PD-L1 Expression Heterogeneity in Non–Small Cell Lung Cancer: Defining Criteria for Harmonization between Biopsy Specimens and Whole Sections

INTRODUCTION: Determination of programmed death ligand 1 (PD-L1) expression defines eligibility for treatment with pembrolizumab in patients with advanced NSCLC. This study was designed to better define which value across core biopsy specimens from the same case more closely reflects the PD-L1 expression status on whole sections and how many core biopsy specimens are needed for confident classification of tumors in terms of PD-L1 expression. METHODS: We built tissue microarrays as surrogates of biopsies collecting five cores per case from 268 cases and compared PD-L1 staining results obtained by using the validated clone SP263 with the results obtained by using whole tumor sections. RESULTS: We found an overall positivity in 39% of cases at a cutoff of 1% and in 10% of cases at a cutoff of 50%. The maximum value across cores was associated with high concordance between cores and whole sections and the lowest number of false-negative cases overall. To reach high concordance with whole sections, four and three cores are necessary at cutoffs of 1% and 50%, respectively. Importantly, with 20% as the cutoff for core biopsy specimens, fewer than three cores showed high sensitivity and specificity in identifying cases with 50% or more of tumor cells positive for PD-L1 on whole sections. Specifically, for PD-L1 expression values of 20% to 49% on cores, the probabilities of a tumor specimen expressing PD-L1 in at least 50% of cells on a whole section were 46% and 24% with one and two biopsy specimens, respectively. CONCLUSIONS: An accurate definition of the criteria to determine the PD-L1 status of a given tumor may greatly help in selecting those patients who could benefit from anti-programmed cell death 1/PD-L1 treatment.