Abstract

Background: Immunohistochemical (IHC) staining for HER2 on paraffin sections stored for prolonged periods at room temperature may inaccurately reflect a patient’s HER2 status. This study aims to evaluate HER2 expression on paraffin sections of breast cancer stored at laboratory room temperature (20-25°C) after 1, 2, 4, 8 and 16 weeks. Materials and Methods: Sixty-three cases of HER2-positive (3+) breast cancer were studied. Paraffin sections were stained for HER2 by IHC after storage at 20-25°C for 1, 2, 4, 8, and 16 weeks. HER2 expression was assessed based on staining intensity and the percentage of tumor cells showing complete and strong membrane staining. Results: HER2 expression decreased progressively with longer storage time of paraffin sections at room temperature. After 1 week, all 63 samples (100%) remained HER2 3+. After 2 weeks, the proportion of 3+ cases decreased to 95.24%, with 4.76% converting to 2+ (p = 0.83). After 4 weeks, 85.71% of samples were 3+, with 12.70% showing 2+ and 1.59% showing 1+ (p = 0.04). At 8 weeks, 61.90% were 3+, 33.33% were 2+, and 4.76% were 1+ (p < 0.01). After 16 weeks, only 28.57% remained 3+, with 55.55% at 2+, 14.29% at 1+, and 1.59% at intensity 0 (p < 0.01). The mean percentage of tumor cells with complete and strong membrane staining (relative to week 0) decreased from 100% to 95.03%, 86.24%, 64.69%, 35.12% and 16.34% after 1, 2, 4, 8, and 16 weeks, respectively. Conclusion: This is the first study in Vietnam on the influence of storage time of paraffin sections on HER2 expression in breast cancer. Sections should be stained immediately after cutting or stored for no longer than 2 weeks at laboratory room temperature (20-25°C).