Dosimetric effects of sectional adjustments of collimator angles on volumetric modulated arc therapy for irregularly-shaped targets

Abstract

PurposeTo calculate an optimal collimator angle at each of sectional arcs in a full-arc volumetric modulated arc therapy (VMAT) plan and evaluate dosimetric quality of these VMAT plans comparing full-arc VMAT plans with a fixed collimator angle.MethodsSeventeen patients who had irregularly-shaped target in abdominal, head and neck, and chest cases were selected retrospectively. To calculate an optimal collimator angle at each of sectional arcs in VMAT, integrated MLC apertures which could cover all shapes of target determined by beam’s-eye view (BEV) within angular sections were obtained for each VMAT plan. The angular sections were 40°, 60°, 90° and 120°. When the collimator settings were rotated at intervals of 2°, we obtained the optimal collimator angle to minimize area size difference between the integrated MLC aperture and collimator settings with 5 mm-margins to the integrated MLC aperture. The VMAT plans with the optimal collimator angles (Colli-VMAT) were generated in the EclipseTM. For comparison purposes, one full-arc VMAT plans with a fixed collimator angles (Std-VMAT) were generated. The dose-volumetric parameters and total MUs were evaluated.ResultsThe mean dose-volumetric parameters for target volume of Colli-VMAT were comparable to Std-VMAT. Colli-VMAT improved sparing of most normal organs but for brain stem, compared to Std-VMAT for all cases. There were decreasing tendencies in mean total MUs with decreasing angular section. The mean total MUs for Colli-VMAT with the angular section of 40° (434 ± 95 MU, 317 ± 81 MU, and 371 ± 43 MU for abdominal, head and neck, and chest cases, respectively) were lower than those for Std-VMAT (654 ± 182 MU, 517 ± 116 MU, and 533 ± 25 MU, respectively).ConclusionsFor an irregularly-shaped target, Colli-VMAT with the angular section of 40° reduced total MUs and improved sparing of normal organs, compared to Std-VMAT.

Optimal collimator rotation based on the outline of multiple brain targets in VMAT

“…In a previous study, we demonstrated the potential of optimal collimator angles during the VMAT delivery for abdomen, head and neck, and chest cases, all of which had an irregularly shaped target, showing high reduction of the total MUs and improvement of dosimetric plan quality, compared to a fixed collimator angle. In that study, Colli-VMAT Pre plans with angular sections of 40° could cover the large and irregularly shaped target, and by using a DICOM-RT format file of the conformal arc plan with a collimator setting of 0°, the optimal collimator angles could be calculated effectively [ 16 ]. However, as mentioned above, MLC positions in the conformal arc plans generated with a collimator setting of 0° could not fully express the target outline.…”

“…In our previous study, we determined the optimal collimator angles with various angular sections using the conformal arc plans with a fixed collimator angle of 0° and then demonstrated that VMAT plans with the optimal collimator angles with angular sections of 40° and 60° (Colli-VMAT Pre (40°), Colli-VMAT Pre (60°)) reduced the total MUs and improved the sparing of normal organs, compared to Colli-VMAT Pre (90°), Colli-VMAT Pre (120°), and Std-VMAT [ 16 ]. Std-VMAT plan was one full-arc VMAT plans with a fixed collimator angle, having an angular section of 360°.…”

“…In the same manner as our previous study [ 16 ], some of the calculated optimal collimator angles for sectional arcs were re-determined by adding 90° rotation to those calculated optimal collimator angles before VMAT plan optimization. The reason for adding 90° rotation is that the collimator angle added to 90° rotation could potentially reduce the burden of MLC control for modulating photon beam intensity owing to the maximum leaf span of the MLCs (15 cm).…”

“…In a previous study, we analyzed the dosimetric effects of optimal collimator angles at each sectional arc in the VMAT for an irregularly shaped target in abdominal, head and neck, and chest cases [ 16 ]. In that study, the optimal collimator angles were calculated using an integrated MLC aperture from a conformal arc plan generated with a fixed collimator setting of 0°.…”

“…In that study, the optimal collimator angles were calculated using an integrated MLC aperture from a conformal arc plan generated with a fixed collimator setting of 0°. Although we demonstrated considerable improvements of dosimetric plan quality and MU efficiency using the optimal collimator angles, the integrated MLC apertures with a fixed collimator setting of 0° had limitations that could not fully express the shape of the targets defined by the MLCs [ 16 ]. For multiple brain targets, the conformal arc plans with a fixed collimator angle of 0° used in our previous study were not appropriate.…”