Abstract: Objective：To compare the dosimetric differences between volumetric modulated arc therapy and intensity modulated radiotherapy for breast cancer patients after breastconserving surgery. Methods: Ten patients who received radiotherapy after breast-conserving surgery were selected. Eclipse planning system was used to design volumetric rotating intensity-modulated (2FRapidArc) and two field intensity-modulated radiation therapy (2F-IMRT) planning for each patient. 2F-RapidArc plans were made using two partial arcs with gantry rotation from 287°－293° to 152°－162°, and 0° to 90° was avoidance sector. The gantry angle of 2F-IMRT were 301°－311° and 125°－135°. The prescription dose was 46 Gy/23 fractions. All plans required 95% of the target volume receiving the prescription dose. The dose distribution of the target, organs at risk, machine unit (MU) and treatment time were compared. Results: 2F-RapidArc and 2FIMRT plans’ uniformity index was 1.12±0.02 and 1.11±0.03 (P=0.282), respectively; conformal index was 0.80±0.03 and 0.65±0.04 (P＜0.001), respectively. V110 of plan target volume was 20.98%±14.47% and 10.43%±10.49% (P=0.030), respectively. Compared with the 2F-IMRT, 2F-RapidArc plans had a higher dosimetric parameters for left lung: V5 (48.06%±17.32% vs. 24.23%±6.56%，P=0.001), V10 (28.89±9.28 vs.17.07±4.78 %，P=0.004), Dmean ［(9.70±2.14) Gy vs. (6.86±1.77) Gy，P=0.002］, increased the double lung: V5 (22.85%±7.55% vs. 11.01%±2.95%，P=0.001), V10 (13.16%±4.33% vs. 7.76%±2.16%，P=0.006), Dmean ［(4.66±0.95) Gy vs. (3.17±0.82) Gy，P=0.001］, reduced the left lung: V40 (3.58%±1.46% vs. 6.19%±3.04%，P=0.006), reduced the double lung: V40 (1.61%±0.64% vs. 2.81%±1.39%，P=0.005), increased cardiac: V5 (39.3%±17.19% vs. 8.79%±4.24%，P＜0.001), V10 (21.31%±13.8% vs. 5.73%±3.42%，P=0.002), V20 (7.80%±6.08% vs. 4.05%±2.85%，P=0.018), Dmean ［(0.64±0.25) Gy vs. (0.29±1.39) Gy，P＜0.001］，reduced the heart: V40(0.50%±0.40% vs. 1.86%±1.94%，P=0.037)，increased the contralateral breast Dmean ［(1.63±1.26) Gy vs. (0.09±0.05) Gy, P=0.004］. Compared with 2F-IMRTplan, 2F-RapidArc increased the treatment time ［(132.9±7.2) s vs. (140.3±11.6) s，P=0.030］. Both the machine units were almost the same ［(467.0±30.4) MU vs. (494.7±44.9) MU，P=0.094］. Conclusion: Both 2F-RapidArc and 2F-IMRT plans could reach the clinical requirements. 2F-RapidArc had a better conformal index, reduced the high dose area, but increased the low dose regions of the lung, heart, body area, and increased the average dose of the contralateral breast. The treatment time of 2F-RapidArc was longer than that of 2F-IMRT, and the MU of 2F-RapidArc and 2F-IMRT plans were almost the same.

Key words: Breast cancer, Intensity-modulated radiotherapy, Volume modulated arc radiotherapy, Dosimetry