Study design and setting

This was a retrospective cohort study conducted via chart review at the Huntsman Cancer Institute (HCI), which is part of the University of Utah Healthcare System and the only National Cancer Institute (NCI)-designated Comprehensive Cancer Center serving Utah, Idaho, Montana, Nevada, and Wyoming within the Intermountain West region in the United States of America.

Data sources

Data sources that were utilized for this study include the University of Utah Clinical Enterprise Data Warehouse (EDW), the Utah Cancer Registry (UCR) and Huntsman Cancer Institute Tumor Registry (HCI-TR). The EDW is a database encompassing medical, financial, and administrative data of patients across the University of Utah Healthcare System and HCI with records dating back to 1993. The UCR and HCI-TR track cancer cases in Utah and HCI, respectively, and collect information on diagnosis, follow-up, and mortality. UCR is also part of the Surveillance, Epidemiology, and End Results (SEER) Program of the NCI.

Study population

The population of interest was patients with advanced breast cancer (stages IIIB, IIIC and IV) who were tested for HER2 receptor status. The inclusion criteria were (i) aged 18 years and older at the time of advanced breast cancer diagnosis; (ii) diagnosis of breast cancer reported to the HCI-TR between January 1, 2010 and December 31, 2019; (iii) at least 2 encounters with relevant ICD-9 or ICD-10 codes for breast cancer (174.x, C50.x) on two dates separated by ≥30 days between January 1, 2010 and December 31, 2019 in EDW; (iv) stages IIIB, IIIC, and IV documented in the HCI-TR based on pathologic classification or if unavailable, clinical classification. Patient eligibility was based on staging at advanced diagnosis regardless of whether the tumor was potentially resectable. Patients were excluded if a diagnosis of a primary cancer other than breast cancer was reported to the HCI-TR between January 1, 2010 and December 31, 2019. Patients were followed up from the date of advanced diagnosis to death, last documented follow-up or December 31, 2021, whichever occurring earlier.

Study procedures

Eligible patients were identified from the HCI-TR using a combination of ICD-9 and ICD-10 codes for breast cancer (174.x, 233.0; C50.x, D05.x) and staging information. HER2 testing was identified using Current Procedural Terminology (CPT) codes (2007332, 0049178, 0049174, 2008603) and HER2 status was ascertained through chart review of laboratory reports and clinical notes using an electronic text-searching tool. Patient data was extracted from electronic health records via chart review.

Outcomes of interestPrevalence of HER2 categories

Pathology reports post advanced stage diagnosis were reviewed chronologically to collect data on pathology assessment. All study subjects were categorized based on the results of pathological assessments between the date of advanced disease diagnosis and prior to first line treatment initiation. If multiple assessments were conducted prior to treatment initiation, the highest HER2 expression or copy number was utilized. Patients with an IHC score of 0 were classified as HER2 IHC0; patients with an IHC score of 3+ or an IHC score of 1+/2+ and a positive FISH result were classified as HER2 positive; while patients with an IHC score of 1+ and a negative or unknown FISH result or an IHC score of 2+ and a negative FISH result were classified as HER2-low. Patients with an IHC score of 2+ but no FISH results documented were considered to have insufficient pathology results to be classified based on our study algorithm and excluded from analysis (Supplementary Information 1). Guidelines by the American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) to interpret IHC and FISH have slightly changed in 2013 and 2018. IHC. However, IHC and FISH interpretation in our study was based on contemporary guidelines at the time of HER2 testing, which would have guided decisions on the use of HER2-directed therapy for our study participants. All pathology testing was also performed by accredited laboratories with appropriately reactive controls and adhered to the necessary standards.

Baseline demographic and clinical characteristics

Baseline demographic characteristics of interest included: Age, ethnicity, insurance type, region of residence and smoking status. Baseline clinical characteristics of interest included: menopausal status, TNM staging at advanced diagnosis, histological type and grade, site of metastasis, hormone receptor (HR) status, performance status and prevalence of individual comorbidities within the Charlson Comorbidity Score. Characteristics were reported by HER2 category. Patients were HR positive if IHC results indicate at least 1% of tumor nuclei stain positive for either ER and/or PR.

Treatment patterns

To examine contemporary treatment patterns, first and second line treatment patterns relative to diagnosis of advanced disease were assessed for patients with HER2-low disease and received treatment for advanced disease in the last five years of the study period, from January 1, 2017 onwards. The period for analysis was limited to account for major changes in the standard of care for advanced breast cancer patients with HER2-negative, HR-positive disease where by 2017, two CDK4/6 inhibitors had been approved for use in this patient subpopulation. A new line of therapy was defined as a distinct change of an entire regimen due to intolerance, adverse events or disease progression based on documentation in the medical records. Maintenance therapy was not considered a new line of therapy. Among patients who were diagnosed with locally advanced (stage IIIB or IIIC) disease, some patients presented with potentially resectable tumors and were managed with neoadjuvant therapy, followed by surgery if response was deemed adequate. However, as surgery data was not collected in our study, it was not feasible to isolate these patients. Therefore, treatment patterns were reported by staging at diagnosis of advanced breast cancer (IIIB or IIIC vs IV) in addition to hormone receptor status. Treatment patterns of the full patient cohort is also included in Supplementary Information 2.

Survival outcomes

Overall survival (OS) and progression-free survival (PFS) of patients with HER2-low disease and received treatment for advanced disease in 2017 or later were evaluated. OS was defined as the time from initiation of first line treatment after diagnosis of advanced disease to death, with censoring performed at date of last follow-up or the end of study (December 31, 2021), whichever occurring earlier. PFS was defined as the time from initiation of treatment (first or second line after diagnosis of advanced disease) to death or disease progression, with censoring performed at the date of last follow-up or the end of study, whichever occurring earlier. Assessment of disease progression was based on clinician documentation in clinical notes. Survival outcomes of the full patient cohort is also included in Supplementary Information 3.

Statistical analysis

The baseline characteristics and prevalence of HER2 categories of all advanced breast cancer patients were summarized with descriptive statistics, including means and standard deviations for continuous variables and counts and percentages for categorical variables. The baseline demographic and clinical characteristics of patients with HER2-low disease were compared against those who were HER2 IHC0 using t-tests, Wilcoxon rank-sum tests, Chi-squared and Fisher’s exact tests depending on the type of variable evaluated. Treatment patterns of patients with HER2-low disease were also presented descriptively with Sankey diagrams illustrating treatment pathways from first line to second line settings, stratified by stage IIIB/IIIC vs stage IV disease and hormone receptor status. Kaplan-Meier analysis was conducted to quantify OS and PFS of patients with HER2-low disease and had first line treatment initiated in 2017 or later. Median survival time and OS and PFS rate at 1, 2 and 3 years, stratified by staging and hormone receptor status, were presented.