The risk of weekend biopsy: Impact of specimen source and fixation status on HER2 assessment in the treatment of advanced gastric cancer (The HER_WEEKEND study).

Accurate evaluation of human epidermal growth factor receptor type 2 (HER2) expression is crucial for determining chemotherapy regimens in gastric cancer. However, formalin fixation status has been identified as an important factor affecting HER2 assessment reliability. This retrospective cohort study aimed to investigate the correlation between sample collection day (weekday vs. weekend) and source (biopsy vs. surgical specimens) in assessing HER2 expression in patients with unresectable advanced/recurrent gastric cancer. Data were collected from gastric cancer patients who received chemotherapy at a single public hospital in Japan from 2008 to 2021. The analysis included 177 patients (109 men, 68 women) with a median age of 68.0 (21-88) years, and the primary outcome was the HER2 positivity rate. The overall HER2 positivity rate was 18.1%, with higher rates on weekdays (20.0%) compared to weekends (12.8%). Biopsies had higher positivity rates on weekdays (23.9%) but lower rates on weekends (11.1%) than surgical specimens. Significant differences were observed in formalin fixation times between weekdays and weekends for both biopsies and surgical samples. The study findings suggest that longer formalin fixation times on weekends may lead to underestimating HER2 expression, particularly in biopsies. Therefore, it is crucial to be cautious of excessive formalin fixation when collecting samples, especially during weekend biopsies.