40 Frequency of Documented IHC Score in Patients With HER2-Negative Breast Cancer in the US: An Observational Study Using Guardian Research Network Data

40 Frequency of Documented IHC Score in Patients With HER2-Negative Breast Cancer in the US: An Observational Study Using Guardian Research Network Data

Background/Significance

For patients with breast cancer, HER2 expression based on immunohistochemistry (IHC) and In Situ Hybridization (ISH) test scores has been interpreted as negative (IHC0, IHC1+, and IHC2+/ISH-) or positive (IHC2+/ISH+ and IHC3+). Data from the DESTINY-Breast04 and DESTINY-Breast06 trials indicate clinical relevance in identifying patients with low (IHC1+ and IHC2+/ISH-) and ultralow (IHC0 with membrane staining) HER2 expression because they could benefit from HER2-directed therapy. The objective of this study was to investigate how frequently HER2 IHC expression is documented in patient medical records and to quantify the proportion of patients with HER2-negative breast cancer who exhibit low and ultralow levels of HER2 expression.

Materials and Methods

This retrospective cohort study utilized electronic medical record data from the Guardian Research Network, which includes data from 7 community health care organizations throughout the US. Patients were eligible for inclusion if they were aged 18 years or older, diagnosed with breast cancer (ICD10 code C50), which was classified as HER2-negative, and had 1 or more clinical evaluation related to their cancer from January 1, 2018, to November 15, 2023. A natural language processing algorithm was applied to structured and unstructured medical record fields to identify HER2-negative patients and documentation of their IHC discrete scores. Patients were classified into subgroups based on the highest IHC score documented during the study period and their breast cancer stage at the time of this score.

Results

The analytic cohort comprised 13,100 patients whose breast cancer was classified as HER2-negative; 10,746 (82.0%) had early-stage breast cancer, 355 (2.7%) locally advanced, and 1,999 (15.3%) metastatic. Most patients (11,174, 85.3%) had documented IHC scores, of which 37.0% were IHC0, 41.3% IHC1+, 20.5% IHC2+, and 1.3% IHC3+. Only one of the 7 health care organizations with patients in the analytic cohort documented the percentage of HER2 IHC staining. Of 514 HER2-negative patients with IHC0 at this center, 51 (9.9%) had 0% staining, 140 (27.2%) had membrane staining (>0% ≤10%), and 323 (62.8%) did not have documented percentage staining.

Conclusion

Results from this study indicate that, despite clinical recommendations at the time of the study, 1 in 6 HER2-negative patients do not have a documented discrete IHC score, and very few have granular information on the observed percentage of HER2 expression.