P-68 Impact of specimens and fixation status on HER2 assessment in the treatment of advanced gastric cancer (HER_WEEKEND study)

HER2 evaluation of unresectable advanced/recurrent gastric cancer (AGC) is affected by the sample condition. We conducted a single-institutional, retrospective, cohort study to investigate the effect of the formalin fixation conditions, especially the sample submission day (weekday [WD] versus weekend [WE]), and the difference between biopsy specimens (Bx) and surgical specimens (SS) in assessing HER2 expression during chemotherapy for unresectable advanced/recurrent gastric cancer (AGC). This survey included all patients with AGC who underwent HER2 testing at a single Japanese institution from January 2008 to December 2021. We retrospectively collected data on the HER2 status, the specimens used for evaluation, and the day of the week when HER2 testing by reviewing the electronic medical records. The primary endpoint was the HER2 positivity rate, and the difference in expression according to the sample type (Bx versus SS) on sample day (WD and WE) was evaluated. The secondary endpoints were the HER2 positivity rate among all patients; a comparison of the fixation time (FXT) in each sample type (Bx or SS) according to the sample collection date (WD or WE); and FTX and the HER2 positivity rate according to the sample type and collection date. Among the 178 patients, the overall HER2 positivity rate was 19.1% (n = 34). The HER2 positivity rates on WD (n = 131) and WE (n = 47) were 20.6% (n = 27) and 14.9% (n = 7), respectively. On WD, the HER2 positivity rate of SS (n = 42) was inferior to that of Bx (n = 89) [11.9% (n = 5) vs. 24.7% (n = 22)], whereas in WE, the HER2 positivity rate of Bx (n = 27) was inferior to that of SS (n = 20) [11.1% (n = 3) vs. 20.0% (n = 4)]. The median FXT for specimens submitted on WE was 73.3 hours, significantly longer than 29.1 hours on WD (P < 0.01). The FXT on WE is significantly longer than on WD, and there is especially a risk of underestimation of HER2 expression in Bx due to the over-fixation of specimens submitted on weekends.