HER2 Amplification Extends Across Tumor Types, Highlighting New Opportunities for Precision Oncology

Key Takeaways

• HER2 amplification was identified in 5.7% of nearly 90,000 solid tumors in Japan, extending well beyond traditional breast and gastric cancers.
• Prevalence varied widely by tumor type, with the highest rates observed in esophagogastric, urothelial, biliary, and uterine cancers.
• Detection rates differed substantially by testing modality and platform, raising important considerations for clinical interpretation and treatment selection.

A large, nationwide genomic analysis from Japan is reshaping the understanding of HER2 amplification, revealing that this actionable alteration is distributed across a far broader range of cancers than previously appreciated. Drawing on comprehensive genomic profiling (CGP) data from 89,374 patients with solid tumors, investigators provide one of the most expansive real-world assessments of HER2 amplification to date.

The study, based on the Center for Cancer Genomics and Advanced Therapeutics (C-CAT) database, found HER2 amplification in 5.7% of all tumors analyzed. While this figure aligns with prior estimates in aggregate, the scale of the dataset allows for a more granular view of how HER2 alterations are distributed across tumor types.

As expected, high prevalence was observed in esophagogastric cancers (12.9%) and breast cancer (9.5%). However, the analysis highlights several additional tumor types with notable HER2 amplification rates, including bladder and urinary tract cancers (10.6%), biliary tract cancers (8.4%), and uterine cancers (8.4%). These findings, visualized in the bar chart on page 7, underscore that HER2 is not confined to a narrow set of malignancies but instead spans a diverse oncologic landscape.

At a more detailed level, certain rare or histologic subtypes exhibited strikingly high rates of amplification. For example, salivary duct carcinoma reached rates exceeding 40%, while extramammary Paget disease approached 28%, reinforcing the concept of HER2 as a tumor-agnostic biomarker in select contexts.

Beyond prevalence, the study provides insight into how testing methodology shapes detection. Tissue-based CGP accounted for the majority of assays (82.8%) and identified HER2 amplification in 6.5% of cases, compared with just 1.8% using plasma-based testing. Differences across assay platforms were also pronounced: the FoundationOne CDx platform demonstrated a detection rate of 7.4%, substantially higher than other platforms, which ranged from 1.5% to 2.3%.

These discrepancies are not merely technical—they carry clinical implications. Variability in assay sensitivity and amplification thresholds may influence which patients are classified as HER2-positive and therefore eligible for targeted therapies such as trastuzumab deruxtecan. The authors caution that reliance on a single platform or amplification metric, without contextual interpretation, could lead to over- or under-identification of candidates for treatment.

The genomic context of HER2 amplification further complicates the picture. As shown in the heat map on page 10, HER2-amplified tumors exhibited a broader and more complex pattern of co-occurring alterations compared with non-amplified tumors. Notably, KRAS mutations were less frequent in HER2-amplified cancers (13% vs. 28%), suggesting a degree of mutual exclusivity, though not absolute. This observation is clinically relevant, as KRAS mutations have been associated with resistance to HER2-targeted therapies in prior studies.

Importantly, the study also highlights how CGP results translate into clinical decision-making. Among patients with HER2 amplification, 62.2% were recommended genomically matched therapies by expert panels, yet only 23.5% ultimately received such treatments. This gap reflects ongoing challenges in access, timing, and integration of precision oncology into routine care.

Taken together, these findings reinforce a shift toward tumor-agnostic treatment paradigms. HER2 amplification, long associated with breast and gastric cancers, is increasingly recognized as a cross-cutting biomarker that may inform therapy across a spectrum of malignancies. At the same time, the study underscores the importance of standardized testing approaches and careful interpretation of genomic data.

As HER2-targeted therapies continue to expand, large-scale real-world analyses such as this provide critical guidance—highlighting both the promise of precision oncology and the complexities that accompany its implementation.