Association of Oncotype-DX HER2 Single Gene Score With HER2 Expression Assessed by Immunohistochemistry in HER2-low Breast Cancer.

Background/aim: "HER2-low" is an emerging subtype of breast cancer, with a documented role in predicting response to treatment with novel antibody-drug conjugates. It is defined based on immunohistochemistry, but increasing evidence is challenging this approach as appropriate for identifying the HER2-low subgroup, due to both interobserver variability and limitations of the method itself.

Patients and methods: We retrospectively analyzed data from 430 patients from our departmental databases who had been subjected to an Oncotype-DX score and assessed the correlation of the Oncotype-DX HER2 single-gene score with the HER2 expression on immunohistochemistry. The Oncotype-DX Recurrence Score was also evaluated in the HER2-0 versus HER2-low subgroups.

Results: The HER2 single-gene score was found to accurately correlate with the HER2 result on immunohistochemistry, with a statistically significant difference both between HER2-0 and HER2 +1 tumors (p<0.0001), as well as between HER2 +1 and +2 tumors (p<0.0001). There was no statistically significant difference in the recurrence score between the HER2-0 and the HER2-low subgroups.

Conclusion: Oncotype-DX single-gene scores for HER2 are a potential surrogate marker for assessing the precise HER2 status, with better reproducibility and less interobserver variance compared to immunohistochemistry. The use of rt-PCR emerges as an alternative method of assessment of the HER2-low subgroup.