Few-leaf Electron Collimator Research Articles

A source model for modulated electron radiation therapy using dynamic jaw movements

The development of fast and accurate source models (SMs) might be of crucial importance for the future clinical implementation of modulated electron radiation therapy (MERT). In this study, a SM is presented for reconstructing phase-space information of modulated electron beams using a few-leaf electron collimator (FLEC) and the photon jaws. During a FLEC-based delivery, two collimation devices (jaws and FLEC) modulate the electron beam characteristics dynamically. The SM separates the beam into a primary and a scattered component. The primary component is derived by a fast Monte Carlo (MC) transport calculation in air using the EGSnrc/BEAMnrc code. The scattered beam is modeled analytically. The accelerator was decomposed into its individual leaf components and the scattered beam was characterized at various levels of the accelerator. Scattered particles are assigned an energy and position by sampling pre-calculated probability distributions. The direction is estimated by geometrical arguments. Particles were assumed to emerge from tunable virtual sources on the side of each collimator leaf. A leaf-hit algorithm was developed to dynamically reject particles that are incident on any collimating leaf. Electron transport in air between the two collimation levels was calculated based on a MC-modified version of the Fermi-Eyges scattering theory. Correlations between direction and position were observed and taken into account at the final collimation level. To validate the model, reconstructed phase-space data were compared with the full accelerator MC phase-space data. The model accurately reproduced the beam characteristics and preserved important correlations. Depth and profile dose distributions in water were derived for square, rectangular, and off-axis field sizes and for a range of clinical energies. Discrepancies in the dose distributions and dose output were within 3% in all cases. Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.

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

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. 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. 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. 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.

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 purposeTo 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 methodsFourteen 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. ResultsDose evaluation volume (DEV) coverage and homogeneity were best for MERT (D98=9.77Gy, D2=11.03Gy) followed by VMAT (D98=9.56Gy, D2=11.07Gy) and DE (D98=9.81Gy, D2=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). ConclusionsMERT 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.

SU‐E‐T‐665: Analytical Source Model for Modulated Electron Radiation Therapy

Purpose: To implement a dynamic, analytical source model for fast and accurate derivation of the beam characteristics on the exit plane of a linear accelerator for modulated electron radiation therapy using a Few Leaf Electron Collimator (FLEC). Methods: An analytical source model (SM) is derived for modulated electron radiation therapy (MERT) by the use of source fluence kernels directly on the linear acceleratorˈs exit plane. The BEAMnrc code was used to identify the most significant radiation sources in MERT. “Virtual” BEAM models were then created, where the different components of the sources were isolated to each other. Monte Carlo simulations of these models were performed to derive the fluence and energy electron distributions on the exit plane. The data were then stored as histograms on a 2D fluence grid (source kernels). Since the source kernels were derived by simulation of a precise accelerator model it is expected that the scattered particles would be accurately predicted. An arbitrary rectangular field can be recreated by superposition of the appropriate source kernels directly on the output plane. Results: Fluence and energy distributions were derived for three field sizes (8 × 8, 2 × 2 and 2 × 8 cm2) and 20 MeV electron beam energy with the SM and compared with the PH‐SP data. Scattered particles from the FLEC agreed well in all cases, while moderate discrepancies were encountered for the primary beam and scattered particles from the jaws. The total reconstructed beam deviation between PH‐SP and SM was less than 1.5% for all cases. Conclusions: The proposed method has shown to reproduce the fluence and energy distributions of small and/or arbitrary rectangular fields within acceptable tolerance. The model is expected to help significantly improve MERT optimization speed while maintaining accuracy in the dose calculations.Alexander S. Onassis Public Benefit Foundation

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.

Poster — Thur Eve — 01: Dynamic Aperture Optimization in MERT Using Direct Aperture Optimization

The advantage of modulated electron radiation therapy (MERT) comes from the defined electron range and sharp fall‐off offered with electron beams, combined with complex inverse planning techniques to conform dose to the target and reduce dose to organs at risk (OAR) beyond the target. Recent studies have evaluated the feasibility of various electron collimator devices for shallow tumor treatments. Despite the promising MERT studies, a MERT system comparable to IMRT is not commercailly available. In this work we investigate a dynamic aperture optimization process, which dynamically optimizes the aperture shapes and weights using direct aperture optimization (DAO) and is applicable to the McGill MERT delivery process using the Few‐Leaf‐Electron‐Collimator (FLEC). This study presents the optimization code (DADAO), and a plan comparison to commercially available photon beam optimization algorithms using a basic target and organ at risk geometry. A FLEC‐DADAO plan was benchmarked to plans generated from TomoTherapy and Varian Eclipse IMRT and RapidArc in order to establish a baseline level of confidence. Results were analyzed using dose volume histograms (DVH) and isodose plots. The DADAO plan was comparable to the clinical plans, competitive in its ability to reduce the dose to OAR but weaker in its ability to highly conform the dose to the target objectives. The DADAO code for MERT demonstrates potential to optimize electron planning and reduce the low‐dose irradiated volume including the low dose to the OAR at a slight cost in target coverage as compared to photon techniques.

TU‐FF‐A2‐03: Construction and Dosimetry of a Prototype Automated Few‐Leaf Electron Collimator (FLEC) for Delivery of Energy‐Intensity Modulated Electron Therapy

Purpose: To design and implement an automated “few leaf electron collimator” (FLEC) used for energy modulated electron therapy (EMET) using Monte Carlo‐based treatment planning. This study investigates the optimization of the physical design of the FLEC and provides preliminary validation results of comparison between Monte Carlo calculations and film measurements for complex intensity/energy modulated electron fields using a manual replica of the FLEC. Methods and Materials: The FLEC design was guided by Monte Carlo studies to optimize the physical configuration of the collimator leaves within the linear accelerator. To validate our Monte Carlo‐based treatment planning system, we used a manual FLEC to acquire output of jaws collimated fields using ionization chamber and compared them with Monte Carlo calculation. Calculated two dimensional dose distributions were compared to film measurements obtained using energy‐independent GafChromic® HS films. A gamma‐index analysis quantifying the difference between calculation and measurements is discussed. Results: A prototype of the automated FLEC was constructed with 1.1 cm thick and 3 cm width copper leafs of. The field collimated by the FLEC is backed up by jaw openings following the FLEC projection that provides an optimum field quality. The compact design of the FLEC, including motors, encoders, switches, and necessary wiring, is suitable to be attached to a clinical electron applicator. Software was developed to facilitate remote control of the FLEC system. A comparison between Monte Carlo calculation and film measurements of complex intensity maps shows agreement within tolerated accuracy measures (3% for 3 mm) based on gamma index analysis. Conclusions: The results of this work confirm most of our design objectives and support the potential of using the prototype FLEC for modulated electron therapy delivery. QA procedures for the EMET treatment delivery using the FLEC integrated with the linear accelerator are being investigated.

52 Monte Carlo-Based Inverse Planning of Modulated Electron Beam Therapy using a Few-Leaf Electron Collimator Provides Highly Conformal Treatment and Decreases Whole Body Dose

52 Monte Carlo-Based Inverse Planning of Modulated Electron Beam Therapy using a Few-Leaf Electron Collimator Provides Highly Conformal Treatment and Decreases Whole Body Dose

74 The implementation of automated few-leaf electron collimator used in inverse Monte Carlo-based energy modulated electron therapy

74 The implementation of automated few-leaf electron collimator used in inverse Monte Carlo-based energy modulated electron therapy

Energy modulated electron therapy using a few leaf electron collimator in combination with IMRT and 3D‐CRT: Monte Carlo‐based planning and dosimetric evaluation

Energy modulated electron therapy (EMET) based on Monte Carlo dose calculation is a promising technique that enhances the treatment planning and delivery of superficially located tumors. This study investigated the application of EMET using a novel few-leaf electron collimator (FLEC) in head and neck and breast sites in comparison with three-dimensional conventional radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) techniques. Treatment planning was performed for two parotid cases and one breast case. Four plans were compared for each case: 3D-CRT, IMRT, 3D-CRT in conjunction with EMET (EMET-CRT), and IMRT in conjunction with EMET (EMET-IMRT), all of which were performed and calculated with Monte Carlo techniques. For all patients, dose volume histograms (DVHs) were obtained for all organs of interest and the DVHs were used as a means of comparing the plans. Homogeneity and conformity of dose distributions were calculated, as well as a sparing index that compares the effect of the low isodose lines. In addition, the whole-body dose equivalent (WBDE) was estimated for each plan. Adding EMET delivered with the FLEC to 3D-CRT improves sparing of normal tissues. For the two head and neck cases, the mean dose to the contralateral parotid and brain stem was reduced relative to IMRT by 43% and 84%, and by 57% and 71%, respectively. Improved normal tissue sparing was quantified as an increase in sparing index of 47% and 30% for the head and neck and the breast cases, respectively. Adding EMET to either 3D-CRT or IMRT results in preservation of target conformity and dose homogeneity. When adding EMET to the treatment plan, the WBDE was reduced by between 6% and 19% for 3D-CRT and by between 21% and 33% for IMRT, while WBDE for EMET-CRT was reduced by up to 72% when compared with IMRT. FLEC offers a practical means of delivering modulated electron therapy. Although adding EMET delivered using the FLEC results in perturbation of target conformity when compared to IMRT, it significantly improves normal tissue sparing while offering enhanced target conformity to the 3D-CRT planning. The addition of EMET systematically leads to a reduction in WBDE especially when compared with IMRT.

MO-E-T-617-02: Dosimetric Evaluation of Inverse Monte Carlo-Based Modulated Electron Beam Treatment Planning and Delivery Using a Few Leaf Electron Collimator

Purpose: To investigate the potential of improving the treatment planning and delivery of treatment to superficially located tumors using energy modulated electron therapy (EMET) based on inverse techniques and Monte Carlo dose calculation algorithms. This study investigates the application of EMET using a few-leaf electron collimator (FLEC) in head & neck, sarcoma, and breast sites in comparison with three dimensional conventional radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) techniques. Method and Materials: Treatment planning was performed for a parotid case, a sarcoma case, and a breast case. Three Monte Carlo calculated plans were compared for each case: 3D-CRT, IMRT, and 3D-CRT in conjunction with EMET (EMET-CRT). For all patients, dose volume histograms (DVHs) were obtained for organs of interest. For each plan, homogeneity and conformity indices of dose distributions, sparing index (SPIN50/10) that quantifies the conformity of the low isodose lines, and the whole-body dose equivalent (WBDE) were analyzed. Results: Adding EMET delivered with the FLEC to 3D-CRT preserves target conformity and dose homogeneity and improves sparing of normal tissues. For the head & neck case the mean dose to the contralateral parotid and brain decreased relative to IMRT by 43% to 84%, and by 57% to 71%, respectively. Improved normal tissue sparing is also quantified as an increase in sparing index of 47% and 30% for the head & neck and the breast cases, respectively. The WBDE for EMET-CRT was reduced by up to 72% when compared with IMRT. Conclusion: EMET delivered with the FLEC could be a valuable addition to currently existing treatment techniques especially when applied to superficially located tumors that are inherently difficult to plan using IMRT. The addition of EMET systematically leads to a reduction in WBDE especially when compared with IMRT.

Monte Carlo based modulated electron beam treatment planning using a few-leaf electron collimator—feasibility study

Energy modulated electron beam therapy with conventional clinical accelerators has lagged behind photon IMRT despite its potential to achieve highly conformal dose distributions in superficial targets. One of the reasons for this is the absence of an automated collimating device that allows for the flexible delivery of a series of variable field openings. Electron-specific multileaf collimators attached to the bottom of the applicator require the use of a large number of motors and suffer from being relatively bulky and impractical for head and neck sites. In this work, we investigate the treatment planning aspects of a proposed ‘few-leaf’ electron collimator (FLEC) that consists of four motor-driven trimmer bars at the end of the applicator. The device is designed to serve as an accessory to standard equipment and allows for the shaping of any irregular field by combination of rectangular fieldlets. Using a Monte Carlo model of the FLEC, dose distributions are optimized using a simulated annealing (SA) inverse planning algorithm based on a limited number of Monte Carlo pre-generated, realistic phantom-specific dose kernels and user-specified dose–volume constraints. Using a phantom setup with an artificial target enclosed by organs at risk (OAR) as well as using a realistic patient case, we demonstrate that highly conformal distributions can be generated. Estimates of delivery times are made and show that a full treatment fraction can be kept to 15 min or less.