PD-L1 as a biomarker in gastric cancer: Concordance between IHC methods.

e15075 Background: The programmed cell death protein 1 ligand (PD-L1) has emerged as a biomarker that determines the progression and prognosis of gastric cancer (GC). Therefore, its detection through immunohistochemical (IHC) studies is essential for the appropriate selection of treatment. Methods: An analytical observational study of a 5-year ambispective cohort involving patients diagnosed with gastric cancer at an oncology center in Colombia, who underwent immunohistochemistry (IHC) studies to detect PD-L1 expression and assess the Combined Positive Score (CPS). This study aimed to compare the detection rate of positivity between the 28-8 and 22C3 assays and their concordance. A descriptive analysis of the variables was conducted. Univariate analysis was performed to determine the frequency of PD-L1 expression and CPS. The kappa index was used to determine the concordance between the results obtained by 28-8 and 22C3. Results: A total of 175 patients diagnosed with gastric cancer (GC) were included in the study, all of whom had a request for positive PD-L1 expression testing. Complete pathological information was available for 155 of these patients. Among those with complete data, 39.4% underwent the 28-8 assay and 60.6% underwent the 22C3 assay. The assays yielded positive results in 34.4% and 28.7% of the cases, respectively. Among the patients with follow-up data (n = 34), the 22C3 assay was conducted, revealing positive PD-L1 expression in 35.3% of cases. In the subsequent clone comparison analysis, 20 patients exhibited PD-L1 positivity in 60% of cases with 28-8. Within this group, 25% had a Combined Positive Score (CPS) of 1-4, 16.7% had a CPS of 5-9, and 58.3% had a CPS greater than 10. The 22C3 assay identified PD-L1 positivity in 40% of cases, with 37.5% having a CPS of 1-4 and 62.5% having a CPS greater than 10. Notably, no cases had a CPS of 5-9. The concordance index between the results of the 28-8 and 22C3 assays was 61%. Conclusions: The antibody 28-8 is able to identify a larger number of patients with positive PD-L1 expression compared to the 22c3 antibody, which may be related to the fact that 28-8 identifies a greater number of cases with a CPS of 5-9 than 22c3. However, both methods agree in 61% of the cases.