Survival impact of adjuvant chemotherapy regimens for small (T1mi/a/b), node-negative (pN0), triple-negative breast cancer (TNBC).

546 Background: While triple negative breast cancer (TNBC) represents an aggressive subtype of breast cancer worldwide, clinically low-risk cases are still encountered. When managing such cases, questions remain regarding the true added benefit of adjuvant chemotherapy on survival outcomes given the absence of dedicated prospective trials for this population. Additionally, in the event that systemic therapy is pursued for these patients, it remains unclear whether anthracycline-containing treatments lead to improved long-term outcomes. Methods: Given the rarity of stage 1 triple negative breast cancer, we extracted recurrence & survival data from within the ASCO-developed CancerLinQ database to perform an in-depth retrospective analysis involving women with small (pT1mi/a/b), node-negative (pN0), TNBC who underwent curative breast surgery and were diagnosed between the years 2002-2023. For the adjuvant chemotherapy recipients, only those who received a regimen of either taxane chemotherapy plus cyclophosphamide (TC) or an anthracycline & cyclophosphamide combined followed by taxane chemotherapy (AC-T) were included in this study. Our co-primary objectives were to compare the invasive recurrence-free survival (iRFS) and overall survival (OS) of patients who received adjuvant TC or AC-T versus those receiving locoregional therapy alone. Our secondary outcome included an iRFS comparison between AC-T, TC, & locoregional therapy. Clinicopathologic variables were compared with appropriate tests for the categorical and continuous variables. Results: Among the 159 patients identified with T1mi/a/b N0 TNBC who met inclusion criteria, 42 had undergone locoregional therapy alone, 77 had received TC chemotherapy, and 40 received AC-T. Baseline demographics found that the locoregional group had a higher proportion of T1mi/a vs T1b patients (p < 0.001) & a higher average age (p < 0.002). No differences were seen between groups in terms of germline mutations (BRCA1, BRCA1, PALB2, CHEK2, & ATM), tumor grade, lymphovascular invasion, surgery type, race, ethnicity, or average body-mass index. After a median follow up period of 57.2 months overall, we found there was a significant benefit in both iRFS (HR 2.52, 95% CI 1.1-5.83, p = 0.025) & OS (HR 6.95, 95% CI 1.62-29.79, p = 0.0027) for those who received adjuvant chemotherapy (TC or AC-T) compared to locoregional therapy alone. The 5-year iRFS was 89.9% with AC-T, 77.1% with TC, & 69.1% with locoregional therapy, whereas the 5-year OS was 96.9%, 96.3%, 85.8%, respectively. Conclusions: These findings suggest that a recurrence & survival benefit is seen with the application of adjuvant chemotherapy, even among this clinically low-risk population. However, whether it needs to be AC-T or TC appears less significant.