With the volumetric modulated arc therapy widely used in clinical, there are many literatures at home and abroad comparing its difference with the intensity modulated radiation therapy, including dosimetry, treatment efficiency and dose validation pass rate. A number of previous studies have shown that in head and neck tumors and esophageal cancer, the dose distribution of VMAT is equal to or slightly better than that of IMRT, which can greatly shorten the treatment time and improve the treatment efficiency. But, the previously reported target structure is often relatively simple. The research of Guckenberger and Bortfeld shows that VMAT may not provide enough intensity modulation for more complex targets [13–14, 19–21]. In order to obtain a shorter treatment time, single arc VMAT may over sacrifice the quality of dose distribution; increasing the number of subfields or rotating arcs can improve the quality of dose distribution, However, the treatment time will increase correspondingly .
In the comparative study of radiotherapy techniques for prostate cancer, different conclusions were drawn according to the target areas of different structures and shapes. Studies only including prostate or prostate and seminal vesicle, Bedford et al  have shown that single-arc VMAT had better PTV coverage and less OAR exposure dose than 5-field IMRT; Boylan et al  have shown that single arc VMAT can better protect OAR compared with 5-field IMRT, but the PTV coverage was worse; other reports had shown that VMAT and IMRT have similar dose distribution, and with the number of fixed fields increases, the dose distribution of IMRT will gradually be equal to or even better than VMAT. Yoo et al  used sequential irradiation to compare the dose distribution and treatment efficiency of IMRT with single arc and double arc VMAT. The primary planning target volume contained prostate, seminal vesicles, and pelvic lymph node with a margin. The results showed that IMRT could protect bladder, rectum and small intestine, and had similar HI and CI to 2ARC, slightly better than 1ARC. For the second course plan, 2ARC and IMRT had similar dose distribution. The results of our study aresimilar to Yoo et al: the target area coverage of IMRT is no less than or slightly better than 1ARC, similar to 2ARC; except for Dmean of left and right femoral head and Dmean of bladder, other parameters of OAR are better than 1ARC and 2ARC, and significantly better than 1ARC, which is better protection of the bladder, rectum and small intestine. In addition, those plans were normalized such that the prescription dose covered at least 95% of the PTV, after which one plan with D5 of PTV ≤ 110% and better OAR sparing was selected for each technique. As shown in Fig. 1, IMRT will be exposed to relatively more low fluenceregion of 20 Gy and 30 Gy, and the smoothness of dose curve will be worse. Compared with IMRT, double arc VMAT can significantly reduce the radiation dose of OAR, which is different from this result, the reason may be that the IMRT used in this study is 5F and the number of subfields is less. In general, the more field shots, the better the result.
The biggest advantage of VMAT is to greatly shorten the treatment time. This study shows that compared with IMRT, the average treatment timeof 1ARC and 2ARCshorten by 81.7% and 61%, and also reduce the number of MUs by 70.0% and 67.2% respectively, which can reduce the loss of the treatment machine. Those results are similar to the results reported by Yoo et al , but Quan et al  used AIP algorithm on Pinnacle v9.0 system to compare with VMAT and IMRT, VMAT had 30% more MUs than 8-field IMRT, but the treatment time was reduced 3 minutes. In addition, the three-dimensional dose validation results of three groups plans in this study meet the clinical requirements (γ ≥ 90%). Although VMAT plan is more complex than IMRT plan and involves more parameters (collimator angle, multi-leaf grating, dose rate, gantry rotation speed) in the process of treatment implementation, the measurement results of VMAT plan are better than IMRT, which may be because Arccheck is cylindrical for the phantom, the subfield in VMAT is smaller, and the angle difference of probe dose response is smaller, which makes it more suitable for the measurement of VMAT plan [23–24].
In conclusion, under the condition that the prescription dose covered at least 95% of the PTV standard, whether using IMRT or VMAT technology in simultaneous integrated boost radiotherapy for prostate cancer can satisfy the needs of OAR and have a good dose verification pass rate. Compared with the IMRT plan, VMAT plan can reduce the treatment time significantly and improve the treatment effectively; 2ARC plan has similar target coverage; but the protection of bladder, rectum and small bowel is worse; the less the number of VMAT arcs, the worse the OAR protection, With the number of arcs increases the quality of plan improves, but at the same time, it the number of MUs and treatment time also increases. The results of this study show that for the complex target structure including prostate, seminal vesicle and pelvic lymph node drainage area, the use of IMRT technology can significantly improve the quality of planning, and can better protect the OAR, and is more suitable for the simultaneous integrated boost radiotherapy of prostate cancer pelvic radiation prevention. However, considering that the sample size used in this study is small the results need to be further verified by expanding the sample size.The plan designer needs to compare the advantages and disadvantages of VMAT and IMRT first with a larger sample size for cases with different target size or structure, weigh the gains and losses, and finally select a more appropriate treatment technology.