The survival benefit of anti-HER2 treatment in the management of small (T1mic, T1a, T1b, T1c), node-negative HER2+breast cancer.

532 Background: Limited compelling prospective and retrospective data regarding the added benefit of anti-HER2 therapy in the management of small, node-negative HER2-positive breast cancer (HER2+BC) exists, in part due to differences in outcome reporting, unmatched analyses, and a lack of head-to-head comparisons. As a result, national guideline committees find themselves unable to confidently recommend anti-HER2 therapy and clinicians are left to exercise clinical judgement on whether the use of anti-HER2 therapy should be considered for such patients. Methods: Our team performed a multi-institutional retrospective analysis using the ASCO CancerLinQ database, with a focus on clinical data from small, node-negative HER2+BC patients diagnosed between 2010 to 2021. We compared clinical outcomes between those who received adjuvant trastuzumab therapy, with or without chemotherapy, to those who did not, with our primary outcomes being invasive disease-free survival (iDFS) and overall survival (OS). We performed both a univariate and multivariate analysis, using a Cox proportional hazard model to control for factors including age, ethnicity, body mass index, hormone status, tumor grade, histology type, BRCA status, region, and smoking history. Additionally, a three-arm univariate analysis was performed comparing untreated patients to trastuzumab alone versus combination therapy. Results: In total, 1206 patients met inclusion criteria, including 779 patients who received trastuzumab with or without chemotherapy. We found a statistically significant improvement in both iDFS (HR 0.73, p = 0.01) and OS (HR 0.63, p = 0.027) on univariate analysis for those receiving anti-HER2 therapy. Similarly on multivariate analysis, iDFS (HR 0.75, p = 0.030) and OS (HR 0.61, p = 0.029) were improved in those who received therapy, regardless of tumor size. Our three-arm univariate analysis involving no treatment (n = 427), trastuzumab monotherapy (n = 169), and combination therapy (n = 578) found that iDFS was significantly improved for both treatment arms compared to observation alone (p = 0.006), whereas OS trended towards significance in the treatment arms but did not reach this target (p = 0.061). No significant difference was noted between treatment arms. Conclusions: Our analysis found a statistically significant improvement in iDFS and OS when patients with small, node negative, HER2+BC received adjuvant anti-HER2 therapy with or without chemotherapy as compared to observation. From our univariate three-arm comparison, it appears that trastuzumab provides the majority of benefit to patients in terms of DFS, but this result is exploratory. Further investigation is warranted, including meta-analyses to better characterize the degree of benefit seen with anti-HER2 treatment. For now, this data adds to evidence suggesting added benefit with therapy over observation.