Impact of small MU/segment and dose rate on delivery accuracy of volumetric‐modulated arc therapy (VMAT)

Abstract

Volumetric‐modulated arc therapy (VMAT) plans may require more control points (or segments) than some of fixed‐beam IMRT plans that are created with a limited number of segments. Increasing number of control points in a VMAT plan for a given prescription dose could create a large portion of the total number of segments with small number monitor units (MUs) per segment. The purpose of this study is to investigate the impact of the small number MU/segment on the delivery accuracy of VMAT delivered with various dose rates. Ten patient datasets were planned for hippocampus sparing for whole brain irradiation. For each dataset, two VMAT plans were created with maximum dose rates of 600 MU/min (the maximum field size of 21×40 cm2) and 1000 MU/min (the maximum field size of 15×15 cm2) for a daily dose of 3 Gy. Without reoptimization, the daily dose of these plans was purposely reduced to 1.5 Gy and 1.0 Gy while keeping the same total dose. Using the two dose rates and three different daily doses, six VMAT plans for each dataset were delivered to a physical phantom to investigate how the changes of dose rate and daily doses impact on delivery accuracy. Using the gamma index, we directly compared the delivered planar dose profiles with the reduced daily doses (1.5 Gy and 1.0 Gy) to the delivered planar dose at 3 Gy daily dose, delivered at dose rate of 600 MU/min and 1000 MU/min, respectively. The average numbers of segments with MU/segment≤1 were 35±8, 87±6 for VMAT‐600 1.5 Gy, VMAT‐600 1 Gy plans, and 30±7 and 42±6 for VMAT‐1000 1.5 Gy and VMAT‐1000 1 Gy plans, respectively. When delivered at 600 MU/min dose rate, the average gamma index passing rates (1%/1 mm criteria) of comparing delivered 1.5 Gy VMAT planar dose profiles to 3.0 Gy VMAT delivered planar dose profiles was 98.28%±1.66%, and the average gamma index passing rate of comparing delivered 1.0 Gy VMAT planar dose to 3.0 Gy VMAT delivered planar dose was 83.75%±4.86%. If using 2%/2 mm and 3%/3 mm criteria, the gamma index passing rates were greater than 97% for both 1.5 Gy VMAT and 1.0 Gy VMAT delivered planar doses. At 1000 MU/min dose rate, the average gamma index passing rates were 96.59%±2.70% for 1.5 Gy VMAT planar dose profiles and 79.37%±9.96% for 1.0 Gy VMAT planar dose profiles when compared to the 3.0 Gy VMAT planar delivered dose profile. When using 2%/2 mm and 3%/3 mm criteria, the gamma index passing rates were greater than 93% for both 1.5 Gy VMAT and 1.0 Gy VMAT planar delivered dose. Under a stricter gamma index criterion (1%/1 mm), significant differences in delivered planar dose profiles at different daily doses were detected, indicating that the known communication delay between the MU console and MLC console may affect VMAT delivery accuracy.PACS number(s): 87.56.bd, 87.55.‐x

A study of minimum segment width parameter on VMAT plan quality, delivery accuracy, and efficiency for cervical cancer using Monaco TPS

“…When designing VMAT plans to treat cervical cancer, optimization often results in some long and narrow segments that may have a notable impact on plan delivery, and can sometimes lead to a low verification passing rate and even an interruption. The impact of VMAT planning parameters, such as small monitor unit (MU) per segment, dose rate, and control point spacing, on plan quality have been evaluated for a Pinnacle 3 treatment planning system (TPS) using the Elekta Synergy/Varian Trilogy linear accelerator 9, 10. However, there have been no reports regarding MSW optimization in terms of VMAT plan quality, delivery, accuracy, and efficiency.…”

Verification of an optimizer algorithm by the beam delivery evaluation of intensity-modulated arc therapy plans

“…al have made comparisons for cranial irradiation plans, focused on the changing dose rate and MU values and they found that plans with low daily dose, very high dose rate have to be handled carefully. 48 Our results show that the speed parameters of delivery do not predict the results of gamma analysis, as it is illustrated in Figure 4 .…”

Effect of the number of control points on the plan quality of intensity-modulated radiotherapy for nasopharyngeal carcinoma

“…The CP25 group had the lowest plan delivery efficiency, which may be because the small number of MUs increased the complexity of plan delivery. In VMAT planning, an increase in the number of segments with a small area and a small number of MUs has been reported to increase the complexity of plan delivery; this may lead to inaccurate delivery of the planned VMAT dose and may even cause the plan delivery to be interrupted (Huang et al, 2016;Sutton et al, 2012). Our results show that reducing the number of segments and the number of MUs can shorten the time of radiotherapy without significantly reducing the plan quality; this may reduce uncertainty due to patient movement during radiotherapy and increase the biological efficacy of the treatment (Peguret et al, 2013;Zhang et al, 2017).…”