Dosimetric dependence on the collimator angle in prostate volumetric modulated arc therapy

Muhammad Isa,Muhammad Afzal,Jalil Rehman,James Chow

doi:10.14319/ijcto.0204.19

Abstract

Purpose: The purpose of this study is to investigate the dose-volume variations of planning target volume (PTV) and organs-at-risk (OARs) in prostate volumetric modulated arc therapy (VMAT) when varying collimator angle. The collimator angle has the largest impact and is worth considering, so, its awareness is essential for a planner to produce an optimal prostate VMAT plan in a reasonable time frame. Methods : Single-arc VMAT plans at different collimator angles (0 o , 15 o , 30 o , 45 o , 60 o , 75 o and 90 o ) were created systematically using a Harold heterogeneous pelvis phantom. The conformity index (CI), homogeneity index (HI), gradient index (GI), machine monitor units (MUs), dose-volume histogram and mean and maximum dose of the PTV were calculated and analyzed. On the other hand, the dose-volume histogram and mean and maximum doses of the OARs such as the bladder, rectum and femoral heads for different collimator angles were determined from the plans. Results: There was no significant difference, based on the planned dose-volume evaluation criteria, found in the VMAT optimizations for all studied collimator angles. A higher CI (0.53) and lower HI (0.064) were found in the 45 o collimator angle. In addition, the 15 o collimator angle provided a lower value of HI similar to the 45 o collimator angle. Collimator angles of 75 o and 90 o were found to be good for rectum sparing, and collimator angles of 75 o and 30 o were found to be good for sparing of right and left femur, respectively. The PTV dose coverage for each plan was comparatively independent of the collimator angle. Conclusion: Our study indicates that the dosimetric results provide support and guidance to allow the clinical radiation physicists to make careful decisions in implementing suitable collimator angles to improve the PTV coverage and OARs sparing in prostate VMAT.

Highlights

This study has been carried out on a Harold phantom and clinically acceptable Volumetric modulated arc therapy (VMAT) plans satisfying a minimum of 99% prescribed coverage to PTV were achieved
It was found that a 30o collimator angle showed as lower Gradient index (GI) value of GI that was closer to unity while higher values were found at 0o collimator angle
This work explores the impact of different collimator angles on a dosimetric scoring function

Summary

Introduction

Volumetric modulated arc therapy (VMAT) has become a standard delivery option in the field of prostate radiotherapy, due to its shortened delivery time and the smaller monitor units (MUs), as compare to step-and-shoot intensity modulated radiotherapy (IMRT).[1,2,3,4,5,6] Patient dosimetry between prostate VMAT and IMRT has been extensively studied, which reveals that prostate VMAT can produce comparable or even improved target coverage and normal tissue (bladder, rectum and femoral heads) sparing.[7,8,9,10,11]VMAT encloses more dose delivery parameters such as dynamic multileaf collimator movement, dose rate, and gantry speed with single or multiple photon arcs in the treatment, 12-15which requires a more powerful machine, patient quality assurance procedures, dose calculation algorithm, and dosimetric evaluation for the treatment.[16,17,18,19]Until the availability of the Elekta linear accelerator VMAT in 200820, the only commercially available treatment planning system (TPS) was ERGO++ (3D Line Medical Systems/ Elekta Ltd, Crawley, UK), which needed an initial definition of sub-arcs and had manual version of the multileaf collimator (MLC) before automatic weight optimization and was not considered a full inverse planning system.11, 21 ,22 In December 2009, two manufacturers introduced a new system of VMAT delivery that employed a VMAT treatment planning tool, implemented in Oncentra with Master-plan v3.3 (Nucletron BV, Veenendal, The Netherlands) with VMAT application on a Synergy linac (Elekta Ltd, Crawley, UK). VMAT encloses more dose delivery parameters such as dynamic multileaf collimator movement, dose rate, and gantry speed with single or multiple photon arcs in the treatment, 12-15which requires a more powerful machine, patient quality assurance procedures, dose calculation algorithm, and dosimetric evaluation for the treatment.[16,17,18,19]. Until the availability of the Elekta linear accelerator VMAT in 200820, the only commercially available treatment planning system (TPS) was ERGO++ (3D Line Medical Systems/ Elekta Ltd, Crawley, UK), which needed an initial definition of sub-arcs and had manual version of the multileaf collimator (MLC) before automatic weight optimization and was not considered a full inverse planning system.11, 21 ,22. In December 2009, two manufacturers introduced a new system of VMAT delivery that employed a VMAT treatment planning tool, implemented in Oncentra with Master-plan v3.3 (Nucletron BV, Veenendal, The Netherlands) with VMAT application on a Synergy linac (Elekta Ltd, Crawley, UK). The Synergy linac was used for a limited number of patients.[8, 23]

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Discussion

Conclusion