The Effect of Maximum MLC Speed and Dose Rate Constraints on VMAT Plan Quality, Accuracy and Efficiency
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Volumetric modulated arc therapy (VMAT) is an efficient and conformal radiation therapy technique. It accomplishes this by dynamically varying multi-leaf collimator (MLC) positions, dose rates and gantry velocity. This work investigated the effect of varying the maximum MLC speed and maximum dose rate on the quality, efficiency and accuracy of treatment plans. The Pinnacle3 SmartArc treatment planning software was used to generate plans on prostate and head and neck (H&N) sites. A range of maximum MLC leaf speeds (0.55 cm/s to 2.20 cm/s) and maximum dose rates (200 MU/min to 600 MU/min) restrictions were applied to each plan to investigate their effect on the treatment quality, efficiency and accuracy. Each plan had their monitor units (MU) per fraction, delivery time, mean dose rate and leaf speed analysed. The dose volume histogram (DVH) data was used in the assessment of the conformity, homogeneity and plan quality. The treatments were delivered on Varian iX accelerator equipped with 120-leaf millennium MLC. Quality assurance measurements were performed using the ArcCHECK™ 3D diode array and results were assessed based on gamma analysis of dose fluence maps, beam delivery statistics and Dynalog data. The number of VMAT fields was found to be a key factor in how significant the maximum MLC leaf speed affected the plan parameters investigated. Single arc treatments were shown to have lower MU, dose rate and plan quality, while also exhibiting a slight increase in estimated delivery time. For dual arc treatments, MU, delivery time, dose rate and plan quality were largely independent of the maximum MLC speed allowed. The QA showed that higher MLC leaf speeds were prone to an increase in the discrepancy between planned and delivered control point (CP) fluence and higher MLC positioning errors. None of these were at a clinically significant level, and the overall fluence distribution and point dose comparisons were independent of maximum MLC leaf speed. The only clinically significant effect that modulation of the maximum dose rate had was on the delivery time. Lower maximum dose rates resulted in longer treatment delivery, which is an important consideration in minimising the intra-fractional motion during treatment. The results of the MLC leaf speed evaluation showed that the lower the maximum leaf speed the more accurate the delivered treatment, -however the quality of the plan is reduced. This indicates that there could be an optimum maximum MLC leaf speed which produces high quality plans that can be accurately delivered. Based on this work a maximum MLC leaf speed of 1.38 cm/s was shown to have no reduction in plan quality however it showed improvement in delivery accuracy. There was no justification found for reducing the maximum dose rate below the recommended 600 MU/min.