Defining the patient population with HER2-low breast cancer in Hawaii

e18560 Background: About half of HER2-negative tumors (immunohistochemistry (IHC) 0, IHC 1+, or IHC 2+ without in-situ hybridization (ISH) amplification) are HER2-low (IHC 1+ or IHC2+ without ISH amplification). These tumors have gained attention due to their responsiveness to HER2-directed antibody-drug conjugates. Most studies have found a positive association between HER2-low and hormone receptor (HR) expression. Few studies report the demographics of patients that develop HER2-low tumors, and those that do were performed in ethnically homogeneous populations. Our study aims to characterize patients with HER2-low tumors in Hawaii, the US state with the highest diversity index, to better understand the patient population affected by HER2-low tumors. Methods: A retrospective chart review at a major health system in Hawaii evaluated all breast cancer specimens with HER2 testing performed between January 2021 and June 2022. Patient demographics at the time of diagnosis were collected, including age, menopausal status, body mass index (BMI), race/ethnicity. Tumor characteristics were identified by searching pathology reports and included: HER2 status-IHC score and ISH amplification, HR status, histologic type and grade, clinical stage, and 21-gene recurrence score. Results: Clinical and pathologic data was available for 332 cases of HER2-low breast cancers out of 1,478 cases. Of these, 309 (93.1%) were HR+, and 23 (6.9%) were HR-. In the HER2-low, HR+ group 53 (17.2%) were age < 50 and 256 (82.8%) were age 50+, compared to the HER2-low, HR- group with 2 (8.7%) patients and 21 (91.3%) patients respectively. Of all patients with HER2-low tumors, 99 (29.8%) were Japanese, 64 (19.3%) were White, 60 (18.1%) were Filipino, and 47 (14.2%) were Native Hawaiian. This distribution remained constant irrespective of HR status. Those with a BMI of < 25 kg/m 2 made up 39.7% of HER2-low patients, and those with a BMI > 25 kg/m 2 made up 54%. Of 159 HR+ cases that had a 21-gene recurrence score, 133 (83.7%) of these had a low score (£25). Most HER-2 low tumors were < 2cm, grade 1 or 2, and stage 1A. Conclusions: Our data confirmed the strong relationship between HER2-low and HR+ expression. As expected, most patients were age 50+, although there were more patients younger than 50 in the HER2-low, HR+ group compared to the HR- group. The ethnic distribution of HER2-low tumors is consistent with that of the general population of Hawaii, there does not appear to be any group more at risk than another. [Table: see text]