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.

中文翻译：

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周末活检的风险：标本来源和固定状态对晚期胃癌治疗中 HER2 评估的影响（HER_WEEKEND 研究）

准确评估人表皮生长因子受体2型（HER2）表达对于确定胃癌化疗方案至关重要。然而，福尔马林固定状态已被确定为影响 HER2 评估可靠性的重要因素。这项回顾性队列研究旨在探讨样本采集日（工作日与周末）和来源（活检与手术标本）之间的相关性，以评估不可切除的晚期/复发性胃癌患者的 HER2 表达。数据收集自 2008 年至 2021 年在日本一家公立医院接受化疗的胃癌患者。分析包括 177 名患者（109 名男性，68 名女性），中位年龄为 68.0（21-88）岁，主要研究对象为结果是 HER2 阳性率。总体 HER2 阳性率为 18.1%，工作日 (20.0%) 的阳性率高于周末 (12.8%)。与手术标本相比，工作日活检的阳性率较高（23.9%），但周末的阳性率较低（11.1%）。对于活检和手术样本，在工作日和周末之间观察到福尔马林固定时间的显着差异。研究结果表明，周末较长的福尔马林固定时间可能会导致低估 HER2 表达，尤其是在活检中。因此，在收集样本时，特别是在周末活检期间，必须小心过度福尔马林固定，这一点至关重要。