SU‐E‐T‐31: Alternative VMAT Technique Reduces Total Monitor Units for Lung SBRT

Abstract

Purpose: To investigate an alternative approach to VMAT optimization for hypofractionation lung treatment which increases average aperture opening and results in lower total Monitor Units (MU) without significantly sacrificing plan quality. Methods: Benchmark Volumetric Modulated Arc Therapy (bVMAT) plans were generated for 10 lung Stereotactic Body radiotherapy (SBRT) cases using Eclipse Version 11.0.42 (Varian Medical Systems) without a maximum MU constraint. Prescriptions ranged from 40 to 54Gy in 3 to 5 fractions. AAA dose calculation and PRO fluence based optimization was utilized. Two comparison VMAT plans were generated for each case, one that forced an initial “open” mlc aperture conformal to the tumor as a starting condition (oVMAT) with similar optimization parameters and arc geometries, and one that repeated the bVMAT optimization but added a maximum MU constraint (muVMAT). All plans used two arcs with lengths between 168 to 230 degrees. PTV D 95% and Dmean, lung V20 Gy, chest wall V30 Gy, average aperture opening and MU's were compared. Statistical significance was evaluated using Wilcoxon signed rank test. Results: Average PTV D(95), PTV mean and lung V20Gy over all plans was 99.2 ± 1.7%, 103.3 ± 0.6% and 7.8 ± 2.4% respectively. The average chest wall V30Gy was 61 ± 61 cc and ranged between 0 to 166 cc. There were no significant differences between the three techniques for the dosimetric quantities. MUs were reduced by 11 ±11% (p<0.01) and 25 ± 5% (p<0.01) and the average aperture size was increased by 13.7 ± 14% (p=0.02) and 35.8 ± 10% (p<0.01) with muVMAT and oVMAT, respectively, compared to bVMAT. Conclusion: oVMAT and muVMAT techniques were both able to increase average aperture size and reduce total MU compared to the benchmark VMAT plan, but the magnitude of the changes observed for oVMAT was larger.

Verification by treatment plan and physical measurement of the monitor unit (MU) objective function for stereotactic ablative body radiotherapy (SABR) lung planning

“…From our results, it can be seen that there is no major difference in the plan quality with and without the MU objective function in the optimisation which agrees with the work of Clemente et al (2013), Happersett et al (2014) and Huang et al (2015). This has been verified both in terms of treatment plan and measurement comparison.…”

“…It is The total number of MUs were reduced by 23% by using the MU objective function. This is significantly higher than what has been reported in the study by Happersett et al (2014) study (11%). Similarly, average MU/Gy reduced from 269.0 to 204.9 which is significantly better than Huang et al (2015) (228 MU Gy −1 ) have reported.…”

“…The purpose of this study was to investigate, by physical measurement on a treatment machine, the accuracy of real plan deliveries following the use of the MU objective feature in the Eclipse TM TPS to reduce plan complexity. This was performed for NSCLC SABR plans; the planning technique used was similar to the papers described above (Clemente et al 2013, Crowe et al 2014, Happersett et al 2014 however the plans were calculated using the Acuros ® XB algorithm. The impact on delivered accuracy was assessed using the Compass TM system (MatriXX array in conjunction with the Compass TM software, IBA Dosimetry, Schwarzenbruck, Germany).…”