Understanding PD-L1 Testing in Breast Cancer: A Practical Approach

Area of Science:

• Oncology
• Immunology
• Pathology

Background:

• Immune checkpoint inhibitors (ICIs) have significantly advanced cancer therapy.
• PD-L1 expression testing is a prerequisite for some approved ICIs.
• Variability in PD-L1 assays, clones, scoring, and cut-offs presents diagnostic challenges.

Purpose of the Study:

• To summarize recommendations and address technical/histopathological issues of diagnostic PD-L1 assessment.
• To focus on PD-L1 testing in invasive breast cancer (IBC).
• To explain current PD-L1 assays and scores relevant for ICI therapy.

Main Methods:

• Review of diagnostic PD-L1 assessment methodologies.
• Emphasis on immunohistochemical assays and antibody clones.
• Analysis of scoring methods and cut-offs for PD-L1 expression.

Main Results:

• Selecting the appropriate PD-L1 assay and antibody clone is critical.
• In-house assay validation, training, and quality assurance are essential.
• Specific recommendations for PD-L1 testing in IBC are provided.

Conclusions:

• PD-L1 testing requires careful consideration of pre-analytical factors.
• Harmonization studies are needed to validate alternative PD-L1 tests for predicting ICI response.
• Standardized and validated PD-L1 assessment is key for effective ICI therapy in breast cancer.