Role of Immunohistochemistry in Lung Cancer
Immunohistochemistry is essential in lung cancer for two critical purposes: accurate histologic subtyping of non-small cell lung cancer (NSCLC) when morphology alone is insufficient, and screening for specific targetable molecular alterations (ALK, ROS1, NTRK) that directly determine treatment selection and patient survival.
Primary Diagnostic Role: Histologic Subtyping
When IHC is Mandatory for Subtyping
IHC must be used when NSCLC cannot be subtyped by morphology alone, with the goal of reducing NSCLC-not otherwise specified (NOS) diagnoses to <10% of cases. 1 This is critical because treatment decisions—particularly the use of pemetrexed chemotherapy and targeted therapies—depend on distinguishing adenocarcinoma from squamous cell carcinoma.
Recommended IHC Panel for NSCLC Subtyping
Use a limited two-marker panel to preserve tissue for molecular testing: 1
- TTF-1 (Thyroid Transcription Factor 1): Positivity indicates adenocarcinoma
- p40: Positivity indicates squamous cell carcinoma
- If both negative: Diagnosis remains NSCLC-NOS
Critical caveat: Only 60% of poorly differentiated and metastatic lung adenocarcinomas stain positive for TTF-1. 1 When TTF-1 is negative but lung adenocarcinoma is suspected (particularly with CK7 positivity), consider SMARCA4 staining, as many TTF-1-negative lung adenocarcinomas show loss of SMARCA4 nuclear staining. 1
Tissue Conservation Principle
Pathologists must use only two tissue sections for IHC NSCLC subtyping and avoid excessive IHC investigation. 1 This preserves precious tissue for mandatory molecular testing that follows histologic diagnosis.
Predictive Biomarker Role: Screening for Targetable Alterations
ALK Testing
Positive ALK IHC with an appropriately validated assay may be used directly to prescribe ALK inhibitors without molecular confirmation. 1 This represents the most clinically actionable use of IHC in lung cancer, as ALK-positive patients have dramatically improved outcomes with targeted therapy versus chemotherapy.
ROS1 Testing
ROS1 IHC may be used as a screening test in advanced lung adenocarcinoma, but positive results must be confirmed by a molecular or cytogenetic method before treatment. 1 Unlike ALK, ROS1 IHC alone is insufficient for treatment decisions.
NTRK Testing
If next-generation sequencing (NGS) is used as the primary NTRK screening tool, IHC confirmation should be considered. 1 Conversely, positive NTRK IHC requires molecular confirmation before prescribing NTRK inhibitors.
What NOT to Use IHC For
EGFR IHC and EGFR FISH have no clinical utility and should not be tested. 1 EGFR mutations must be detected by sequencing methods (NGS or PCR-based assays) to identify specific sensitizing mutations (exon 19 deletions, L858R) that predict TKI response.
Distinguishing Primary Lung Cancer from Metastases
For Suspected Lung Adenocarcinoma
When evaluating adenocarcinoma in the lung, use an appropriate IHC panel to exclude metastatic carcinoma: 1
- TTF-1: Supports primary lung adenocarcinoma (but remember only 60% sensitivity in poorly differentiated cases) 1
- Napsin A: Can be useful in combination with TTF-1, but has limited value when TTF-1 is negative 1
For Cancer of Unknown Primary with Lung Involvement
When evaluating biopsies with adenocarcinoma and suspected lung origin in the context of cancer of unknown primary: 1
- CK7 and CK20 patterns provide initial localization clues
- TTF-1 positivity suggests lung origin
- If CK7-positive and TTF-1-negative with clinical suspicion of lung primary, add SMARCA4 staining
PD-L1 Testing for Immunotherapy
PD-L1 IHC is essential for selecting patients for immune checkpoint inhibitor therapy, but requires drug-specific companion or complementary diagnostic assays. 1, 2 Three separate FDA-approved tests exist:
- 22C3 assay (companion test for pembrolizumab)
- 28-8 assay (complementary test for nivolumab)
- SP142 assay (complementary test for atezolizumab)
Each uses different antibodies, scoring systems, and cutoffs, creating practical challenges in clinical implementation. 1
Practical Workflow Algorithm
Obtain adequate tissue via core biopsy when possible (superior to fine needle aspiration for both IHC and molecular testing) 1
Establish morphologic diagnosis with H&E staining first 1
If NSCLC subtype unclear morphologically: Apply limited IHC panel (TTF-1 and p40 only) 1
Immediately proceed to molecular testing for all adenocarcinomas and NSCLC-NOS for EGFR, ALK, ROS1, BRAF, and other actionable mutations 1
For ALK screening: Positive IHC is sufficient for treatment; for ROS1/NTRK: positive IHC requires molecular confirmation 1
Order PD-L1 testing with the appropriate drug-specific assay based on available treatment options 1, 2
Common Pitfalls to Avoid
- Using EGFR IHC or FISH: These have no role in selecting patients for EGFR TKI therapy 1
- Excessive IHC panels: This wastes tissue needed for molecular testing 1
- Treating ROS1 or NTRK IHC-positive patients without molecular confirmation: Only ALK IHC can stand alone for treatment decisions 1
- Assuming TTF-1 negativity excludes lung adenocarcinoma: 40% of poorly differentiated lung adenocarcinomas are TTF-1 negative 1
- Using the wrong PD-L1 assay: Each checkpoint inhibitor has specific companion/complementary diagnostic requirements 1