superior vena cava syndrome. The dyspnea is often insidious in onset and progressive, particularly with central tumors causing airway narrowing.

• Chest pain: Occurs in 20-40% of patients and typically reflects pleural involvement (sharp, pleuritic pain worsened by breathing or coughing) or chest wall invasion. Bone pain results from osseous metastases, particularly to ribs and spine. Central tumors may cause mediastinal pain or shoulder pain if Pancoast tumors (superior sulcus lesions) involve the brachial plexus.

• Hoarseness: Results from recurrent laryngeal nerve involvement by tumors in the left hilum or mediastinum (the nerve descends around the aortic arch) and occurs in 2-5% of patients. Hoarseness persisting >3 weeks warrants laryngoscopy to exclude other etiologies.

• Paraneoplastic syndromes: SCLC in particular produces neuroendocrine mediators causing systemic manifestations:
• SIADH/hyponatremia (most common paraneoplastic endocrinopathy, occurring in 10-15% of SCLC): Results from ectopic secretion of vasopressin by neuroendocrine cells. Presents with confusion, seizures, or coma if severe (Na+ <125 mEq/L); subtle hyponatremia may be asymptomatic.
• Ectopic Cushing syndrome (ACTH-secreting, 2-3% of SCLC): Produces hypokalemia, hypertension, hyperglycemia, and proximal muscle weakness from rapid cortisol excess.
• Lambert-Eaton myasthenic syndrome (LEMS, 2-3% of SCLC): Autoimmune attack on voltage-gated calcium channels (VGCC) on presynaptic nerve terminals, causing proximal leg weakness that improves with continued effort (opposite of myasthenia gravis), areflexia, and autonomic dysfunction.

• Systemic symptoms: Constitutional symptoms including unintentional weight loss (>10% body weight), fever, night sweats, and fatigue occur in 20-40% and typically indicate advanced disease with systemic involvement. These symptoms reflect tumor necrosis, inflammatory mediators (IL-6, TNF-α), and metabolic derangements from cachexia.

• Symptoms from metastatic disease at presentation:
• Brain metastases (10-30% at diagnosis, particularly SCLC): Cause headache, focal neurologic deficits, seizures, or altered

Initial imaging

• Chest radiograph: first test in a smoker with persistent cough, hemoptysis, or weight loss. Always compare with prior films — for a solid nodule, stability for 2+ years or a benign calcification pattern (central, laminated, popcorn) needs no further workup. This rule does not apply to subsolid nodules: per the Fleischner Society, ground-glass and part-solid lesions require surveillance for at least 5 years, because indolent lepidic adenocarcinoma can appear unchanged for years.
• Contrast-enhanced CT of the chest through the adrenal glands: defines size, borders (spiculation, pleural tag), central versus peripheral location, cavitation, nodal enlargement, and adrenal/liver metastases. Incidental nodules are managed by the Fleischner Society criteria (size, solid vs subsolid, risk category); screen-detected nodules are reported with Lung-RADS.

Tissue diagnosis is the gold standard — biopsy the site that confers the highest stage

• Bronchoscopy with EBUS-guided transbronchial needle aspiration: preferred for central lesions and simultaneous mediastinal nodal staging.
• CT-guided transthoracic core needle biopsy: peripheral lesions; pneumothorax is the main risk.
• Thoracentesis with cytology: if an effusion is present, sample it first — malignant cytology establishes M1a disease and precludes curative resection.
• Mediastinoscopy: confirmatory when EBUS is negative but nodal disease is suspected.

Histology and immunohistochemistry

• Adenocarcinoma: peripheral, glandular/lepidic growth, TTF-1 and napsin A positive.
• Squamous cell carcinoma: central, keratin pearls and intercellular bridges, p40/p63 and CK5/6 positive, TTF-1 negative.
• SCLC: small cells with scant cytoplasm, nuclear molding, salt-and-pepper chromatin, crush artifact; synaptophysin, chromogranin, CD56 positive with Ki-67 typically >70%.

Staging

• Whole-body FDG PET-CT plus brain MRI with contrast (NCCN recommends brain imaging for all SCLC and for stage II or higher NSCLC).
• NSCLC and SCLC are both staged by AJCC TNM (8th edition); SCLC is additionally described by the VA classification — limited stage fits within one tolerable radiation port, extensive stage does not.

Immediate stabilization (before oncologic planning)

• Malignant spinal cord compression: corticosteroid (dexamethasone) immediately, emergent MRI of the whole spine, then radiation or decompressive surgery.
• Superior vena cava syndrome: elevate head, treat the tumor; endovascular stenting for airway/cerebral edema.
• Symptomatic brain metastases: dexamethasone; antiepileptics only if seizures occur.

Early-stage NSCLC (I–II)

• Lobectomy with mediastinal lymph node dissection is the definitive standard (NCCN); sublobar resection is acceptable for selected small peripheral tumors.
• Stereotactic body radiotherapy (SBRT) for medically inoperable stage I disease.
• Preoperative physiologic assessment per ACCP: spirometry and DLCO with predicted postoperative values — markedly reduced predicted postoperative FEV1 or DLCO signals prohibitive surgical risk.
• Adjuvant platinum doublet chemotherapy (cisplatin-based) for resected stage II–IIIA; adjuvant osimertinib after resection of EGFR-mutant disease (ADAURA). Neoadjuvant chemoimmunotherapy is now standard for selected resectable stage II–III disease.

Metastatic NSCLC — driver mutation dictates first-line therapy (NCCN/ASCO)

• EGFR mutation: third-generation TKI, osimertinib.
• ALK rearrangement: alectinib or another next-generation ALK TKI (CNS-penetrant).
• ROS1: entrectinib or crizotinib. BRAF V600E: dabrafenib plus trametinib.
• KRAS G12C: not a first-line targeted indication — treat first-line exactly as driver-negative disease (chemo-immunotherapy); sotorasib or adagrasib is reserved for subsequent-line therapy after progression on platinum chemotherapy and/or immunotherapy.
• No driver, PD-L1 high: single-agent pembrolizumab; otherwise platinum doublet plus pembrolizumab.

SCLC (NCCN)

• Limited stage: concurrent platinum plus etoposide with thoracic radiotherapy, followed by consolidation durvalumab in patients without progression (ADRIATIC) — the SCLC parallel of the PACIFIC paradigm; surgery only for the rare T1–2N0 nodule.
• Extensive stage: platinum/etoposide plus a PD-L1 inhibitor (atezolizumab or durvalumab).
• Prophylactic cranial irradiation is offered to responders, with MRI surveillance an accepted alternative.

Contraindicated/avoid

• Bevacizumab in squamous histology — cavitation and fatal hemoptysis; also avoid with recent hemoptysis.
• Pemetrexed in squamous histology (ineffective).
• Checkpoint inhibitors with caution in active autoimmune disease or solid-organ transplant.
• Smoking cessation counseling is indicated at every visit.

Oncologic emergencies

• Superior vena cava syndrome: central tumor or bulky right paratracheal nodes obstruct venous return — facial/upper-extremity swelling, distended neck and chest wall veins, worse on bending forward. Stridor or altered mental status makes it a true emergency.
• Malignant spinal cord compression: vertebral metastasis with epidural extension — progressive back pain worse when supine, then weakness, sensory level, and late bowel/bladder dysfunction. Steroids and MRI before neurologic deficit becomes fixed.
• Massive hemoptysis: tumor erosion into a bronchial artery — position the bleeding lung dependent, secure the airway, then bronchial artery embolization. Death is from asphyxiation, not exsanguination.
• Hypercalcemia of malignancy: PTHrP from squamous cell carcinoma (also lytic bone metastases) — confusion, constipation, polyuria, short QT. Treat with isotonic saline, then a bisphosphonate or denosumab; calcitonin for rapid bridging.
• Febrile neutropenia: chemotherapy-induced marrow suppression — empiric antipseudomonal beta-lactam (cefepime) within one hour per IDSA.

Disease-related

• Malignant pleural effusion: pleural seeding; exudative, often bloody — indicates M1a and unresectability. Indwelling catheter or pleurodesis for recurrence.
• Post-obstructive pneumonia and lobar collapse: endobronchial obstruction with impaired mucus clearance; recurrent pneumonia in the same lobe in a smoker is cancer until proven otherwise.
• Pancoast (superior sulcus) tumor: invades the sympathetic chain and lower brachial plexus — Horner syndrome (ptosis, miosis, anhidrosis) with C8–T1 arm pain and hand wasting.
• Hypertrophic pulmonary osteoarthropathy: digital clubbing with painful periostitis of long bones, classically adenocarcinoma.
• Venous thromboembolism: tumor-associated tissue factor and a hypercoagulable state.

Treatment-related

• Cisplatin: nephrotoxicity, ototoxicity, peripheral neuropathy, severe emesis — requires aggressive hydration and antiemetics.
• Radiation pneumonitis: dyspnea and cough weeks to months after thoracic RT with ground-glass opacity confined to the radiation port; corticosteroids. Radiation esophagitis causes odynophagia during treatment.
• Immune-related adverse events (checkpoint inhibitors): loss of peripheral tolerance produces organ-specific autoimmunity. Management differs by organ, per ASCO guidance:
• Organ-inflammatory irAEs (pneumonitis, colitis, hepatitis, nephritis, myocarditis): hold the drug and give corticosteroids, escalating to infliximab or other immunosuppression if steroid-refractory. Pneumonitis and colitis with perforation are emergencies.
• Endocrine irAEs (thyroiditis, hypophysitis): generally treated with hormone replacement — levothyroxine, physiologic hydrocortisone — with continuation of the checkpoint inhibitor; high-dose steroids are reserved for mass effect or adrenal crisis. Always replace glucocorticoid before thyroid hormone in suspected hypophysitis.
• EGFR/ALK TKIs: acneiform rash and diarrhea; interstitial lung disease is rare but potentially fatal.

• Location predicts histology: squamous cell and small cell are central (hilar, endobronchial); adenocarcinoma and large cell are peripheral. Squamous cavitates; adenocarcinoma is the most common subtype overall and in never-smokers.
• Paraneoplastic pairings are the single most tested association: squamous → PTHrP hypercalcemia; small cell → SIADH, ectopic ACTH/Cushing, and Lambert-Eaton (proximal weakness that improves with repeated use, areflexia, autonomic symptoms — the mirror image of myasthenia gravis, which worsens with use and responds to edrophonium).
• Solitary pulmonary nodule — best next step is to obtain prior imaging. For a solid nodule, two years of stability or benign calcification ends the workup; a subsolid (ground-glass or part-solid) nodule needs at least 5 years of surveillance per Fleischner Society principles, since lepidic adenocarcinoma grows indolently. A growing or spiculated nodule goes to CT then tissue.
• New pleural effusion in known or suspected lung cancer → thoracentesis with cytology. Positive cytology equals M1a: the patient is no longer a surgical candidate. Do not send them to lobectomy.
• Pancoast tumor = shoulder/ulnar arm pain plus Horner syndrome; hoarseness means left recurrent laryngeal nerve involvement, and hemidiaphragm elevation means phrenic nerve involvement — all signs of local invasion, not automatically metastasis.
• SCLC is not a surgical disease. It is exquisitely chemo- and radiosensitive but relapses quickly; treat limited stage with concurrent platinum/etoposide plus thoracic radiation followed by consolidation durvalumab in non-progressors, and extensive stage with platinum/etoposide plus a PD-L1 inhibitor (NCCN).
• Common distractors: bevacizumab is contraindicated in squamous histology (fatal hemoptysis) and pemetrexed does not work there; TTF-1 does not distinguish adenocarcinoma from SCLC (both can be positive) — use p40 for squamous and synaptophysin/chromogranin/CD56 for small cell. KRAS G12C is not a first-line targeted target — sotorasib/adagrasib come after progression on chemo-immunotherapy.
• Screening stem: a 55-year-old with a 30 pack-year history who quit 5 years ago gets annual low-dose CT, not chest radiography and not sputum cytology (USPSTF grade B).