Abstract Background: Treatment delays in breast cancer may adversely affect survival. However, the impact of such delays on survival across stages remains poorly characterized. Objectives: To evaluate the association between stage-specific treatment delay after diagnosis and 5-year survival among women with breast cancer. Methods: A retrospective cohort study was conducted using the Korean-Clinical Data Utilization network for Research Excellence (K-CURE) breast cancer public library sample data (2012-2022). Breast cancer was identified using the International Classification of Diseases for Oncology, 3rd Edition (ICD-O-3 code, C50). Treatment delay was defined as the interval from diagnosis to the initiation of the first treatment, categorized into four groups: no treatment, 0-30 days (reference), 31-60 days, and ≥61 days. Patients were followed from the date of first diagnosis until death, 5 years after diagnosis or the end of study period whichever occurred first. Covariates included demographics, several lifestyle factors, and comorbidities. Stage-stratified adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) for death were estimated using Cox proportional hazards models. Sensitivity and stratified analyses were conducted based on prior cancer history and treatment combinations received during follow-up. Results: The study included 7, 335, 6, 554, 1, 941, and 665 patients with stage I, II, III, and IV breast cancer, respectively. Among treated patients, 5-year survival ranged from 96. 9% to 97. 5% in stage I, 93. 1% to 94. 5% in stage II, 71. 9% to 82. 0% in stage III, and 34. 8% to 46. 5% in stage IV. Compared with treatment initiation within 30 days, no treatment was significantly associated with poorer prognosis across all stages, whereas initiation after 31-60 days or ≥61 days was not. However, among patients with stage I or II who received combined surgery, radiotherapy, and chemotherapy, treatment delays were associated with a slightly higher risk of death (stage I with a 31-60-day delay: aHR 1. 99, 95% CI 1. 00-3. 95; stage II with a ≥61-day delay: aHR 1. 91, 95% CI 1. 07-3. 41). Conclusions: Untreated patients had poorer outcomes across all stages compared with treated patients. By contrast, prognosis did not differ significantly between patients who initiated treatment within 1 month of diagnosis and those who initiated treatment thereafter. However, treatment delays among patients requiring triple-modality therapy may adversely affect survival, warranting further large-scale studies to validate these findings. Citation Format: Hee-Jin Kim, Ji Yoon Baek, Soobin Lee, Aesun Shin. Association between treatment delay and prognosis among breast cancer patients: A population-based cohort study abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB393.
Kim et al. (Fri,) studied this question.