Population based assessment of HER-2 and PD-L1 positivity in gastric and gastroesophageal junction adenocarcinomas in a Canadian population.

Abstract

e16038 Background: Current standard of care for advanced gastric and gastroesophageal cancers combines systemic chemotherapy with either human epidermal growth factor receptor 2 (HER-2) or programmed death ligand 1 (PD-L1) targeting therapy. New data is emerging that for patients whose tumours are positive for both HER-2 overexpression and have a PD-L1 combined positive score (CPS) >1%, combining all three treatment modalities results in superior outcomes. There is limited international data, and no Canadian data, exploring the frequency of patients who have tumours that are both HER-2 and PD-L1 positive who will be eligible for this novel treatment approach. Objective: To determine the proportion of patients in Nova Scotia with HER-2 positive gastric and GEJ adenocarcinomas who have a PD-L1 CPS score above established clinical thresholds. Methods: A retrospective review of PD-L1 and HER-2 status for all patients diagnosed with gastric and GEJ adenocarcinomas in Nova Scotia between June 3, 2022 and August 13, 2023 was completed. PD-L1 CPS was evaluated using the 28-8 assay and was considered positive if >1%. HER-2 overexpression was evaluated by immunohistochemistry and in cases with equivocal expression, status was based on fluorescence in situ hybridization. Results: Over the study period, 56 cases of gastric and 42 cases of GEJ adenocarcinoma were identified for a total of 98 cases. The median age of the study population was 73 years (range 38 - 91). 73 patients (74%) were male and 25 patients (26%) were female. HER-2 expression was evaluated in 92 cases, and PD-L1 CPS was evaluated in 95 cases. 19 cases (21%) were HER-2 positive and 86 cases (91%) were PD-L1 positive. 17 HER-2 positive cases were also PD-L1 positive (89%). Conclusions: This study provides population level data in a Canadian context on patients with gastric and GEJ adenocarcinoma with respect to HER-2 overexpression and PD-L1 status. This data will be helpful in planning allocation of clinical and financial resources as a new standard of care for these patients is established.