Two-step intensity modulated arc therapy (2-step IMAT) with segment weight and width optimization

Abstract

Background2-step intensity modulated arc therapy (IMAT) is a simplified IMAT technique which delivers the treatment over typically two continuous gantry rotations. The aim of this work was to implement the technique into a computerized treatment planning system and to develop an approach to optimize the segment weights and widths.Methods2-step IMAT was implemented into the Prism treatment planning system. A graphical user interface was developed to generate the plan segments automatically based on the anatomy in the beam's-eye-view. The segment weights and widths of 2-step IMAT plans were subsequently determined in Matlab using a dose-volume based optimization process. The implementation was tested on a geometric phantom with a horseshoe shaped target volume and then applied to a clinical paraspinal tumour case.ResultsThe phantom study verified the correctness of the implementation and showed a considerable improvement over a non-modulated arc. Further improvements in the target dose uniformity after the optimization of 2-step IMAT plans were observed for both the phantom and clinical cases. For the clinical case, optimizing the segment weights and widths reduced the maximum dose from 114% of the prescribed dose to 107% and increased the minimum dose from 87% to 97%. This resulted in an improvement in the homogeneity index of the target dose for the clinical case from 1.31 to 1.11. Additionally, the high dose volume V105 was reduced from 57% to 7% while the maximum dose in the organ-at-risk was decreased by 2%.ConclusionsThe intuitive and automatic planning process implemented in this study increases the prospect of the practical use of 2-step IMAT. This work has shown that 2-step IMAT is a viable technique able to achieve highly conformal plans for concave target volumes with the optimization of the segment weights and widths. Future work will include planning comparisons of the 2-step IMAT implementation with fixed gantry intensity modulated radiotherapy (IMRT) and commercial IMAT implementations.

Aperture generation based on threshold segmentation for intensity modulated radiotherapy treatment planning

“…Although this two‐step process used for IMRT treatment planning is simple, it results in decreased treatment quality. To improve the quality of treatment planning, direct aperture optimization (DAO) was developed …”

“…DAO methods can be classified into three types, and these methods usually begin with different initial aperture shapes. The first type of method is a stochastic search‐based method, which initializes simpler apertures such as those based on beam‐eye‐view projections of the target . With this type of approach, the aperture generated in the first iteration is typically large, compromising the treatment plan conformality.…”

“…To improve the quality of treatment planning, direct aperture optimization (DAO) was developed. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] An important step in DAO pertains to the initial aperture shapes. DAO methods can be classified into three types, and these methods usually begin with different initial aperture shapes.…”

“…The first type of method is a stochastic search-based method, which initializes simpler apertures such as those based on beam-eye-view projections of the target. [1][2][3][4][5][6][7] With this type of approach, the aperture generated in the first iteration is typically large, compromising the treatment plan conformality. The second type of method is local gradient-based, and involves initial leaf sequencing after optimizing the beamlet intensities.…”