This study aimed to determine the optimal Biological Effective Dose (BED)-based compensation strategy for treatment interruptions in left-sided breast cancer radiotherapy, with a focus on evaluating cardiac substructures to address a previously unmet clinical need.

Twenty patients with left-sided breast cancer who had received radiotherapy were retrospectively enrolled.

Simulations assumed treatment interruptions (number of interruption days) occurred after the first week, ranging from 1 to 10 days. Three BED-based compensation strategies were evaluated: (A) maintaining total fractions and days while delivering twice-daily treatments; (B) maintaining total days while increasing the dose per fraction; and (C) keeping the dose per fraction constant while extending the overall treatment course. Original uninterrupted plans served as the baseline. BEDs for the planning target volume (PTV), simultaneous integrated boost (SIB), cardiac substructures and other organs at risk (OARs) were calculated. Physical and BED differences among the schemes were systematically compared.

Compared to the original scheme, physical doses to PTV and SIB were lower in Scheme B but higher in Scheme C. As interruptions increased from 1 to 10 days, PTV and SIB doses in Scheme B decreased to minimum values of 42.71 Gy and 50.58 Gy, respectively, while Scheme C resulted in maximum values of 58.60 Gy and 67.15 Gy. Analysis of BED changes (ΔBED) in OARs revealed that the left anterior descending artery (LAD) was the most affected cardiac substructure, with ΔBED values of 0.41, –1.20 and 0.60 for Schemes A, B and C, respectively, at 10 interruption days. Among other OARs, the left lung showed the highest ΔBED changes (0.39, –0.30 and 0.32, respectively). Most OAR comparisons reached statistical significance (ANOVA, p < 0.05).

Compensation strategies for radiotherapy interruptions significantly influence the BED of OARs, particularly in the LAD and left lung. Scheme B most effectively reduced the BED of OARs but requires replanning. Schemes A and C offer clinical convenience at the cost of a higher BED of OARs. The choice of compensation strategy should be individualised based on clinical priorities and patient-specific anatomy.