Which molecular studies should be obtained for a patient with adenocarcinoma of the gastro‑esophageal junction?

Molecular Studies for GE Junction Adenocarcinoma

All patients with gastroesophageal junction adenocarcinoma should undergo mandatory testing for HER2, PD-L1 CPS, and MSI/MMR status at diagnosis, with CLDN18.2 testing strongly recommended if advanced/metastatic disease is documented or suspected. 1, 2

Essential Biomarker Panel

The 2024 Upper GI Oncology Summit guidelines provide strong evidence (Level C) for a comprehensive molecular workup that directly impacts survival outcomes through targeted therapy selection 1:

1. HER2 Testing (Mandatory)

• Method: Immunohistochemistry (IHC) with reflex to FISH if IHC 2+
• Positive definition: IHC 3+ OR IHC 2+ plus FISH-positive
• Clinical significance: HER2-positive tumors occur in approximately 32% of GE junction adenocarcinomas (higher than the 21% seen in gastric cancer) 1
• Treatment impact: Trastuzumab-based therapy shows HR 0.67 for overall survival specifically in GE junction tumors, superior to gastric cancer (HR 0.76) 1
• Timing: Should be performed on biopsy or resection specimens before initiating therapy; cell blocks from FNA are acceptable alternatives 3

2. PD-L1 Combined Positive Score (CPS) - Mandatory

• Method: PD-L1 IHC 28-8 pharmDx assay
• Calculation: (PD-L1+ tumor cells + immune cells) / total viable tumor cells × 100
• Threshold: CPS ≥5 predicts benefit from nivolumab plus chemotherapy
• Clinical significance: ~60% of patients have CPS ≥5 1
• Treatment impact: In GE junction tumors with CPS ≥5, nivolumab plus chemotherapy shows HR 0.82 for overall survival 1

3. MSI/MMR Status (Universal Testing Required)

• Method: PCR/NGS for MSI OR IHC for MMR proteins (MLH1, MSH2, MSH6, PMS2)
• Prevalence: 3-5% in gastric adenocarcinoma; lower frequency in GE junction tumors 1
• Treatment impact: MSI-high tumors show dramatic benefit from immunotherapy (HR 0.38 for overall survival with nivolumab plus chemotherapy vs. 0.78 for microsatellite-stable) 1
• Note: Pembrolizumab is highly effective in MSI-high tumors regardless of line of therapy 1

4. CLDN18.2 Testing (Strongly Recommended for Advanced Disease)

• Method: IHC with specific CLDN18.2 antibody
• Positive definition: ≥75% tumor cells with moderate-to-strong membranous staining
• Prevalence: 38.4% of gastric/GE junction adenocarcinomas 4
• Treatment impact: Zolbetuximab plus chemotherapy in CLDN18.2-positive, HER2-negative tumors 1
• Important caveat: Subgroup analysis shows HR 1.07/1.01 in GE junction tumors (less benefit than gastric cancer at 0.67/0.72), but still represents a treatment option 1
• Current limitation: Not yet commercially available in all regions 1

Testing Algorithm by Disease Stage

Early-Stage/Resectable Disease:

• HER2, MSI/MMR status at diagnosis 2, 5
• Consider NGS via validated assay 2
• PD-L1 and CLDN18.2 testing optional but increasingly relevant with emerging perioperative immunotherapy data 5, 6

Advanced/Metastatic Disease:

• All four biomarkers (HER2, PD-L1 CPS, MSI/MMR, CLDN18.2) before first-line therapy 1, 2
• NGS should be strongly considered to identify additional actionable alterations 2, 7

Critical Technical Considerations

Specimen requirements 4, 3:

• Biopsy or resection specimens preferred (92.9% concordance between types)
• Primary tumor or metastatic sites acceptable
• Cell blocks from FNA are acceptable for HER2
• CLDN18.2 scoring applies only to invasive adenocarcinoma, not precursor lesions (intestinal metaplasia/dysplasia)

Common pitfalls to avoid 4:

• CLDN18.2: Do not score precursor lesions; requires moderate-to-strong membranous staining in ≥75% of tumor cells
• HER2: Use gastric-specific scoring criteria (different from breast cancer)
• PD-L1: Must include both tumor cells and immune cells in calculation
• MSI/MMR: If IHC shows loss of MLH1/PMS2, reflex to BRAF V600E and MLH1 promoter methylation to distinguish Lynch syndrome from sporadic cases

Treatment Selection Hierarchy

Sequential testing approach based on results 1:

1. If HER2-positive: Trastuzumab + chemotherapy (first priority)
2. If HER2-negative + PD-L1 CPS ≥5: Nivolumab + chemotherapy
3. If HER2-negative + CLDN18.2-positive: Zolbetuximab + chemotherapy (note limited benefit in GE junction subgroup)
4. If MSI-high: Pembrolizumab monotherapy or with chemotherapy (effective across all lines)

Emerging Considerations

Next-generation sequencing provides comprehensive profiling including HER2 amplification, MSI status, tumor mutational burden, and additional actionable alterations (PIK3CA, FGFR2, MET amplification) 2, 8, 7. Feasibility studies show 89% of patients have at least one actionable alteration by NGS 7.

Molecular heterogeneity: GE junction tumors demonstrate both esophageal-like (31%) and gastric-like (69%) molecular profiles, with implications for ERBB2/EGFR targeting 9, 10. This biological diversity underscores the importance of comprehensive biomarker testing rather than extrapolating from gastric or esophageal cancer paradigms.