Is PD‑L1 testing required for a patient with early‑stage triple‑negative breast cancer who is planned for curative surgery and will receive neoadjuvant pembrolizumab (Keytruda) plus carboplatin and paclitaxel followed by pembrolizumab plus doxorubicin and cyclophosphamide?

PD-L1 Testing is NOT Required for Early-Stage TNBC Receiving Neoadjuvant Pembrolizumab

For patients with early-stage triple-negative breast cancer (stage II-III) receiving neoadjuvant pembrolizumab plus chemotherapy followed by adjuvant pembrolizumab, PD-L1 testing is not required. The benefit of pembrolizumab in this setting is independent of PD-L1 expression status.

Evidence-Based Rationale

Early-Stage TNBC: PD-L1 Testing Not Needed

The KEYNOTE-522 trial, which established pembrolizumab as standard of care for high-risk early-stage TNBC, enrolled patients regardless of tumor PD-L1 expression 1. The FDA approval for pembrolizumab in this indication does not require PD-L1 testing 2.

Key findings from KEYNOTE-522:

• Patients experienced event-free survival benefit regardless of tumor PD-L1 status 3
• The benefit from pembrolizumab is independent of PD-L1 status 4
• Overall survival improvement was demonstrated at 75 months follow-up without PD-L1 stratification 5

The 2025 NCCN Guidelines explicitly state that the preferred regimen for stage II-III TNBC is pembrolizumab plus carboplatin and paclitaxel, followed by pembrolizumab plus doxorubicin/cyclophosphamide, then adjuvant pembrolizumab—with no mention of PD-L1 testing requirements 1.

Critical Distinction: Early-Stage vs. Metastatic Disease

This is where confusion often arises. PD-L1 testing requirements differ dramatically based on disease stage:

Metastatic TNBC: PD-L1 Testing IS Required

For locally recurrent unresectable or metastatic TNBC, PD-L1 testing with an FDA-approved test (22C3 assay, CPS ≥10) is mandatory to determine eligibility for pembrolizumab plus chemotherapy 6, 2. The KEYNOTE-355 trial showed benefit was limited to patients with CPS ≥10 7.

Early-Stage TNBC: PD-L1 Testing NOT Required

The neoadjuvant/adjuvant setting operates under completely different evidence, where pembrolizumab benefits all patients regardless of PD-L1 status 4, 3, 8.

Clinical Implementation Algorithm

For your patient with early-stage TNBC planned for curative surgery:

1. Confirm eligibility criteria:

   • Stage II or III disease (tumor >1 cm with nodal involvement OR tumor >2 cm regardless of nodes) 2
   • Triple-negative status confirmed
   • No contraindications to immunotherapy (active autoimmune disease requiring systemic therapy, medical conditions requiring immunosuppression) 4, 2

2. Proceed directly to treatment—no PD-L1 testing needed:

   • Neoadjuvant: 4 cycles pembrolizumab + carboplatin + paclitaxel
   • Then: 4 cycles pembrolizumab + doxorubicin/cyclophosphamide (or epirubicin/cyclophosphamide)
   • Surgery
   • Adjuvant: 9 cycles pembrolizumab monotherapy 1

3. Continue adjuvant pembrolizumab regardless of pathologic response 4, 9

Common Pitfalls to Avoid

Pitfall #1: Ordering unnecessary PD-L1 testing in early-stage disease

• This delays treatment initiation without clinical benefit
• PD-L1 expression shows substantial intratumoral heterogeneity in TNBC, making small biopsy assessment unreliable 10
• Testing adds cost without changing management

Pitfall #2: Withholding pembrolizumab based on negative PD-L1 results

• If PD-L1 testing was performed and is negative, still proceed with pembrolizumab in early-stage disease
• The survival benefit is PD-L1-independent in this setting 3, 5

Pitfall #3: Confusing metastatic and early-stage guidelines

• Always verify disease stage before determining PD-L1 testing requirements
• Metastatic disease = PD-L1 testing required (CPS ≥10)
• Early-stage disease = PD-L1 testing not required

Nuances in Special Circumstances

Stage I TNBC (T1c N0): The standard pembrolizumab regimen is generally reserved for stage II-III disease. For T1c N0 tumors, neoadjuvant therapy may be considered when tumor measures ≥2 cm 9, but the anthracycline-free docetaxel/carboplatin/pembrolizumab regimen is listed as "useful in certain circumstances" rather than preferred 1.

Patients ineligible for pembrolizumab: Those with contraindications to immunotherapy should receive standard chemotherapy regimens without pembrolizumab—again, no PD-L1 testing needed as it won't change management 1.

Monitoring Considerations

While PD-L1 testing isn't required for treatment decisions, patients receiving pembrolizumab require close monitoring for immune-related adverse events throughout treatment, including permanent endocrinopathies 4, 9. Approximately 44% of patients experience immune-related adverse reactions 3.