LONG DETAILED ANSWERS

1. WHO Classification of Lung Tumors - Clinical Features, Morphology, and Diagnostic Approach

Introduction

2021 WHO Classification of Malignant Epithelial Lung Tumors (Simplified)

Pathogenesis

• Early: Loss of heterozygosity on chromosome 3p (tumor suppressor loci) - occurs even in benign bronchial epithelium of smokers
• Intermediate: KRAS mutation (~30% of adenocarcinomas), TP53 mutation
• Adenocarcinoma-specific: EGFR activating mutations (especially in non-smoking women, Asian populations) - these tumors respond to EGFR inhibitors (erlotinib, gefitinib). EGFR and KRAS mutations are mutually exclusive.
• Other targetable mutations: ALK rearrangements, ROS1, HER2, MET (4-6% each)

• Smoking: ~90% of lung cancers occur in current or recent smokers. Risk is 60x higher in those smoking 2 packs/day for 20 years vs. non-smokers
• Asbestos: 5-fold risk in non-smokers; ~55-fold risk when combined with heavy smoking
• Uranium mine exposure, arsenic, chromium, nickel, vinyl chloride
• P-450 gene polymorphisms modify risk by altering carcinogen activation

Individual Tumor Types

A. Adenocarcinoma

• Lepidic pattern: Neoplastic cells grow along intact alveolar walls (formerly bronchioloalveolar carcinoma); best prognosis
• Acinar: Gland-forming
• Papillary/Micropapillary: Papillary structures; micropapillary pattern carries poor prognosis
• Solid: Sheets of cells with little gland formation
• Mucinous: Tall columnar cells with abundant intracytoplasmic mucin; spreads aerogenously

B. Squamous Cell Carcinoma (SCC)

• Preceded by squamous metaplasia → dysplasia → carcinoma in situ → invasive carcinoma
• Shows keratinization (keratin pearls), intercellular bridges
• Cavitation is common (central necrosis)
• Spreads locally before metastasizing

C. Small Cell Carcinoma (SCLC)

• Small cells, scant cytoplasm, finely granular ("salt-and-pepper") chromatin, absent or inconspicuous nucleoli
• Nuclear molding
• Crush artifact on biopsy specimens
• High mitotic rate
• Necrosis common
• Neuroendocrine markers (dense core granules on EM)

• SIADH (ectopic ADH) → hyponatremia
• Cushing syndrome (ectopic ACTH)
• Eaton-Lambert syndrome (anti-VGCC antibodies)
• Not amenable to surgery; treated with chemotherapy ± radiation

D. Large Cell Carcinoma

E. Carcinoid Tumors (Neuroendocrine)

Diagnostic Approach

• Symptoms: cough, hemoptysis, dyspnea, chest pain, weight loss, hoarseness (recurrent laryngeal nerve), Horner syndrome (Pancoast tumor: superior sulcus + ptosis + miosis + anhidrosis)
• Paraneoplastic syndromes (SIADH, Cushing, hypercalcemia, Eaton-Lambert)

• CXR: Hilar mass, peripheral nodule, atelectasis, pleural effusion
• CT chest with contrast: Defines extent, mediastinal nodes
• PET-CT: Staging, detects occult metastases
• Brain MRI: All SCLC and selected NSCLC

1. Sputum cytology (central tumors, SCC, SCLC)
2. Bronchoscopy + BAL + transbronchial biopsy
3. CT-guided percutaneous needle biopsy (peripheral lesions)
4. EBUS/EUS for mediastinal nodes
5. Thoracoscopy / surgical resection (when above fail)

• H&E morphology
• IHC panel: TTF-1, p40, synaptophysin, chromogranin, Ki-67
• Molecular testing (all advanced NSCLC): EGFR, KRAS, ALK, ROS1, BRAF, MET, RET, PD-L1 expression

2. Chronic Gastritis

Definition and Causes

1. Helicobacter pylori infection - the most common cause worldwide
2. Autoimmune gastritis - most common in patients without H. pylori
3. Chronic NSAID use - increasingly prevalent as H. pylori rates fall

A. H. pylori Gastritis

Epidemiology

Pathogenesis

1. Flagella - allow motility in viscous mucus
2. Urease - generates ammonia from urea, elevating local pH and protecting bacteria from acid
3. Adhesins - enhance bacterial adherence to foveolar cells
4. Toxins - CagA and CagE stimulate IL-8 release (potent neutrophil chemotactic factor), initiating innate and adaptive immune responses

Morphology (H. pylori Gastritis)

• H&E/Giemsa: Curved bacilli concentrated in mucus overlying foveolar cells in surface and neck regions
• Inflammation: Variable neutrophils in lamina propria; intraepithelial neutrophils; pit abscesses (neutrophils in gland lumina)
• Chronic inflammation: Plasma cells in clusters/sheets + lymphocytes + macrophages in superficial lamina propria
• Lymphoid aggregates with germinal centers (submucosal MALT) - potential to transform to MALT lymphoma
• Intestinal metaplasia: Goblet cells + columnar absorptive cells - associated with increased risk of gastric adenocarcinoma
• Note: H. pylori shows tropism for gastric foveolar epithelium; NOT found in areas of intestinal metaplasia → antral biopsies preferred

Diagnosis of H. pylori

• Non-invasive: Serology (H. pylori IgG antibodies), stool antigen test, urea breath test (gold standard non-invasive)
• Invasive (biopsy-based): Rapid urease test (CLO test), bacterial culture, PCR for H. pylori DNA, histology with Giemsa/IHC stain

B. Autoimmune Gastritis

Features

• Antibodies to parietal cells (H+/K+ ATPase) and intrinsic factor - detectable in serum and gastric secretions
• Reduced serum pepsinogen I levels
• Antral endocrine cell hyperplasia (G cells responding to achlorhydria)
• Vitamin B12 deficiency → pernicious anemia + subacute combined degeneration of cord
• Achlorhydria (impaired gastric acid secretion)

Pathogenesis

Morphology

• Body/fundus: Diffuse atrophy of oxyntic (acid-secreting) mucosa
• Loss of parietal and chief cells → replaced by mucous glands
• Intestinal metaplasia may be present
• Endocrine cell (ECL) hyperplasia in antrum due to hypergastrinemia → may lead to gastric carcinoid tumors (Type I NETs)

Complications

• Pernicious anemia (B12 deficiency)
• Increased risk of gastric adenocarcinoma (due to atrophy + intestinal metaplasia)
• Type I gastric neuroendocrine tumor (carcinoid)

C. Complications of Chronic Gastritis

3. Classification of Intestinal Adenomas + Colon Carcinoma (with IHC)

Classification of Intestinal Polyps

I. Non-Neoplastic Polyps

• Most common polyp type; usually in left colon and rectum
• Benign epithelial proliferations with star-shaped crypts and "saw-tooth" luminal surface
• No malignant potential (must be distinguished from sessile serrated polyps)

• Form from chronic cycles of injury and healing (as seen in IBD)
• Pseudopolyps in ulcerative colitis

• Occur sporadically or as part of genetic syndromes:
  • Juvenile polyps: Cystic spaces with dilated mucin-filled glands, lamina propria expanded by inflammatory infiltrate. Juvenile polyposis syndrome (SMAD4, BMPRIA mutations): 30-50% risk of adenocarcinoma by age 45
  • Peutz-Jeghers polyps: Arborizing smooth muscle core extending into lamina propria; small intestine >> colon. Associated with STK11/LKB1 mutation. Mucocutaneous hyperpigmentation. Lifetime cancer risk ~40% (colon, pancreatic, breast, ovarian, uterine, testicular)

II. Neoplastic Polyps (Adenomas)

Morphologic Classification of Adenomas

• Pedunculated (slender fibromuscular stalk)
• Smooth surface; predominantly tubular architecture (>75% tubular glands)
• Hallmarks: Nuclear hyperchromasia, elongation, stratification; reduced goblet cells; prominent nucleoli; eosinophilic cytoplasm
• Low-grade dysplasia most common
• Malignant transformation uncommon if <1 cm

• Usually sessile, large, broad-based; surface like "velvety" fronds
• Predominantly villous architecture (>75% villous)
• Most commonly in rectum/sigmoid
• Highest risk of malignancy (~40% have invasive carcinoma at time of diagnosis)
• May secrete large amounts of mucus → hypokalemia, hyponatremia ("secretory diarrhea")

• Mix of tubular and villous patterns (25-75% villous)
• Intermediate malignant potential

• Flat/sessile, often right colon
• Shares morphologic features with hyperplastic polyps: serrated crypts
• Key distinction: serration involves crypt bases (ectatic/dilated crypt bases); lacks typical cytologic dysplasia
• Progresses via methylator/CIMP pathway (BRAF mutation + CpG island hypermethylation)
• Can lead to MSI-high colorectal cancer via MLH1 methylation silencing

Risk Factors for Malignant Transformation in Adenomas

• Size >2 cm
• Villous architecture
• High-grade dysplasia
• Multiple adenomas

Familial Adenomatous Polyposis (FAP)

• Autosomal dominant; APC gene mutation (chromosome 5q21)

• 100 (typically hundreds to thousands) colorectal adenomas

• 100% risk of colorectal cancer if untreated, typically by age 40
• Extraintestinal: Desmoid tumors, epidermoid cysts, osteomas, congenital hypertrophy of retinal pigmented epithelium (CHRPE)
• Treatment: prophylactic colectomy

HNPCC (Lynch Syndrome)

• Autosomal dominant; MSH2 or MLH1 mutations (mismatch repair genes)
• Fewer polyps, older age at diagnosis than FAP
• Amsterdam criteria: ≥3 relatives with CRC; ≥2 consecutive generations; ≥1 before age 50
• High risk of endometrial, ovarian, gastric, and urothelial cancers also

Colorectal Adenocarcinoma

Epidemiology

• Most common malignancy of the GI tract; ~10% of all cancer deaths worldwide
• ~1.2 million new cases/600,000 deaths annually worldwide
• Peaks at 60-70 years; <20% before age 50 (but rising incidence in younger adults)
• Highest incidence: North America, Australia, Europe; lower in South America, India, Africa

Molecular Pathogenesis - Three Major Pathways

• APC is a tumor suppressor that binds β-catenin and promotes its degradation
• Loss of APC → β-catenin accumulates → enters nucleus → activates proliferative genes (MYC, cyclin D1)
• Sequence: APC loss → KRAS mutation → SMAD2/SMAD4 loss (18q21) → TP53 mutation → invasive carcinoma
• Characterized by chromosomal instability (CIN): aneuploidy, LOH at multiple loci

• Mutations in MMR genes (MSH2, MLH1, MSH6, PMS2)
• Defective MMR → microsatellite instability (MSI) - mutations accumulate at 50,000-100,000 microsatellite sites
• Typical of HNPCC; also seen in ~15% of sporadic tumors (via MLH1 methylation silencing)
• Tumors tend to be right-sided, poorly differentiated, mucinous, with prominent lymphocytic infiltrate
• Paradoxically better prognosis than CIN tumors

• Accounts for ~15% of sporadic tumors
• Begins with BRAF mutation + aberrant CpG island hypermethylation (epigenetic silencing)
• Silences MLH1 → leads to MSI
• Precursor lesion: sessile serrated polyp

Morphology

• Right colon (cecum, ascending): Polypoid, exophytic fungating masses along one wall; rarely obstruct (large caliber)
• Left colon (sigmoid, descending): Annular "napkin-ring" lesions causing luminal narrowing and obstruction; change in bowel habits

• Most are composed of tall columnar cells resembling dysplastic adenoma epithelium
• Invasive component evokes strong desmoplastic stromal response → firm consistency
• Poorly differentiated tumors form few glands
• Mucinous adenocarcinoma: Abundant extracellular mucin pooling in intestinal wall → poor prognosis; may cause pseudomyxoma peritonei
• Signet ring cell carcinoma: Rare; composed of signet-ring cells similar to gastric cancer; aggressive

Immunohistochemistry of Colorectal Carcinoma

• Four proteins tested: MLH1, MSH2, MSH6, PMS2
• Loss of nuclear staining = MMR deficient (dMMR) → indicates MSI-High
• MLH1 loss: most common; if combined with BRAF V600E mutation → sporadic (methylation); if MLH1 loss alone → consider germline MLH1 mutation (Lynch)
• MSH2 loss: almost always Lynch syndrome

Clinical Features and Staging

• Right-sided: Iron deficiency anemia, fatigue, weakness; often asymptomatic until advanced
• Left-sided: Visible rectal bleeding, change in bowel habits, cramping, tenesmus, pencil-thin stools, obstruction
• Rectum: Hematochezia, tenesmus, perineal pain

SHORT NOTES

1. Chronic Bronchitis

• Inhaled irritants → goblet cell hyperplasia + hypertrophy of mucous glands in the bronchial wall → excessive mucus production
• Inflammation with neutrophils and macrophages in the bronchial wall and lumen
• Loss of cilia → impaired mucociliary clearance
• Reid index increased (ratio of mucous gland thickness to total bronchial wall thickness from epithelium to cartilage): normal <0.4; chronic bronchitis >0.5
• Bacterial colonization (H. influenzae, S. pneumoniae, Moraxella) → exacerbations
• Progressive bronchiolar narrowing (small airways disease) causes airflow limitation

• Macroscopic: Hyperemic, swollen mucosa; excessive mucus in airways
• Microscopic:
  • Goblet cell hyperplasia (extends beyond normal location in large bronchi into smaller bronchi and bronchioles)
  • Hypertrophy of submucosal mucous glands (increased Reid index)
  • Inflammatory infiltrate: neutrophils + lymphocytes + macrophages
  • Squamous metaplasia of bronchial epithelium
  • Small airway disease: inflammation, fibrosis, goblet cell metaplasia in bronchioles ("blue bloater")

• Chronic productive cough with mucopurulent sputum
• Dyspnea, wheezing on exertion
• Repeated respiratory infections
• "Blue bloater" phenotype: cyanosis, CO2 retention (hypercapnia), cor pulmonale, edema
• PFTs: obstructive pattern (decreased FEV1/FVC)
• ABG: hypoxemia with hypercapnia (V/Q mismatch + hypoventilation)
• Complications: Pulmonary hypertension, cor pulmonale, polycythemia, respiratory failure

2. Pneumoconiosis

• Particle size 1-5 μm is most dangerous (reaches alveoli and is retained)
• Macrophages attempt to phagocytose particles → frustrated phagocytosis
• Macrophage activation → release of ROS, cytokines (TNF-α, IL-1, IL-6, TGF-β, PDGF) → fibroblast proliferation → collagen deposition → progressive massive fibrosis (PMF)
• T-cell-mediated immune responses amplify tissue damage
• Coal dust: less fibrogenic; silica and asbestos: highly fibrogenic (silica most toxic)

A. Coal Workers' Pneumoconiosis (CWP)

• Simple CWP (anthracosis): Carbon pigment deposits in alveolar macrophages along lymphatics; clinically insignificant; seen in all urban dwellers
• Coal macules (1-2 mm): Aggregates of carbon-laden macrophages around respiratory bronchioles
• Coal nodules (>5 mm): Collagen added to macules
• Progressive Massive Fibrosis (PMF): >2 cm lesions, upper lobes; severe restrictive/obstructive defect; associated with rheumatoid arthritis (Caplan syndrome)

B. Silicosis

• Silica particles phagocytosed by alveolar macrophages
• Silica damages phagolysosomal membrane → activates NLRP3 inflammasome → IL-1β release → inflammation
• Macrophage death → release of silica → re-phagocytosis → perpetual cycle
• Progressive fibrosis: silicotic nodules → PMF
• Associated with increased susceptibility to tuberculosis

• Silicotic nodules: Concentric whorls of dense hyalinized collagen with peripheral zone of macrophages and lymphocytes; 1-10 mm; upper lobes and hilar lymph nodes; may calcify ("eggshell calcification" in nodes)
• PMF: Nodules coalesce; large irregular fibrotic masses, upper lobes, may cavitate
• Under polarized light: birefringent silica particles within nodules

• Slowly progressive dyspnea, restrictive PFTs
• CXR: Upper-lobe nodules, "eggshell" hilar lymph node calcification
• Caplan syndrome: Rheumatoid arthritis + pneumoconiosis = Caplan nodules (large necrotic nodules)
• Increased risk of MTB reactivation (silica impairs macrophage killing)
• Slightly increased risk of lung cancer

C. Berylliosis

3. Asbestos-Related Lung Disorders

• Serpentine (curly) fibers: Chrysotile (white asbestos) - most commonly used commercially; fibers are long, flexible, curly; less biopersistent; less carcinogenic
• Amphibole (straight, stiff) fibers: Crocidolite (blue), amosite (brown), tremolite - more dangerous; more biopersistent; strongly linked to mesothelioma

• Fibers >8 μm too long to be fully phagocytosed → frustrated macrophages → ROS + cytokine release → fibrosis
• Crocidolite is most carcinogenic (molecular mimicry, iron content)
• Asbestos bodies (ferruginous bodies): asbestos fibers coated with iron-protein complex; dumbbell or beaded rod shape; Golden-yellow/brown on H&E; Prussian blue stain positive

Asbestos-Related Disorders

1. Asbestosis

• Starts in lower lobes and subpleurally (vs. silicosis: upper lobes)
• Diffuse interstitial fibrosis + asbestos bodies (ferruginous bodies) in fibrotic tissue
• Honeycomb lung in advanced cases
• Pleural fibrosis often coexists

2. Pleural Plaques

• Most common manifestation of asbestos exposure
• Discrete, well-demarcated, white, firm plaques on parietal pleura (anterolateral chest wall and diaphragm)
• Composed of acellular, dense, hyalinized collagen; may calcify
• NOT precancerous; may cause mild restrictive defect
• Bilateral pleural plaques on CXR are essentially pathognomonic of asbestos exposure

3. Malignant Mesothelioma

• Arises from mesothelial cells lining the pleura (rarely peritoneum)
• 80% have history of significant asbestos exposure (especially crocidolite)
• Latency period: 25-45 years after initial exposure
• Morphology: Three patterns - epithelioid (most common, best prognosis), sarcomatoid, biphasic (mixed)
  • Diffuse thickening of pleura encasing the lung ("rind-like")
  • Epithelioid type: Tubular/papillary structures resembling adenocarcinoma
• IHC: Calretinin (+), WT-1 (+), CK5/6 (+), D2-40 (+); CEA (-), TTF-1 (-), MOC-31 (-) - helps distinguish from metastatic adenocarcinoma
• Clinical: Chest pain, recurrent pleural effusion, dyspnea; very poor prognosis (median survival 12-18 months)

4. Asbestos-Related Lung Cancer

• Squamous cell carcinoma and adenocarcinoma both increased
• Asbestos exposure alone: 5x increased risk
• Asbestos + heavy smoking: ~55x increased risk (synergistic, not merely additive)

5. Benign Asbestos Pleural Effusion

• Earliest pleural manifestation; exudative, often blood-tinged
• Recurrent, unilateral; diagnosis of exclusion

Summary Table

4. Molecular Pathogenesis and Pathology of GIST

Definition and Epidemiology

• Stomach (~60%): best prognosis
• Small intestine (~30%): intermediate prognosis
• Colorectum (~5%), esophagus (<5%): worst prognosis

Molecular Pathogenesis

1. KIT Mutations (c-KIT / CD117) - Most Common (~70-75%)

• KIT encodes a receptor tyrosine kinase (CD117) that normally requires binding of stem cell factor (SCF) to activate
• Gain-of-function mutations render KIT constitutively active → downstream activation of RAS/MAPK, PI3K/AKT, STAT3 pathways → uncontrolled proliferation and survival
• Exon 11 mutations (juxtamembrane domain): Most common (~70% of KIT mutations); most likely to respond to imatinib
• Exon 9 mutations (extracellular domain): Less common; somewhat less likely to respond (~40-50% response); associated with small intestinal GISTs
• Exon 13 and 17 mutations: Less common; reduced imatinib sensitivity

2. PDGFRA Mutations (~5-10%)

• PDGFRA (platelet-derived growth factor receptor alpha) is also a receptor tyrosine kinase
• Gain-of-function mutations are mutually exclusive with KIT mutations
• Most common mutation: PDGFRA exon 18 D842V substitution
• GISTs with PDGFRA mutations are insensitive to imatinib (imatinib-resistant)
• PDGFRA mutations common in gastric epithelioid GISTs with myxoid stroma

3. Wild-Type GISTs (~10-15%)

• Most common wild-type subtype; most pediatric GISTs + some adult GISTs
• Mutations in SDHA, SDHB, SDHC, SDHD genes
• Associated with Carney triad (GIST + pulmonary chondromas + extra-adrenal paragangliomas) and Carney-Stratakis syndrome
• IHC: Loss of SDHB staining
• Clinically indolent despite having metastatic potential

• Patients with neurofibromatosis type 1 (NF1) may develop multiple small intestinal GISTs
• KIT overexpression but unmutated KIT/PDGFRA genes
• Loss of NF1 tumor suppressor → increased RAS signaling

Pathology

Gross Features

• Well-circumscribed, fleshy tumors arising from the bowel wall
• May be intramural, pedunculated, or exophytic
• Large tumors show areas of hemorrhage, necrosis, cystic degeneration
• Most common in stomach (submucosal/intramural)

Microscopic Features

Immunohistochemistry

Risk Stratification (NIH/Modified Fletcher Criteria)

Treatment Implications of Molecular Pathology

• Imatinib (Gleevec): First-line tyrosine kinase inhibitor; blocks KIT and PDGFRA signaling. Dramatically improves outcomes in metastatic/unresectable GIST.
  • Best response: KIT exon 11 mutations
  • Partial response: KIT exon 9, 13, 17; wild-type (up to 30% sensitive)
  • Resistant: PDGFRA exon 18 D842V mutation
• Avapritinib: Approved specifically for PDGFRA D842V-mutant GISTs
• Sunitinib/Regorafenib: Second/third-line therapy for imatinib-resistant GISTs
• Surgical resection remains the primary treatment for localized disease; adjuvant imatinib is given for high-risk tumors