Implementation of 2-Step Intensity Modulated Arc Therapy

Intensity modulated arc therapy is a novel treatment technique that has shown great potential to be superior to conventional intensity modulated radiotherapy, both in terms of treatment plan quality as well as treatment delivery. Based on previous literature, a simplified technique called two-step intensity modulated arc therapy (2-step IMAT) was implemented into a treatment planning system. In order to automatically generate treatment plans for this technique, a beam portal shaping method was developed to generate beam segments. A sensitivity analysis was carried out on a geometric phantom to determine optimal parameters for the 2-step IMAT implementation for that particular phantom. The segment weights were optimized using the dose-based and dose-volume-based objective functions. The optimal solution search was based on the gradient-descend algorithm. The dose-based objective function was implemented using a so-called lambda-value-dose-based objective function developed in this work in order to increase both speed and flexibility of the optimization. The successful implementation demonstrated the feasibility of automatic 2-step IMAT treatment planning. A comparison of conventional arc therapy and 2-step IMAT showed improvements in the target dose uniformity by about 50% for both geometric phantom and clinical paraspinal tumor case, whilst also improving the organ sparing. The comparisons between the lambda-value-dose-based and dose-volume-based optimizations showed a speed advantage of the former by a factor of over five in the phantom study. The current beam portal shaping approach can be improved by optimizing the segment width and including multiple organs-at-risk in the segment generation algorithm. Future work will also include the implementation of a stochastic optimization to minimize the chance of getting trapped in local minima during the segment weight optimization. In summary, the work of this research showed that the automatic 2-step IMAT planning is a viable technique that can result in highly conformal plans while keeping the treatment planning and delivery simple and straightforward

Development of magnetic resonance imaging based prostate treatment planning

Research Doctorate - Doctor of Philosophy (PhD)Radiotherapy is one of the main methods used to treat prostate cancer. Radiotherapy treatment relies on accurate planning and simulation before any radiation is administered. Currently this is mainly based on CT (computed tomography) imaging, although MR (magnetic resonance) imaging provides superior soft-tissue contrast and is therefore often used to assist with accurate organ delineation. The overall treatment planning workflow and performance can be improved if the entire workflow is solely performed using MR images. In order to achieve such MR-only treatment planning, three main challenges need to be overcome: 1) the geometric accuracy of MR images needs to be assured ; 2) the MR simulator needs to be commissioned and evaluated ; 3) electron density information required for dose calculation needs to be generated from MR images. This thesis examines each of these challenges. First, a pelvic shape phantom was used to quantify the geometric distortion arising in prostate treatment. The CT image was acquired as the gold reference and the distortion of the MR image was corrected with the vendor’s built-in algorithm. Using the image registration method, the maximum geometric distortion was reduced from nearly 8 mm to within the radiotherapy tolerance level. Second, commercial radiotherapy-dedicated equipment was implemented on the Siemens Skyra 3 Tesla MR scanner. This involved a hard flat tabletop which mimicked the flat radiotherapy treatment table, and coil mounts to lift the MR coil above the patient’s body and minimise coil-induced disagreement between the MR planning and treatment geometry. A reduction in image quality was observed on the MR simulator, but no clinically significant difference was found in the accuracy of organ delineation. Furthermore, use of the MR simulator eliminated patient positioning error associated with conventional MR scanner design and thus reduced the systematic dosimetric error. The entire workflow of MR-based planning was tested using an anthropomorphic phantom and no significant difference was found between MR- and CT-based plans. Finally, substitute (also known as synthetic or pseudo) sCT images were generated from MR images using a multi-atlas local weighted voting method. Validation was conducted on 39 patients and the sCT images were in high level agreement with the CT images. In summary, MR-based radiotherapy planning for treating prostate cancer has been thoroughly tested and evaluated in this study. This may provide an important stepping stone for the future clinical implementation

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

Abstract Background 2-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. Methods 2-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. Results The 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%. Conclusions The 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.</p

Automatic substitute computed tomography generation and contouring for magnetic resonance imaging (MRI)-alone external beam radiation therapy from standard MRI sequences

Purpose: To validate automatic substitute computed tomography CT (sCT) scans generated from standard T2-weighted (T2w) magnetic resonance (MR) pelvic scans for MR-Sim prostate treatment planning. Patients and Methods: A Siemens Skyra 3T MR imaging (MRI) scanner with laser bridge, flat couch, and pelvic coil mounts was used to scan 39 patients scheduled for external beam radiation therapy for localized prostate cancer. For sCT generation a whole-pelvis MRI scan (1.6 mm 3-dimensional isotropic T2w SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolution] sequence) was acquired. Three additional small field of view scans were acquired: T2w, T2*w, and T1w flip angle 80° for gold fiducials. Patients received a routine planning CT scan. Manual contouring of the prostate, rectum, bladder, and bones was performed independently on the CT and MR scans. Three experienced observers contoured each organ on MRI, allowing interobserver quantification. To generate a training database, each patient CT scan was coregistered to their whole-pelvis T2w using symmetric rigid registration and structure-guided deformable registration. A new multi-atlas local weighted voting method was used to generate automatic contours and sCT results. Results: The mean error in Hounsfield units between the sCT and corresponding patient CT (within the body contour) was 0.6 ± 14.7 (mean ± 1 SD), with a mean absolute error of 40.5 ± 8.2 Hounsfield units. Automatic contouring results were very close to the expert interobserver level (Dice similarity coefficient): prostate 0.80 ± 0.08, bladder 0.86 ± 0.12, rectum 0.84 ± 0.06, bones 0.91 ± 0.03, and body 1.00 ± 0.003. The change in monitor units between the sCT-based plans relative to the gold standard CT plan for the same dose prescription was found to be 0.3% ± 0.8%. The 3-dimensional γ pass rate was 1.00 ± 0.00 (2 mm/2%). Conclusions: The MR-Sim setup and automatic sCT generation methods using standard MR sequences generates realistic contours and electron densities for prostate cancer radiation therapy dose planning and digitally reconstructed radiograph generation