• Provide a structured, exam-ready answer formatted the way MD/MS pathology examiners expect (introduction, classification, morphology, microscopy, clinical correlations, tables/diagrams where relevant)
• Draw from authoritative pathology textbooks (Robbins, Rosai & Ackerman, Sternberg, etc.) available in my medical library
• Supplement with recent evidence where relevant
• Tailor the depth and length to match what's appropriate for a university exam answer (typically 8-10 marks or 5-mark short answers)

1. Upload the question as text or image - either works
2. Let me know the marks allotted if visible (helps calibrate answer length)
3. Mention any specific focus your university tends to emphasize (e.g., morphology-heavy, lots of IHC/molecular, lab diagnosis, etc.)
4. Let me know your university/exam board if you're comfortable - different universities (AIIMS, MCI-affiliated, state universities) sometimes have different answer expectations

• Clean, point-format answers ready to write in an exam
• Key diagrams/flowcharts described where examiners expect them
• Important buzzwords and morphological descriptors that fetch marks
• Differential diagnoses and clinicopathological correlations where needed

Role of PD-L1 and PD-1 Testing in Lung Carcinoma

I. Introduction

II. Biology of the PD-1/PD-L1 Axis

• A co-inhibitory receptor expressed on activated T cells, B cells, monocytes, and NK cells
• A member of the CD28 superfamily

• Expressed on tumor cells, macrophages, dendritic cells, and other stromal cells
• Under normal physiology: PD-L1 - PD-1 interaction prevents autoimmunity (peripheral tolerance)
• In cancer: Tumor cells hijack this pathway - overexpressing PD-L1 to ligate PD-1 on tumor-infiltrating T cells, leading to:
  • T-cell exhaustion
  • Inhibition of cytotoxic effector function
  • Immune evasion

III. Approved ICIs in Lung Cancer

Note: Pembrolizumab and nivolumab target the receptor (PD-1) on T cells; atezolizumab and durvalumab target the ligand (PD-L1) on tumor cells.

IV. PD-L1 Testing - The Pathologist's Role

A. Why Test?

B. When to Test?

• All patients with advanced/metastatic NSCLC - testing is mandatory at diagnosis
• Squamous cell carcinoma patients should also be tested (ICI benefit is histology-independent for PD-L1-high tumors)
• Testing before initiating first-line therapy is essential

C. Specimen Requirements

• Formalin-Fixed Paraffin-Embedded (FFPE) tissue
• Core biopsy preferred over FNA to ensure adequate tissue
• May be performed on primary tumor, lymph node, or metastatic site

D. FDA-Approved IHC Assays and Antibody Clones

Key exam point: Each drug uses a different clone on a potentially different platform - this creates challenges for interassay comparability.

V. Scoring Systems

1. Tumor Proportion Score (TPS)

• Used for NSCLC (pembrolizumab/22C3)
• Defined as: the percentage of viable tumor cells showing partial or complete membrane staining at any intensity
• Formula: TPS = (No. of PD-L1 positive tumor cells / Total viable tumor cells) × 100%

2. Combined Positive Score (CPS)

• Used for gastric, cervical, HNSCC, esophageal cancers (not standard for NSCLC)
• Formula: CPS = (PD-L1 staining cells [tumor cells + lymphocytes + macrophages] / Total viable tumor cells) × 100

3. Immune Cell (IC) Score

• Used with SP142 assay (atezolizumab)
• Measures PD-L1 expression on tumor-infiltrating immune cells (not tumor cells)
• IC ≥ 1% qualifies for atezolizumab in some settings

VI. Clinical Significance of TPS in NSCLC - Key Trials

VII. Limitations of PD-L1 Testing

1. Interassay variability - Different clones (22C3 vs 28-8 vs SP142) give discordant results; SP142 is known to stain less than the others
2. Intratumoral heterogeneity - PD-L1 expression varies within a tumor and between primary and metastatic sites
3. Dynamic expression - PD-L1 levels change over time and with prior therapy
4. Imperfect predictor - PD-L1-negative patients can respond to ICI (especially with high Tumor Mutational Burden); PD-L1-high patients may not respond
5. Driver-mutated tumors - Patients with EGFR mutations or ALK rearrangements have lower tumor mutational burden and derive less benefit from ICI despite high PD-L1; TKIs remain preferred even in PD-L1-high EGFR/ALK-positive NSCLC

VIII. Other Biomarkers Used Alongside PD-L1

IX. Treatment Algorithm (NSCLC - First Line, No Driver Mutation)

Advanced NSCLC - No EGFR/ALK/ROS1
         |
    PD-L1 TPS testing
         |
  ┌──────┴──────┐──────────────┐
TPS ≥ 50%   TPS 1-49%    TPS < 1%
   |              |             |
Pembrolizumab  Pembro +     Chemo +
monotherapy    Platinum     Pembro/Atezo
(KEYNOTE-024)  chemo        combination

X. Testing in Special Scenarios

• Small cell lung cancer (SCLC): PD-L1 testing is less established; atezolizumab + chemotherapy is approved for extensive-stage SCLC regardless of PD-L1 status (IMpower 133)
• Neoadjuvant setting: ICI use in early-stage NSCLC (Stage I-IIIA) is being actively studied; PD-L1 testing is relevant here too
• Liquid biopsy: ctDNA-based PD-L1 analysis is emerging but not yet standard

XI. Summary

PD-L1/PD-1 testing in lung carcinoma represents the interface between pathology and precision oncology. The pathologist's role is to accurately quantify PD-L1 expression using the correct IHC assay (clone 22C3 for pembrolizumab), apply the TPS scoring method correctly, and communicate results in a clinically actionable manner. While PD-L1 TPS ≥50% is the strongest predictor of benefit from ICI monotherapy, it must be interpreted alongside driver mutation testing (EGFR, ALK, ROS1) and emerging biomarkers like TMB to guide optimal therapy selection in NSCLC.

Molecular Testing in Lung Cancer

I. Introduction

II. Why Molecular Testing? - Rationale

1. Driver mutations are present in ~50% of nonsquamous NSCLC - identifying them allows matched targeted therapy with dramatically superior outcomes
2. TKIs (Tyrosine Kinase Inhibitors) targeting driver mutations yield response rates of 60-80% vs ~30% with chemotherapy
3. Precludes unnecessary immunotherapy - EGFR/ALK-mutated tumors respond poorly to ICIs despite high PD-L1 expression (lower tumor mutational burden, fewer neoantigens)
4. Resistance profiling - molecular testing at progression identifies acquired resistance mechanisms and guides second-line selection

III. Relative Frequency of Driver Mutations in NSCLC

IV. Specimen Requirements

• Tissue type: FFPE (Formalin-Fixed Paraffin-Embedded) core biopsy is preferred over FNA - provides adequate tumor cellularity for both histology and molecular studies
• Minimum tumor content: Generally ≥20-30% tumor nuclei required for reliable NGS
• Sites: Primary tumor, mediastinal lymph node biopsy (EBUS-guided), or metastatic site (bone, liver, adrenal, pleural effusion cell block)
• Repeat biopsy: Required at disease progression to identify acquired resistance mutations

V. Recommended Molecular Targets and Testing Methods

A. EGFR (Epidermal Growth Factor Receptor)

• Gene: Chromosome 7p11.2; receptor tyrosine kinase
• Alterations tested:
  • Exon 19 deletions (most common, ~45%)
  • Exon 21 L858R point mutation (~40%)
  • Exon 20 insertions (resistance; treated differently)
  • T790M (acquired resistance mutation; ~60% of 1st/2nd gen TKI failures)
  • Less common: G719X (exon 18), S768I (exon 20), L861Q (exon 21)
• Testing method: PCR-based assays, Sanger sequencing, or preferably NGS
• Targeted drugs:
  • 1st generation: Erlotinib, Gefitinib (reversible binders)
  • 2nd generation: Afatinib, Dacomitinib (irreversible)
  • 3rd generation (preferred 1st line): Osimertinib - targets L858R/exon 19 del AND T790M; superior CNS penetration (FLAURA trial: median PFS 18.9 vs 10.2 months)

B. ALK (Anaplastic Lymphoma Kinase)

• Alteration: EML4-ALK gene fusion (chromosomal inversion 2p); results in constitutively active kinase
• Clinical profile: Young, never-smokers, adenocarcinoma; predilection for CNS metastases
• Testing methods:
  • FISH (Fluorescence In Situ Hybridization) - gold standard historically; uses break-apart probe
  • IHC - D5F3 clone (Ventana) is highly sensitive and specific; used as screening tool
  • NGS - preferred for comprehensive fusion detection
• Targeted drugs:
  • 1st line: Alectinib (preferred; superior CNS penetration) or Brigatinib
  • After resistance: Lorlatinib (3rd generation; broad resistance coverage)
  • Others: Ceritinib, Crizotinib (1st generation; less preferred now)

C. ROS1

• Alteration: Gene rearrangement (multiple fusion partners); ~2% NSCLC
• Similar profile to ALK - young, never-smokers, adenocarcinoma
• Testing: FISH (break-apart probe), IHC (D4D6 clone), NGS
• Drugs: Crizotinib, Entrectinib, Ceritinib; Lorlatinib for resistance

D. BRAF

• Alteration: V600E point mutation (exon 15); ~2% NSCLC adenocarcinoma
• More common in smokers (unlike EGFR/ALK)
• Testing: PCR, allele-specific assays, NGS
• Drug: Dabrafenib + Trametinib combination (BRAF inhibitor + MEK inhibitor; required combination due to MAPK pathway feedback)

E. RET

• Alteration: Gene rearrangement (~2%); multiple fusion partners (KIF5B most common)
• Testing: FISH, IHC (not reliable), NGS (preferred)
• Drugs: Selpercatinib, Pralsetinib (highly selective; superior to older multikinase inhibitors)

F. MET

• Alterations:
  • Exon 14 skipping mutation (~3-4%): Splicing alteration; removes regulatory juxtamembrane domain; targetable
  • MET amplification: Primary (de novo) or secondary (acquired resistance to EGFR TKI)
• Testing: RNA-based NGS preferred for exon 14 skipping; FISH for amplification
• Drugs: Capmatinib, Tepotinib (for exon 14 skipping); Crizotinib (historical)

G. KRAS

• Alteration: Point mutations (~32% adenocarcinoma); G12C, G12V, G12D most common
• KRAS G12C (~13% of NSCLC) - now targetable with Sotorasib, Adagrasib
• Testing: NGS; allele-specific PCR
• Important: Other KRAS variants (G12D, G12V) are not yet reliably targetable

H. NTRK (1/2/3)

• Alteration: Gene fusions; <1% NSCLC but pan-tumor target (agnostic approval)
• Testing: IHC (screening), FISH, RNA-based NGS (confirmatory)
• Drugs: Larotrectinib, Entrectinib - tumor-agnostic approvals

I. HER2 (ERBB2)

• Alteration: Exon 20 insertions (~2-3%)
• Testing: NGS (preferred); IHC and FISH can assess amplification
• Drugs: Trastuzumab deruxtecan (T-DXd) - antibody drug conjugate

VI. Testing Platform Comparison

Key exam point: NGS (panel-based) is now the preferred and most cost-effective approach as it detects all targets simultaneously in a single tissue sample - critical when tissue is limited.

VII. Next-Generation Sequencing (NGS) - Details

• DNA-based panel: Detects SNVs (single nucleotide variants), indels, CNVs, MSI, TMB
• RNA-based panel: Required for fusion/rearrangement detection (ALK, ROS1, RET, NTRK) - DNA-based NGS may miss some fusions
• Comprehensive genomic profiling (CGP): e.g., FoundationOne CDx (FDA-approved companion diagnostic; detects alterations in 324 genes + TMB + MSI)

• Tumor Mutational Burden (TMB): Measured in mutations/megabase; high TMB (≥10 mut/Mb) predicts ICI response independently of PD-L1
• Microsatellite Instability (MSI-H): Predicts pembrolizumab response (pan-tumor approval)
• Co-mutations: e.g., TP53 co-mutation with EGFR worsens prognosis

VIII. Liquid Biopsy

• Sensitivity is lower than tissue biopsy (~60-70% sensitivity for known mutations)
• Specificity is high
• A negative liquid biopsy does NOT exclude a mutation - tissue biopsy must follow
• Preferred technique: Cobas EGFR Mutation Test v2 (FDA approved for plasma); ddPCR; NGS-based ctDNA panels

IX. Clinical Testing Algorithm (Pathologist Perspective)

Newly diagnosed advanced NSCLC (adenocarcinoma / adenosquamous / NSCLC NOS)
                    |
         Adequate tissue biopsy (FFPE core)
                    |
         SIMULTANEOUS REFLEX TESTING
              /              \
     NGS Panel             PD-L1 IHC (22C3)
  (DNA + RNA based)         TPS scoring
        |
 Reports: EGFR, ALK, ROS1, BRAF,
 RET, MET, NTRK, HER2, KRAS,
 TMB, MSI
        |
  If tissue insufficient → Liquid biopsy (plasma ctDNA)
  If liquid biopsy negative → Repeat tissue biopsy

X. Important CAP/IASLC/AMP Guideline Points

1. Reflexive testing should be performed at diagnosis for all advanced NSCLC without waiting for oncology request
2. Turnaround time: ≤10 working days recommended
3. ALK IHC with D5F3 clone has sufficient sensitivity/specificity to guide alectinib therapy in resource-limited settings
4. RNA-based fusion testing is mandatory - DNA-based NGS alone may miss up to 20-30% of ALK/ROS1 fusions
5. Squamous cell carcinoma: Molecular testing recommended if never/light smoker, small biopsy (cannot exclude adenocarcinoma component), or clinician request
6. At disease progression on TKI, re-biopsy (tissue or liquid) is mandatory to identify resistance mechanisms

XI. Resistance Mechanisms and Retesting

XII. Summary for Exam

Molecular testing in lung cancer is the cornerstone of precision oncology. The pathologist must ensure adequate tissue procurement, select the appropriate testing platform (NGS preferred), and report results with turnaround time ≤10 days. Key actionable alterations include EGFR mutations (TPS: osimertinib), ALK/ROS1 fusions (alectinib/crizotinib), BRAF V600E (dabrafenib+trametinib), RET fusions (selpercatinib), MET exon 14 skipping (capmatinib), KRAS G12C (sotorasib), and NTRK fusions (larotrectinib). Liquid biopsy using ctDNA is a valuable adjunct, particularly when tissue is limited, but cannot replace tissue biopsy due to lower sensitivity. All molecular testing should be performed alongside PD-L1 IHC to complete the biomarker profile needed for first-line treatment decisions.

Molecular Testing in Lung Cancer

1. Why Test?

• ~50% of nonsquamous NSCLC carry actionable driver mutations
• Matched targeted therapy → response rates 60-80% vs ~30% chemo
• Guides against immunotherapy in EGFR/ALK+ tumors (they respond poorly to ICIs)

2. Who to Test?

• All patients with advanced/metastatic adenocarcinoma, adenosquamous carcinoma, NSCLC-NOS
• Squamous: test if never/light smoker or small biopsy

3. Specimen

• FFPE core biopsy preferred (FNA often insufficient for NGS)
• Re-test at disease progression (resistance mechanism profiling)

4. Key Targets - One Line Each

5. Testing Methods - Simplified

Bottom line: Use NGS (DNA + RNA panel) to detect everything in one shot. RNA-based component is essential - DNA-based NGS alone misses fusions.

6. NGS Also Reports

• TMB (Tumor Mutational Burden) ≥10 mut/Mb → predicts ICI response
• MSI-H → pembrolizumab eligible (pan-tumor)

7. Liquid Biopsy (ctDNA)

• Use when: Tissue insufficient or biopsy risky
• Best for: EGFR T790M detection at progression
• Limitation: Lower sensitivity (~60-70%) → negative result does NOT rule out mutation → follow with tissue biopsy

8. Key Resistance Mutations to Know

9. Mnemonic for Targets

• EGFR, ALK, BRAF, Liquid biopsy (ctDNA), KRAS, Immunotherapy (TMB/MSI), Drivers, MET, NTRK, HER2 (+ RET + ROS1)

10. Exam One-liner Summary

Molecular testing (NGS preferred) is mandatory in all advanced nonsquamous NSCLC to identify actionable driver mutations (EGFR, ALK, ROS1, BRAF, RET, MET, NTRK, HER2, KRAS G12C), guide matched targeted therapy, and determine ICI eligibility alongside PD-L1 testing. Liquid biopsy (ctDNA) is a useful adjunct but cannot replace tissue biopsy.

Molecular Testing in Lung Cancer (10 Marks)

I. Introduction (~1 mark)

• ~50% of nonsquamous NSCLC carry an actionable driver mutation
• Mandatory for all advanced/metastatic NSCLC at diagnosis (CAP/IASLC/AMP guidelines)

II. Specimen Requirements (~1 mark)

FNA alone is usually insufficient for comprehensive molecular testing.

III. Testing Methods (~1 mark)

NGS is the preferred platform - DNA panel detects mutations; RNA panel is essential for fusion/rearrangement detection (ALK, ROS1, RET, NTRK). DNA-based NGS alone misses up to 30% of fusions.

IV. Key Molecular Targets and Targeted Therapies (~4 marks)

A. EGFR (Most important target)

• Frequency: 10-15% Western; 25-30% Asian; women, never/light smokers
• Sensitizing mutations: Exon 19 deletion (~45%), L858R point mutation (~40%), others (G719X, S768I, L861Q)
• Resistance mutation: T790M (exon 20) - acquired in ~60% after 1st/2nd gen TKIs
• Exon 20 insertion: Resistant to standard TKIs; treated differently
• Drugs:

FLAURA trial: Osimertinib → median PFS 18.9 vs 10.2 months (1st gen TKI)

B. ALK Fusion

• Frequency: 3-7%; young, never-smokers, adenocarcinoma; EML4-ALK most common
• Predilection for CNS metastases
• Testing: FISH (break-apart probe), IHC (D5F3 clone - highly sensitive), NGS
• Drugs: Alectinib / Brigatinib (1st line preferred) → Lorlatinib (resistance)

C. ROS1 Fusion

• Frequency: ~2%; similar clinical profile to ALK (young, never-smoker, adenocarcinoma)
• Testing: FISH, IHC (D4D6 clone), NGS
• Drugs: Crizotinib, Entrectinib; Lorlatinib for resistance

D. Other Actionable Targets (summarize in table)

V. Additional Biomarkers from NGS (~1 mark)

• Measured as mutations/megabase (mut/Mb)
• High TMB (≥10 mut/Mb) predicts benefit from immune checkpoint inhibitors independent of PD-L1
• Reported by comprehensive NGS panels (e.g., FoundationOne CDx)

• Less common in NSCLC but pan-tumor predictor of pembrolizumab response
• Detected by IHC (MMR proteins) or PCR/NGS

VI. Liquid Biopsy (~1 mark)

• Sample: Peripheral blood → plasma → ctDNA (circulating tumor DNA)
• Method: ddPCR or NGS-based assays

1. When tissue is insufficient or biopsy too risky
2. Detecting T790M acquired resistance at EGFR TKI progression
3. Monitoring treatment response (serial ctDNA)
4. Minimal residual disease (MRD) detection

• Sensitivity ~60-70% (lower than tissue)
• Negative result does NOT exclude a mutation → must confirm with tissue biopsy
• Sensitivity varies with tumor burden and shedding

VII. Resistance Mechanisms (~1 mark)

At progression: Tissue re-biopsy OR liquid biopsy must be performed to identify resistance mechanism and guide next-line therapy.

VIII. Summary / Closing Statement

Molecular testing is the cornerstone of NSCLC management. NGS (DNA + RNA based) is the preferred platform as it simultaneously identifies all actionable mutations (EGFR, ALK, ROS1, BRAF, RET, MET, NTRK, HER2, KRAS G12C), reports TMB and MSI, and spares limited tissue. Testing must be performed at diagnosis AND at progression. The pathologist's role is central - from ensuring adequate tissue procurement to selecting the correct assay, interpreting results, and integrating molecular findings with PD-L1 IHC for a complete biomarker report that directly guides therapy.