Clinical Implementation of Volumetric Modulated Arc Therapy (VMAT) and Image Guided Radiotherapy (IGRT) Techniques

Abstract

Volumetric Modulated Arc Therapy is a novel treatment delivery technique, used to deliver intensity modulated radiotherapy (IMRT) fluences in dynamic-arc-mode around the patient. VMAT has gained the attention due to its ability to produce complex dose distributions and delivery in much shorter treatment time than conventional IMRT technique. The optimum VMAT dose distributions and efficient delivery depend on the choice of optimization algorithm, user selectable planning parameters and treatment delivery equipment. Many authors reported VMAT studies for conventional Linac equipped with 40 × 40 cm² apertures. This study intended in the search for optimal user selectable VMAT planning parameters for a Linac equipped with the 'Beam-Modulator™' with limited aperture of 21 × 16 cm². Geometric errors induced by inter- and intra- fraction motion may compromise the quality of radiation therapy treatments. Organ motion is a large contributor to treatment uncertainties in radiation therapy which can detrimentally affect the accuracy of treatment dose delivery. There have been many strategies in treatment planning, delivery and online imaging that account for organ motion to reduce effects on delivered doses. The multileaf collimator (MLC) tracking is one of the real-time adaptation techniques, that adapt treatment-beam to the target motion in real-time. We have, therefore, undertaken our studies to bring improvements in the areas of radiation treatment planning and image guidance for the better treatment of the patients. Our studies had been divided into three parts. In the first study, validation of the relative insensitivity of volumetric modulated arc therapy (VMAT) plan quality to gantry angle spacing (GS) was investigated. A quantitative comparison of dose–volume indices (DIs) was made for partial-arc (PA), single-arc (SA) and dual-arc (DA) VMAT plans optimized for 2°, 3° and 4° gantry angle spacing, representing a large variation of deliverable MLC segments. VMAT plans of six prostate cancer and six head-and-neck cancer patients were simulated. All optimization techniques generated clinically acceptable VMAT plans, except single-arc for the headand- neck cancer patients. A GS of 2°, with finest resolution was considered being reference, and was compared with GS 3° and 4°. The differences between the majority of reference and compared DIs were <2%. The metrics, such as treatment plan optimization xvii time and pre-treatment (phantom) dosimetric calculation time, supported the use of a GS of 4°. Therefore a GS of 4° is an optimal choice for minimal usage of planning resources without compromising plan quality. In the second study, dosimetric comparison of VMAT planning techniques and influence of collimator rotation on plan quality was investigated using a collimation system, limited to a maximum field size of 21 × 16 cm² (SynergyS® linear accelerator equipped with Beam Modulator™). A VMAT planning study of fifteen prostate and ten head and neck cancer patients was carried out. Single-arc, dual-arc and two combined independent-single-arcs (ISAs) VMAT techniques were optimized for four collimator angles; .C0°, C15°, C45° and C90° for prostate and C15°, C30°, C45° and C90° for head-and-neck. DA and ISAs provided similar PTV coverage, while DA provided better sparing of organs at risk, and similar treatment delivery times were noted for DA and ISAs techniques. In case of prostate optimizations similar target coverage for C0°, C15°, and C45° is noted, however C45° spared more rectum volumes than rest of collimator angles. A rotation of C45° provided significantly better target coverage and sparing of OARs than a rotation of C90°. In case of head-and-neck optimizations clinically acceptable dose distribution was calculated, and very similar target coverage for C30° and C45° is noted, however C45° spared more OARs volumes than rest of collimator angles. A single arc for the treatment of complex tumor sites like head-and-neck is not feasible while using limited aperture collimation (Beam Modulator™). None of the VMAT techniques optimized at C90° could achieve the defined treatment planning objectives. Thus, an optimal choice of VMAT arc and collimator angle is another degree of freedom to obtain desired PTV dose distributions and sparing of organs at risk. In the third study using prostate VMAT plans, a novel method of dynamic collimator rotation for improved multileaf collimator tracking was investigated. In the first step, two dual arc VMAT plans, one with fixed collimators (45° and 315°) and second with a rotating collimator were optimized for 22 prostate cancer patients. In the second step largescale MLC tracking simulations were done (using 695 motion traces) for all optimized VMAT plans, and thereafter dose reconstruction was performed for 35 motion traces for one patient, and the calculated root-mean-square dose error was compared with the MLC xviii exposure error. Rotating collimator VMAT plans were of similar quality as the fixed collimator plans, but significantly improved MLC tracking with 33% lower MLC exposure errors (p<<0.0001). The reductions in MLC exposure error correlated significantly with dose error reductions. Therefore MLC tracking with rotating collimator were significantly better than fixed collimator and agreement between planned and delivered dose distribution was higher for rotating collimator compared to fixed collimator. Hence, this study provided the improved methods for target dose distribution with optimal sparing of organs at risk, and accurate radiation dose delivery to the moving targets.