Dosimetric comparison of three sequential intensity-modulated radiotherapy planning methods in the Monaco system for postoperative left-sided breast cancer: A planning study

This study aimed to investigate dosimetric differences among three distinct sequential radiotherapy planning techniques within the Monaco treatment planning system for patients undergoing postoperative radiotherapy for left-sided breast cancer. A cohort of 31 patients who underwent breast-conserving surgery for left-sided breast cancer was selected. A baseline initial-phase treatment plan was created for each patient. Subsequently, second-phase sequential plans were designed using three different methods: Bias Plan, Sum Plan, and Rx Plan. A comprehensive comparative analysis of the dose distribution and various planning parameters was conducted using the three methods. A comparative analysis of the three treatment plans revealed statistically significant differences in the dosimetric quality of the PTVboost ( p < 0.05). The metrics assessed were the near-maximum dose (D2 %), conformity index (CI), and homogeneity index (HI). The Bias Plan was found to be superior, demonstrating a significantly higher CI ( p < 0.05) and lower D2 % and HI values ( p < 0.05) than Sum and Rx Plans. Compared directly with the Sum Plan, the Bias Plan improved the CI by 8.6 % and the HI by 5.3 %. Therefore, the Bias Plan provided a more optimal dose distribution with enhanced conformity and homogeneity. An analysis of the dose to organs at risk (OARs) demonstrated significant inter-plan variations ( p < 0.05) for several key metrics: heart D_mean and V 5 Gy , left lung D_mean, right lung V 5 Gy , and spinal cord D_max. In subsequent pairwise analyses, the Bias Plan consistently demonstrated superior sparing of the OARs. The V 5 Gy of the heart was reduced by 2.5 % compared to the Sum Plan and 3.4 % compared to the Rx Plan. Similarly, the mean dose to the ipsilateral (left) lung decreased by 1.0 % and 1.7 %, while the maximum spinal cord dose decreased by 8.2 % and 6.9 % compared to Sum and Rx Plans, respectively. A comparison between Bias and Sum Plans exhibited a statistically significant advantage in heart V 5 Gy ( p = 0.047), although the difference in D_mean was not significant. The analysis also identified that the Rx Plan delivered a significantly higher V 5 Gy to the right lung than the Sum Plan ( p = 0.040). Analysis of dose to normal tissue revealed that the Bias Plan provided the best sparing, with significantly lower irradiated volumes across a range of low-to-intermediate dose levels (V 5 Gy –V 40 Gy ) than the other plans ( p < 0.05). Specifically, the Rx Plan increased the average irradiated normal tissue volume by 2.9 % compared to the Bias Plan. However, this dosimetric advantage was associated with increased plan complexity and reduced delivery efficiency. The number of segments, monitor units (MUs), and estimated treatment delivery time were highest for the Bias Plan and lowest for the Rx Plan. The complexity of the Bias Plan was significantly greater than that of the Rx Plan, requiring 30.97 % more segments, 30.50 % more MUs, and a 7.51 % longer delivery time ( p < 0.001 for all comparisons). These findings highlight the trade-off between optimal protection of normal tissue and treatment delivery efficiency. The treatment planning approach using the Bias Dose method successfully met the target volume dose requirements while enhancing the conformity and rationality of the dose distribution. These findings suggest that the Bias Dose method has considerable potential for clinical application in sequential radiotherapy for breast cancer.