Peri-treatment Creatinine-Based eGFR Trajectories in Patients Receiving Breast Radiotherapy: A Retrospective Cohort Study

BACKGROUND: Longitudinal estimated glomerular filtration rate (eGFR) changes during breast cancer treatment remain incompletely characterized. We examined peri-treatment eGFR trajectories, factors associated with eGFR decline, whether 1-month eGFR change was associated with later decline, and differences by radiotherapy fractionation. MATERIALS AND METHODS: We retrospectively included 741 patients receiving breast radiotherapy from 2023 to 2025. eGFR was assessed at baseline and 1, 3, 6, 9, and 12 months. The primary endpoint was a ≥ 20% decrease from baseline confirmed at 2 consecutive scheduled visits; ≥30% decrease was the sensitivity endpoint. Trajectories were analyzed with a five-segment linear mixed-effects model. Cox regression evaluated associated factors, a temporally separated 1-month landmark analysis assessed later decline, and fractionation comparisons used multivariable adjustment and overlap weighting. RESULTS: 58 patients (7.8%) met the confirmed ≥20% endpoint and 24 (3.2%) met the ≥30% endpoint. Trajectories diverged most clearly within 3 months. N1-3 disease (HR, 2.60; 95% CI, 1.11-6.12) and recorded CDK4/6 inhibitor use (HR, 8.73; 95% CI, 4.68-16.27) were associated with the endpoint. One-month eGFR decline was not associated with subsequent confirmed ≥20% decline (HR, 1.04 per 10% decline; 95% CI, 0.77-1.41; P = 0.786) but was associated with subsequent confirmed ≥30% decline (HR, 1.61; 95% CI, 1.06-2.44; P = 0.025). No significant renal-risk difference was observed by fractionation scheme. CONCLUSION: Peri-treatment creatinine-based eGFR trajectories diverged early in patients who later met the confirmed-decline endpoint. Nodal stage and recorded CDK4/6 inhibitor use were associated with the endpoint; 1-month eGFR change was associated with the ≥30% sensitivity endpoint.