HER2 overexpression/amplification in colorectal cancer: spatial intratumoral heterogeneity as a key determinant of detection discordance and therapy response.

In the IHC cohort (n = 3,585), spatial HER2 expression heterogeneity was comprehensively assessed.

HER2 amplification was detected in 2.53% (34/1,343) of tumors and defined a distinct molecular subset characterized by fewer RAS/KRAS mutations and more frequent CDK12 and TP53 alterations.

Concordance between next-generation sequencing and IHC was higher when applying ToGA criteria (κ = 0.85) compared to the HERACLES criteria, particularly in cases with high-level amplifications (gene copy number ≥ 10).

This heterogeneity represents a key determinant of inter-method discordance and may influence response to HER2-targeted therapy.