In-Breast Tumor Progression During Neoadjuvant Chemotherapy: Impact on and Factors Influencing Distant Recurrence-Free Survival.

Background: Most patients with breast cancer treated with neoadjuvant chemotherapy (NAC) experience clinical benefit, however, a small proportion progress. We aimed to characterize factors predicting in-breast tumor progression and impact on distant recurrence.

Patients And Methods: We reviewed all patients with clinical stage I-III breast cancer treated with NAC in 2006-2021 at our institution. We compared in-breast progressive disease (PD), defined as ≥ 20% increase in tumor size, with stable disease (SD) or response. Distant recurrence-free survival (DRFS) was analyzed using the Kaplan-Meier method and Cox proportional hazards regression.

Results: Of 1403 patients, 70 (5%) experienced in-breast PD, 243 (17%) SD, 560 (40%) partial response (PR), and 530 (38%) breast pathologic complete response (breast pCR, ypT0/Tis). The rate of PD varied by tumor subtype (8% in HR+/HER2-, 5% TNBC, 2% HER2+, p < 0.001). With median 48 months follow-up, the rates of DRFS were significantly different according to clinical breast response as follows: PD 56%, SD 68%, PR 82%, or breast pCR 93%, p < 0.001. In patients with PD on multivariable analysis, post-NAC grade (adjusted HR 2.9, p = 0.002) and ypT3-4 category (adjusted HR 2.4, p = 0.03) were the strongest predictors of DRFS. Combining these factors, 23% had neither, 44% had one, and 33% had both, which stratified outcome in PD with 3-year DRFS of 100%, 77%, and 30%, respectively (p < 0.001).

Conclusions: While in-breast PD during NAC is uncommon (5%), it predicts poor survival. Among patients with in-breast PD, post-NAC tumor grade and T category predict outcomes and may be useful to guide treatment escalation.