A Alexander

Comparison of modulated electron radiotherapy to conventional electron boost irradiation and volumetric modulated photon arc therapy for treatment of tumour bed boost in breast cancer

BACKGROUND AND PURPOSE To compare few leaf electron collimator (FLEC)-based modulated electron radiotherapy (MERT) to conventional direct electron (DE) and volumetric modulated photon arc therapy (VMAT) for the treatment of tumour bed boost in breast cancer. MATERIALS AND METHODS Fourteen patients with breast cancer treated by lumpectomy and requiring post-operative whole breast radiotherapy with tumour bed boost were planned retrospectively using conventional DE, VMAT and FLEC-based MERT. The planning goal was to deliver 10Gy to at least 95% of the tumour bed volume. Dosimetry parameters for all techniques were compared. RESULTS Dose evaluation volume (DEV) coverage and homogeneity were best for MERT (D(98)=9.77Gy, D(2)=11.03Gy) followed by VMAT (D(98)=9.56Gy, D(2)=11.07Gy) and DE (D(98)=9.81Gy, D(2)=11.52Gy). Relative to the DE plans, the MERT plans predicted a reduction of 35% in mean breast dose (p<0.05), 54% in mean lung dose (p<0.05) and 46% in mean body dose (p<0.05). Relative to the VMAT plans, the MERT plans predicted a reduction of 24%, 36% and 39% in mean breast dose, heart dose and body dose, respectively (p<0.05). CONCLUSIONS MERT plans were a considerable improvement in dosimetry over DE boost plans. There was a dosimetric advantage in using MERT over VMAT for increased DEV conformity and low-dose sparing of healthy tissue including the integral dose; however, the cost is often an increase in the ipsilateral lung high-dose volume.

Toward automatic field selection and planning using Monte Carlo-based direct aperture optimization in modulated electron radiotherapy

Modulated electron radiotherapy (MERT) has been proven to produce optimal plans for shallow tumors. This study investigates automated approaches to the field determination process in generating optimal MERT plans for few-leaf electron collimator (FLEC)-based MERT, by generating a large database of pre-calculated beamlets stored as phase-space files. Beamlets can be used in an overlapping feathered pattern to reduce the effect of abutting fields, which can contribute to dose inhomogeneities within the target. Beamlet dose calculation was performed by Monte Carlo (MC) simulations prior to direct aperture optimization (DAO). The second part of the study examines a preliminary clinical comparison between FLEC-based MERT and helical TomoTherapy. A MERT plan for spinal irradiation was not able to conform to the PTV dose constraints as closely as the TomoTherapy plan, although the TomoTherapy plan was taken as is, i.e. not Monte Carlo re-calculated. Despite the remaining gradients in the PTV, the MERT plan was superior in reducing the low-dose bath typical of TomoTherapy plans. In conclusion, the FLEC-based MERT planning techniques developed within the study produced promising MERT plans with minimal user input. The phase-space database reduces the MC calculation time and the feathered field pattern improves target homogeneity. With further investigations, FLEC-based MERT will find an important niche in clinical radiation therapy.

An experimental feasibility study on the use of scattering foil free beams for modulated electron radiotherapy

The potential benefit of using scattering foil free beams for delivery of modulated electron radiotherapy is investigated in this work. Removal of the scattering foil from the beamline showed a measured bremsstrahlung tail dose reduction just beyond R(p) by a factor of 12.2, 6.9, 7.4, 7.4 and 8.3 for 6, 9, 12, 16 and 20 MeV beams respectively for 2 × 2 cm(2) fields defined on-axis when compared to the clinical beamline. Monte Carlo simulations were matched to measured data through careful tuning of source parameters and the modification of certain accelerator components beyond the manufacturers specifications. An accelerator model based on the clinical beamline and one with the scattering foil removed were imported into a Monte Carlo-based treatment planning system (McGill Monte Carlo Treatment Planning). A treatment planning study was conducted on a test phantom consisting of a PTV and two distal organs at risk (OAR) by comparing a plan using the clinical beamline to a plan using a scattering foil free beamline. A DVH comparison revealed that for quasi-identical target coverage, the volume of each OAR receiving a given dose was reduced, thus reducing the dose deposited in healthy tissue.

An investigation into the use of MMCTP to tune accelerator source parameters and testing its clinical application

This paper presents an alternative method to tune Monte Carlo electron beam parameters to match measured data using a minimal set of variables in order to reduce the model setup time prior to clinical implementation of the model. Monte Carlo calculations provide the possibility of a powerful treatment planning verification technique. The nonstandardized and nonautomated process of tuning the required accelerator model is one of the reasons for delays in the clinical implementation of Monte Carlo techniques. This work aims to establish and verify an alternative tuning method that can be carried out in a minimal amount of time, allowing it to be easily implemented in a clinical setting by personnel with minimal experience with Monte Carlo methods. This tuned model can then be incorporated into the MMCTP system to allow the system to be used as a second dose calculation check for IMRT plans. The technique proposed was used to establish the primary electron beam parameters for accelerator models for the Varian Clinac 2100 6 MV photon beam using the BEAMnrc Monte Carlo system. The method is intended to provide a clear, direct, and efficient process for tuning an accelerator model using readily available clinical quality assurance data. The tuning provides a refined model, which agrees with measured dose profile curves within 1.5% outside the penumbra or 3 mm in the penumbra, for square fields with sides of 3 cm up to 30 cm. These models can then be employed as the basis for Monte Carlo recalculations of dose distributions, using the MMCTP system, for clinical treatment plans, providing an invaluable assessment tool. This was tested on six IMRT plans and compared to the measurements performed for the pretreatment QA process. These Monte Carlo values for the average dose to the chamber volume agreed with measurements to within 0.6%. PACS number: 87.55.km

Direct aperture optimization for FLEC‐based MERT and its application in mixed beam radiotherapy

PURPOSE Despite promising research in modulated electron radiotherapy (MERT), an applicator to produce modulated electron beams and associated treatment planning software is still not commercially available. This work investigated an optimization process in treatment planning for the McGill few leaf electron collimator (FLEC) MERT delivery device. In addition, the possibility of combining MERT with photon fields was examined to investigate mixed beam radiotherapy. METHODS A FLEC direct aperture optimization (DAO) method, in which FLEC apertures and weights were iteratively optimized was created. The authors evaluated the performance of DAO against our previous technique for generating FLEC plans and with commercially available photon beam optimization algorithms using a basic target and organ at risk geometry. The authors applied the DAO technique on a sarcoma treatment to evaluate clinical parameters. Finally, the authors examined the merit of mixing the DAO generated FLEC electron fields with photon fields to improve the dosimetry of the sarcoma treatment. RESULTS In relation to the alternative plans, the DAO generated sarcoma MERT plan was competitive in its ability to reduce the dose to OAR but weaker in its ability to highly conform the dose to the target volume. The addition of photon fields improved the quality of the MERT plan in terms of OAR sparing and target conformality. CONCLUSIONS The DAO approach yielded deliverable FLEC-based MERT plans with a limited number of fields. The approach combined with photon optimization added flexibility, where the mutual benefits of each radiation type was used in unison to improve plan quality.

SU‐D‐BRB‐03: All‐Inclusive DOSXYZnrc Source for Monte Carlo QA of External Beam Radiotherapy

Purpose: To extend the versatility and simplicity of external beam Monte Carlo(MC) methods within BEAMnrc/DOSXYZnrc with the introduction of a new universal DOSXYZnrc source for multi‐field simulations. Methods and Materials: A single DOSXYZnrc source, capable of simulating all types of external beam radiotherapy, was developed as an extension to the standard set of sources. The source incorporates dynamic movement of components such as MLC leaves, jaws, gantry, collimator and couch. This was achieved by coupling the field index between the BEAMnrc components with the field index of the DOSXYZnrc components through the LATCH variable, transferred from BEAMnrc to DOSXYZnrc via the phase space information. This allows the state of the accelerator (such as MLC leaves, jaw positions and angles of incidence) to be changed for each history, effectively simulating coupled time‐dependent geometries. The process for RapidArc is as follows. RapidArc fields are defined within the control points of the DICOM plan file. Each point defines a MLC pattern and gantry angle. Within the MC simulation, the MLC pattern was defined in BEAMnrc, while the gantry angles were defined in the new DOSXYZnrc source. Control points for Tomotherapy are defined with the XML and sinogram files. In addition, IMRT and conformal treatments such as SBRT, which are comprised of 1–7 beams, can be merged into a single simulation. For practical considerations, it is more efficient to run a single MC simulation than to manage the submission of multiple beams and recombining them afterwards. We tested the new source within the context of the MMCTP system. Results and Conclusion: This source has enabled MC plan verification of RapidArc and Tomotherapy and simplified the MC calculation of IMRT and SBRT into a single simulation for efficient QA. The new source adds much needed functionality to the existing set of DOSXYZnrc sources.

TH‐E‐224C‐04: MMCTP, a Radiotherapy Research Environment for Monte Carlo and Patient‐Specific Treatment Planning

Purpose: To develop a flexible software package, on low cost hardware with the aim of integrating new patient specific treatment planning with Monte Carlo dose calculation suitable for large‐scale prospective and retrospective treatment planning studies. Programming Philosophy: The McGill Monte Carlo Treatment Planning system (MMCTP) is designed as a software environment for the research development of patient specific treatment planning. The design includes a workstation GUI for treatment planning tools, and anonymous access to standard low cost hardware for MCdose calculation. Results: Before using MMCTP, treatment plans are converted into the so‐called McGill RT format. This new file structure was designed for saving patient plans on the workstation. The current MMCTP features are: (a) DICOM and RTOG imports; (b) transverse/sagittal/coronal slice viewing for contours, CT scans,dose distributions; (c) contouring tools; (d) colour‐wash and isodose line display; (e) DVH analysis, and dose matrix comparison tools; (f) external beam editing; (g) thumbnail CT navigation tool; (h) EGS/Beam calculation and XVMC patient transport for photon and electron beams. MMCTP uses a two‐step process to generate MCdose distributions. The MC module controls egs/Beam and XVMC calculations. Input files, prepared from the beam geometry, are uploaded and run on the cluster using shell commands. Upon completion of XVMC, the GUI downloads individual dose files. Conclusion: The MMCTP GUI provides a flexible research platform for the development of patient specific MCtreatment planning for photon and electron external beamradiation therapy. MMCTP uses an internal storage format that is flexible in that it allows for multi‐instance multi‐modality image information useful in the planning process. The visualization,dose matrix operation and DVH tools offer extensive possibility for plan analysis and comparison to plans imported from commercial treatment planning systems through well‐documented image storage protocols such as DICOM.

Sci—Thur PM: Planning & Delivery — 03: Automated delivery and quality assurance of a modulated electron radiation therapy plan

Modulated electron radiation therapy (MERT) offers the potential to improve healthy tissue sparing through increased dose conformity. Challenges remain, however, in accurate beamlet dose calculation, plan optimization, collimation method and delivery accuracy. In this work, we investigate the accuracy and efficiency of an end-to-end MERT plan and automated-delivery workflow for the electron boost portion of a previously treated whole breast irradiation case. Dose calculations were performed using Monte Carlo methods and beam weights were determined using a research-based treatment planning system capable of inverse optimization. The plan was delivered to radiochromic film placed in a water equivalent phantom for verification, using an automated motorized tertiary collimator. The automated delivery, which covered 4 electron energies, 196 subfields and 6183 total MU was completed in 25.8 minutes, including 6.2 minutes of beam-on time with the remainder of the delivery time spent on collimator leaf motion and the automated interfacing with the accelerator in service mode. The delivery time could be reduced by 5.3 minutes with minor electron collimator modifications and the beam-on time could be reduced by and estimated factor of 2–3 through redesign of the scattering foils. Comparison of the planned and delivered film dose gave 3%/3 mm gamma pass rates of 62.1, 99.8, 97.8, 98.3, and 98.7 percent for the 9, 12, 16, 20 MeV, and combined energy deliveries respectively. Good results were also seen in the delivery verification performed with a MapCHECK 2 device. The results showed that accurate and efficient MERT delivery is possible with current technologies.

Monte Carlo investigation of collapsed versus rotated IMRT plan verification.

IMRT QA requires, among other tests, a time‐consuming process of measuring the absorbed dose, at least to a point, in a high‐dose, low‐dose‐gradient region. Some clinics use a technique of measuring this dose with all beams delivered at a single gantry angle (collapsed delivery), as opposed to the beams delivered at the planned gantry angle (rotated delivery). We examined, established, and optimized Monte Carlo simulations of the dosimetry for IMRT verification of treatment plans for these two different delivery modes (collapsed versus rotated). The results of the simulations were compared to the treatment planning system dose calculations for the two delivery modes, as well as to measurements taken. This was done in order to investigate the validity of the use of a collapsed delivery technique for IMRT QA. The BEAMnrc, DOSXYZnrc, and egs_chamber codes were utilized for the Monte Carlo simulations along with the MMCTP system. A number of different plan complexity metrics were also used in the analysis of the dose distributions in a bid to qualify why verification in a collapsed delivery may or may not be optimal for IMRT QA. Following the Alfonso et al. (1) formalism, the kQclin,Qfclin,fref correction factor was calculated to correct the deviation of small fields from the reference conditions used for beam calibration. We report on the results obtained for a cohort of 20 patients. The plan complexity was investigated for each plan using the complexity metrics of homogeneity index, conformity index, modulation complexity score, and the fraction of beams from a particular plan that intersect the chamber when performing the QA. Rotated QA gives more consistent results than the collapsed QA technique. The kQclin,Qfclin,fref factor deviates less from 1 for rotated QA than for collapsed QA. If the homogeneity index is less than 0.05 then the kQclin,Qfclin,fref factor does not deviate from unity by more than 1%. A value this low for the homogeneity index can only be obtained with the rotated QA technique. PACS number: 87.55.Qr

Sci—Thur PM: YIS — 01: Inverse treatment planning for modulated electrons and mixed photon and electron radiotherapy

Modulated electron radiotherapy (MERT) takes advantage of the low distal dose of electrons to reduce dose to healthy tissue. The dosimetric advantage of MERT is clear when compared against single-field electron irradiation where MERT demonstrates superior target homogeneity and sparing; however the dosimetric advantage is unclear when comparing MERT with photon intensity-modulated radiotherapy (IMRT) where MERT techniques struggle to match the IMRT target homogeneity but with less total energy delivered to healthy tissues. In an effort to improve dosimetric benefits of MERT, this study investigated an inverse planning technique for the creation of hybrid MERT-IMRT mixed beam radiotherapy (MBRT) plans. The optimization process decouples the photon and electron beamlets for combined modality optimization. The input to the optimization algorithm was a series of patient-specific 3D dose distributions for the corresponding electron and photon beamlets, while the output was a list of weights that satisfied the optimization constraints. A photon IMRT Eclipse (Varian, Palo Alto, CA) plan and a MERT plan were created for a patient-specific sarcoma irradiation. The MERT plan was competitive in its ability to reduce dose to organs at risk and total-body dose; however, the plan suffered from poorer target conformity compared with the IMRT plan. The MBRT plan was created by adding two photon fields, divided into beamlets, to the electron beamlets of the MERT plan for reoptimization. The MBRT plan improved MERT target coverage with only minimal cost to healthy tissue dose. The MBRT plan provided clear dosimetric advantages over the IMRT and MERT plan.