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

"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.

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.

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.

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.