Lung Cancer Pathology — Adenocarcinoma, SCC, SCLC, Large Cell

Molecular Carcinogenesis and Driver Mutations

• KRAS mutations: Present in ~30% of adenocarcinomas (especially in smokers) and 10% of SCCs; activate RAF/MEK/ERK and PI3K/AKT pathways; generally confer worse prognosis and chemoresistance
• EGFR mutations: Found in ~20% of adenocarcinomas (higher in never-smokers and Asian populations); activate MAPK and PI3K signaling; tyrosine kinase inhibitors (TKIs) provide durable responses
• ALK translocations: Present in ~5% of adenocarcinomas (rarely in other histotypes); create constitutively active fusion kinase; ALK inhibitors are highly effective
• ROS1 fusions: Found in ~1-2% of adenocarcinomas; similar clinical benefit from ROS1 inhibitors as ALK inhibitors
• TP53 mutations: Nearly universal in SCLC (~90%); common in SCC (~80%) and ADC (~50%); loss of p53 function eliminates G1/S checkpoint control and apoptosis, promoting aggressive behavior and chemoresistance
• RB1 (retinoblastoma) inactivation: Nearly universal in SCLC (~90%); loss of Rb removes E2F repression, driving S-phase progression
• PTEN loss: Frequent in SCLC and contributes to PI3K pathway activation
• PD-L1 expression: Upregulated in many lung cancers; drives immune evasion and predicts response to checkpoint inhibitors

Cellular and Morphological Transformation Pathways

• Adenocarcinoma: Arises from mucin-secreting Clara cells (Club cells) and type II pneumocytes in the peripheral airways and alveoli; characterized by formation of glandular structures, mucinous differentiation, and/or lepidic (alveolar) growth patterns; peripheral location correlates with earlier symptoms related to pleural involvement
• Squamous cell carcinoma: Arises from metaplastic squamous epithelium of proximal airways (mainstem and lobar bronchi) following chronic irritation; progresses through dysplasia (low-grade and high-grade) → carcinoma in situ → invasive carcinoma; central location predisposes to airway obstruction and hemoptysis
• Small cell lung cancer: Arises from neuroendocrine cells (Kulchitsky cells) in the proximal airways; tumors consist of densely packed small cells with scant cytoplasm, finely dispersed chromatin ("salt-and-pepper" pattern), and numerous mitoses; extremely rapid cell division cycle (doubling time 25-75 days); early extensive hematogenous and lymphatic dissemination; neuroendocrine differentiation confers expression of dense-core neurosecretory granules and neuroendocrine markers (synaptophysin, chromogranin, CD56)
• Large cell carcinoma: High-grade malignancy with large, polygonal cells, vesicular nuclei, prominent nucleoli, abundant cytoplasm; lacks specific differentiation features (glandular, squamous, or neuroendocrine); may represent poorly differentiated adenocarcinoma or squamous carcinoma, or primary large cell undifferentiated carcinoma; often associated with neuroendocrine features (now reclassified as large cell neuroendocrine carcinoma—LCNEC)

Epithelial-Mesenchymal Transition (EMT) and Metastatic Potential

• Downregulation of E-cadherin and upregulation of N-cadherin facilitate local invasion and distant dissemination
• Expression of vimentin, fibronectin, and matrix metalloproteinases (MMPs) promote stromal invasion and angiogenesis
• SCLC exhibits the most aggressive EMT phenotype and metastatic potential, with >95% of patients having distant metastases at presentation

Angiogenesis and Tumor Microenvironment

• Upregulation of VEGF (vascular endothelial growth factor) drives tumor neovascularization
• Recruitment of cancer-associated fibroblasts (CAFs) and immune cells creates a permissive microenvironment
• PD-L1+ tumor cells evade T-cell-mediated immunity through PD-1/PD-L1 axis engagement

Major Risk Factors

Tobacco Smoke (Active and Passive)

• Most significant risk factor for all lung cancer histotypes, particularly SCLC and SCC
• Polycyclic aromatic hydrocarbons (PAHs) and nitrosamines are metabolized to carcinogenic intermediates (diol epoxides) that form DNA adducts
• Dose-dependent relationship: lifetime smoking burden (pack-years) correlates with cancer risk
• SCC and SCLC are rare in never-smokers; ADC is increasingly common in never-smokers

Occupational and Environmental Exposures

• Asbestos: Occupational exposure in construction, shipbuilding, mining; risk increased with concurrent smoking
• Radon: Naturally occurring radioactive gas; second-leading preventable cause of lung cancer after smoking
• Crystalline silica: Associated with silicosis and increased lung cancer risk
• Chromium, nickel, beryllium, arsenic: Industrial carcinogens
• Indoor and outdoor air pollution: Particulate matter and diesel exhaust increase risk in never-smokers

Genetic and Host Factors

• Family history of lung cancer: Suggests inherited predisposition or shared environmental exposures
• CHRNA5 polymorphisms: Influence nicotine dependence and lung cancer susceptibility
• TERT and CLPTM1L variants: Associated with increased lung cancer risk in GWAS studies
• Prior lung disease: COPD, idiopathic pulmonary fibrosis (IPF), tuberculosis history increase risk

Adenocarcinoma-Specific Risk Factors

• Never-smoker status: Paradoxically, adenocarcinoma is most common in never-smokers
• Estrogen exposure: Female sex and hormone replacement therapy linked to increased ADC risk (unclear mechanism)
• Oncogenic driver mutations (EGFR, ALK, ROS1): Often occur in never-smokers and drive tumorigenesis independently

SCLC-Specific Risk Factors

• Heavy smoking history: Nearly universal; median 40+ pack-year exposure
• Neuroendocrine differentiation: Arises from neuroendocrine precursor cells predisposed to malignant transformation

Pulmonary Symptoms (Histotype-Dependent)

Early-Stage Disease (Often Asymptomatic)

• Many tumors detected incidentally on imaging (CT screening)
• Peripheral adenocarcinomas and large cell carcinomas may remain silent until pleural involvement or distant spread

Central Airway Obstruction (SCC, SCLC > ADC)

• Persistent cough: Paroxysmal, often productive; SCC involving mainstem/lobar bronchi causes obstruction
• Hemoptysis: Blood-tinged or frank hemoptysis; more common in SCC and SCLC due to central necrotic masses
• Wheeze or stridor: Focal airway narrowing from endobronchial tumor
• Dyspnea: Obstructive airway disease or postobstructive atelectasis

Peripheral Airway/Parenchymal Involvement (ADC)

• Chest wall or shoulder pain: Pancoast tumors (superior sulcus) causing C8 and T1 nerve root invasion; may present with Horner syndrome (miosis, ptosis, anhidrosis)
• Pleural effusion: Exudative fluid with malignant cells; causes dyspnea and pleuritic chest pain
• Pleural thickening and pain: From direct pleural invasion

Systemic and Paraneoplastic Manifestations

• Syndrome of inappropriate antidiuretic hormone (SIADH): Most common; leads to hyponatremia, confusion, seizures
• Lambert-Eaton myasthenic syndrome (LEMS): 3% of SCLC; antibodies against voltage-gated calcium channels (P/Q-type) cause proximal muscle weakness, autonomic dysfunction, and diminished reflexes; antibody testing (anti-VGCC) diagnostic
• Paraneoplastic encephalitis: Antibodies to neuronal antigens (Hu antigen/CRMP5, Yo antigen); causes cognitive decline, ataxia, seizures
• Cushing syndrome: ACTH production by tumor cells causing hypokalemia, hypertension, hyperglycemia, metabolic alkalosis
• Hypertrophic osteoarthropathy: Clubbing and periosteal bone proliferation (rare, more common in ADC)

SCC-Associated Manifestations

• Hypercalcemia: PTHrP (parathyroid hormone-related peptide) secretion; more common in SCC than other histotypes; causes polyuria, polydipsia, confusion, cardiac arrhythmias
• Squamous cell carcinoma antigen (SCC-Ag): May be elevated; correlates with tumor burden

ADC-Associated Paraneoplastic Phenomena

• Hypertrophic osteoarthropathy: Digital clubbing with periosteal new bone formation along long bones; painful and tender; often improves with anti-PD-L1 therapy
• Constitutional symptoms: Fever, weight loss, fatigue (less prominent than in SCLC)

Physical Examination Findings

• Decreased breath sounds over affected lobe or hemithorax (atelectasis, pleural effusion)
• Bronchial breath sounds (consolidated lung from post-obstructive pneumonia)
• Dullness to percussion and decreased tactile fremitus (pleural effusion)
• Horner syndrome: Miosis, ptosis, anhidrosis, enophthalmos (Pancoast tumor with T1 involvement)
• Supraclavicular or cervical lymphadenopathy: Metastatic involvement
• Weight loss and cachexia: Particularly prominent in SCLC and advanced ADC

Constitutional Symptoms

• Unintentional weight loss: Often >10 lbs; reflects tumor metabolic demands and cytokine-mediated effects
• Fatigue and malaise: From anemia (paraneoplastic or chemotherapy-related) and systemic illness
• Fever: May indicate post-obstructive pneumonia or tumor necrosis

Histological Findings and Classification

• Lepidic (formerly bronchioloalveolar): Growth along alveolar septa with single-layer of cuboidal/columnar cells WITHOUT stromal invasion; represents early/non-invasive disease; favorable prognosis
• Acinar: Well-formed glandular structures with central lumens lined by cuboidal cells; mucin may fill glandular spaces
• Papillary: Branching fibrovascular cores lined by columnar cells; resembles benign papillomas but shows nuclear atypia and increased mitotic activity
• Solid with mucin production: Solid nests of cells containing intracellular mucin (PAS-positive, diastase-resistant); often associated with worse prognosis
• Mucinous (colloid): Abundant extracellular mucin pools ("lakes" or "seas" of mucin) with clusters of malignant cells; poor prognosis; associated with lung mucinous adenocarcinoma (formerly bronchioloalveolar carcinoma with mucinous features)

• Glandular differentiation with occasional mucin-filled cells (PAS-positive, mucicarmine-positive)
• Nuclear enlargement, hyperchromasia, irregular membranes
• Increased mitotic activity (varies by grade)
• Variable stromal desmoplasia (fibroblastic response)

• TTF-1 positive (70-80%), CK7 positive, CK20 negative (useful to distinguish from metastatic colorectal adenocarcinoma)
• Mucin stains: PAS, mucicarmine positive if present

• Nests of polygonal cells with abundant eosinophilic cytoplasm
• Prominent intercellular bridges ("desmosomes") creating intercellular connections
• Keratin pearls: Concentric layers of keratinized material in center of nests (pathognomonic but not always present)
• Surface keratinization and parakeratosis: At interface with necrosis
• Marked nuclear pleomorphism, coarse chromatin, prominent nucleoli
• High mitotic rate with abnormal forms
• Tumor necrosis: Often extensive and central

• Well-differentiated: Obvious keratinization, keratin pearls, abundant intercellular bridges
• Moderately differentiated: Focal keratinization, less prominent bridges
• Poorly differentiated: Minimal keratinization, sparse bridges, high-grade nuclei

• p63 positive and strong (sensitive for squamous differentiation)
• CK5/6 positive
• TTF-1 variable (positive in 50-70% of lung SCC, unlike SCC of other sites)
• CK7 positive, CK20 negative

• Small, densely packed cells (size of 2-3 lymphocytes, 10-15 micrometers)
• Scant cytoplasm with high nuclear-to-cytoplasmic ratio
• "Salt-and-pepper" or "finely dispersed" chromatin: Fine, evenly distributed chromatin with absent/inconspicuous nucleoli
• Numerous mitoses (>10 per 10 high-power fields) and apoptotic bodies/single-cell necrosis ("crushed artifact")
• Molding of nuclei due to tight cellular packing
• Necrosis: Extensive and diffuse; often obscures architecture

• Dense-core neurosecretory granules visible on electron microscopy
• Strong immunoexpression of synaptophysin, chromogranin A/B, and CD56 (neuroendocrine markers)

• Pure SCLC: Pure neuroendocrine small cells without other differentiation
• SCLC with large cell component (combined SCLC): Mixed with large cell carcinoma, squamous, or adenocarcinoma elements; slightly better prognosis

• **Synaptophys

Stabilise first — treat the oncologic emergency, not the biopsy result

• Superior vena cava syndrome: elevate the head, obtain contrast CT, and pursue tissue diagnosis before empiric steroids/radiation unless there is airway compromise or cerebral edema; endovascular stenting gives the fastest relief.
• Malignant pleural effusion, hypercalcemia (IV isotonic fluids then a bisphosphonate such as zoledronic acid), symptomatic hyponatremia from SIADH (hypertonic saline, correcting sodium slowly), and cord compression (dexamethasone plus urgent MRI) are managed before systemic therapy.

Non-small cell lung cancer (NCCN NSCLC guideline framework)

• Stage I–II: lobectomy with mediastinal lymph node dissection/sampling is definitive; adjuvant platinum-based chemotherapy for resected node-positive or larger tumors. Resected specimens should also undergo EGFR mutation and PD-L1 testing, because adjuvant osimertinib (EGFR-mutant disease) or adjuvant checkpoint inhibition (selected PD-L1–expressing tumors) may follow platinum-based chemotherapy per NCCN/ASCO. Stereotactic body radiotherapy is the alternative when pulmonary reserve or comorbidity precludes resection.
• Stage III (unresectable): concurrent platinum-doublet chemoradiation followed by consolidation anti–PD-L1 antibody (durvalumab).
• Stage IV — molecular testing and PD-L1 come before chemotherapy, per NCCN and CAP/IASLC/AMP testing recommendations:
• EGFR-mutant: third-generation EGFR tyrosine kinase inhibitor (osimertinib).
• ALK-rearranged: next-generation ALK inhibitor (alectinib); ROS1 fusions respond to crizotinib or entrectinib.
• No driver, high PD-L1: anti–PD-1 monotherapy (pembrolizumab); otherwise chemoimmunotherapy with a platinum doublet (carboplatin plus pemetrexed for nonsquamous, a taxane for squamous) plus a checkpoint inhibitor.
• Contraindicated: pemetrexed in squamous histology (no benefit); bevacizumab in squamous, cavitating, or hemoptysis-prone tumors because of fatal pulmonary hemorrhage; EGFR/ALK TKIs in wild-type tumors; checkpoint inhibitors are used cautiously in active autoimmune disease or after solid-organ transplant and are relatively ineffective in EGFR-mutant disease.

Small cell lung cancer (NCCN SCLC guideline framework)

• Limited stage: concurrent platinum plus etoposide with thoracic radiotherapy; surgery only for the rare node-negative peripheral T1–T2 nodule.
• Extensive stage: platinum/etoposide plus a PD-L1 inhibitor (atezolizumab or durvalumab); second line topotecan or lurbinectedin.
• Prophylactic cranial irradiation is offered to responders, with MRI surveillance as an accepted alternative.
• SCLC is exquisitely chemosensitive but relapses early — chemotherapy, not resection, is the answer on exams.

Local tumor complications

• Superior vena cava syndrome: central tumor (classically SCLC or right upper lobe SCC) compresses the SVC → facial/upper-extremity edema, distended neck and chest wall veins, Pemberton sign. Emergency if there is stridor, laryngeal edema, or altered mentation.
• Post-obstructive pneumonia and lobar atelectasis: endobronchial SCC obstructs a lobar bronchus; recurrent pneumonia in the same lobe in a smoker is cancer until proven otherwise.
• Massive hemoptysis: erosion of a bronchial artery by a cavitating central tumor — airway emergency; position bleeding side down, secure the airway, and pursue bronchial artery embolization.
• Pancoast (superior sulcus) tumor: brachial plexus C8–T1 invasion → shoulder/ulnar arm pain; sympathetic chain involvement → Horner syndrome.
• Malignant pleural or pericardial effusion: exudative, cytology-positive; pericardial involvement can produce tamponade, an emergency signalled by pulsus paradoxus and electrical alternans.

Systemic and metastatic complications

• Brain, bone, liver, adrenal metastases (SCLC and ADC especially): new headache, seizure, focal deficit, or pathologic fracture; cord compression presenting with back pain plus weakness or retention is an emergency.
• Hypercalcemia of malignancy (PTHrP, SCC): polyuria, confusion, short QT — emergent when severe.
• SIADH (SCLC): hypotonic hyponatremia; over-rapid correction risks osmotic demyelination.
• Tumor lysis syndrome: rare but real in bulky, chemosensitive SCLC — hyperkalemia, hyperuricemia, hyperphosphatemia, acute kidney injury.

Treatment-related complications

• Febrile neutropenia after platinum/etoposide: emergency; empiric antipseudomonal beta-lactam within an hour.
• Cisplatin toxicity: nephrotoxicity, ototoxicity, peripheral neuropathy, severe emesis.
• Immune-related adverse events from checkpoint inhibitors: pneumonitis (new dyspnea with ground-glass opacities), colitis, hepatitis, thyroiditis, hypophysitis; myocarditis is the lethal one — treat with high-dose corticosteroids per ASCO immune-related adverse event guidance.
• Radiation pneumonitis and esophagitis; EGFR/ALK TKI toxicity: acneiform rash, diarrhea, QT prolongation, and drug-induced interstitial lung disease.

• Location splits the histology: peripheral = adenocarcinoma and large cell; central = squamous cell and small cell. Nearly every stem encodes the answer in the tumor's location plus smoking history.
• Squamous = "sentral, Smoking, cavitation, hyperCalcemia": keratin pearls and intercellular bridges on histology, PTHrP-mediated hypercalcemia with suppressed PTH — this distinguishes it from bone-metastasis hypercalcemia and from primary hyperparathyroidism.
• Small cell = neuroendocrine: Kulchitsky cells, salt-and-pepper chromatin, nuclear molding, synaptophysin/chromogranin/CD56 positive; the classic paraneoplastic trio is SIADH, ectopic ACTH, and Lambert-Eaton (anti-P/Q-type voltage-gated calcium channel antibodies; strength improves with repeated contraction, unlike myasthenia gravis).
• Adenocarcinoma is the most common lung cancer overall, in women, and in never-smokers; TTF-1 positive with CK7+/CK20−, which separates a lung primary from metastatic colorectal adenocarcinoma (CK7−/CK20+/CDX2+).
• Single best next step for an incidental solitary pulmonary nodule: compare with prior imaging. Stability over years and popcorn (hamartoma) or central/laminated (granuloma) calcification argue benign; spiculation, upper-lobe location, and growth argue malignant.
• The one association examiners love: asbestos exposure raises the risk of bronchogenic carcinoma more than mesothelioma, and the risk is multiplicative with smoking.
• Screening: USPSTF recommends annual low-dose CT for adults 50–80 years with a 20 pack-year history who currently smoke or quit within the past 15 years.
• Common distractors to avoid: TTF-1 is not exclusive to adenocarcinoma — SCLC is usually TTF-1 positive, and only rare lung squamous carcinomas (<10%, clone-dependent) show weak focal staining, so p40 (with p63/CK5-6) is the more specific squamous marker and TTF-1+/p40− versus TTF-1−/p40+ is the working discriminator; lepidic growth without stromal invasion is the non-invasive pattern, not a poor-prognosis one; large cell carcinoma is a diagnosis of exclusion after adequate immunostaining; and SCLC — even when it looks localized — is managed with chemotherapy plus radiation, not resection (the only exception is the rare clinically staged T1–T2, node-negative peripheral nodule, which still receives adjuvant chemotherapy).