Association between treatment delay and prognosis among breast cancer patients: A population-based cohort study

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.