Modulated Electron Radiotherapy Research Articles

Dosimetric properties of an amorphous silicon EPID for verification of modulated electron radiotherapy

To investigate the dosimetric properties of an electronic portal imaging device (EPID) for electron beam detection and to evaluate its potential for quality assurance (QA) of modulated electron radiotherapy (MERT). A commercially available EPID was used to detect electron beams shaped by a photon multileaf collimator (MLC) at a source-surface distance of 70 cm. The fundamental dosimetric properties such as reproducibility, dose linearity, field size response, energy response, and saturation were investigated for electron beams. A new method to acquire the flood-field for the EPID calibration was tested. For validation purpose, profiles of open fields and various MLC fields (square and irregular) were measured with a diode in water and compared to the EPID measurements. Finally, in order to use the EPID for QA of MERT delivery, a method was developed to reconstruct EPID two-dimensional (2D) dose distributions in a water-equivalent depth of 1.5 cm. Comparisons were performed with film measurement for static and dynamic monoenergy fields as well as for multienergy fields composed by several segments of different electron energies. The advantageous EPID dosimetric properties already known for photons as reproducibility, linearity with dose, and dose rate were found to be identical for electron detection. The flood-field calibration method was proven to be effective and the EPID was capable to accurately reproduce the dose measured in water at 1.0 cm depth for 6 MeV, 1.3 cm for 9 MeV, and 1.5 cm for 12, 15, and 18 MeV. The deviations between the output factors measured with EPID and in water at these depths were within ±1.2% for all the energies with a mean deviation of 0.1%. The average gamma pass rate (criteria: 1.5%, 1.5 mm) for profile comparison between EPID and measurements in water was better than 99% for all the energies considered in this study. When comparing the reconstructed EPID 2D dose distributions at 1.5 cm depth to film measurements, the gamma pass rate (criteria: 2%, 2 mm) was better than 97% for all the tested cases. This study demonstrates the high potential of the EPID for electron dosimetry, and in particular, confirms the possibility to use it as an efficient verification tool for MERT delivery.

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.

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.

Poster - Thur Eve - 27: Scattering foil redesign for modulated electron radiotherapy.

This work serves to build on existing work that discussed the advantages of removing the scattering foil from the beamline for modulated electron radiotherapy (MERT) much like flattening filter-free beams in photon IMRT. Due to concerns about the ability of the accelerators transmission ionization chamber to function normally with narrow, minimally-scattered beams as well as a limited maximum field size, this study focused on the design of a new custom scattering foil that was optimized for MERT applications. Different foil parameters such as material, thickness and shape were investigated to produce a minimally scattered beam, as opposed to a flat beam in the case of the clinical foil, while preserving the dosimetric benefits of a reduced thickness foil such as dramatically higher dose rate and reduced photon contamination in the bremsstrahlung tail. Using a Monte Carlo model that was verified against measured data, fluence profiles at the level of the transmission ionization chamber were produced and analyzed for all foil configurations, as well as PDDs and profiles in a water phantom. It was found that a simple foil of slab geometry made of low to medium Z material with the thickness determined by the maximum desirable field size and chosen electron energy produced a sufficiently scattered beam that maintained the dosimetric advantages of a scattering-foil-free beamline. These new foils, when applied to MERT, have the advantage of reducing treatment time due to higher dose rates and reducing dose to healthy tissue due to reduced dose in the bremsstrahlung tail.

Sci-Thur AM: Planning - 07: A fast and accurate source model for energy and intensity modulated electron beams.

The purpose of this study is to develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). An algorithm has been developed for creating phase-space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre-calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from tunable Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction a particle is rejected if it is expected to suffer a leaf-hit. A leaf-hit counter is used to calculate the output of scatter particles based on the pre-calculated scatter probabilities. To account for multiple coulomb scattering in air a MC-corrected version of the Fermi-Eyges scattering theory was implemented. Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off-axis fields. The model agreed with full MC dose distributions within 3 % in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6 % in all cases. The model was 16-22 times faster in generating a phase-space file than a full MC simulation with the BEAMnrc code.

SU-E-T-576: Investigation of Combining Modulated Electron Beams with Intensity Modulated Photons for Radiation Therapy of Breast Cases.

Modulated electron radiation therapy (MERT) can offer significant advantages for breast treatments over conventional radiotherapy in terms of sparing distal critical structures. While intensity modulated radiation therapy (IMRT) has the advantage of achieving better dose homogeneity inside the target combining both MERT and IMRT will be the ideal scenario. The Aim of the present study is to investigate the possibility of further improving breast radiation therapy using combined MERT/IMRT treatment technique. Accurate modeling of a prototype motorized electron multileaf collimator was verified in a separate study. In this work treatment planning was performed by an in house Monte Carlo based inverse planning system. Dose deposition coefficients were calculated using MCPLAN and utilizing real patients CTs. Optimization is then conducted based on an equivalent uniform dose objective function. MERT and IMRT plans were created for different patients. The clinical beneficial outcome for MERT either alone or combined with IMRT was investigated based on isodose distributions and dose volume histograms. It is shown that MERT can give similar dose distributions as IMRT in some cases. For some cases, MERT could be advantageous whenever more skin dose was required. In some cases MERT can be identified as the best option. It was found that MERT compared to IMRT could introduce hot spots inside the target. However this was resolved in combined MERT/IMRT treatment. Dose uniformity can be restored with a reduction in the maximum lung and heart received dose. MERT can improve treatment plan quality for many breast patients. In some cases better results can be obtained with a combined MERT/IMRT treatment, where a homogeneous dose in the target can be achieved with an improvement in the DVH of critical structures. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society.

MO‐G‐213AB‐06: Segment Matching Optimization for Modulated Electron Radiation Therapy

Purpose: Modulated electron radiotherapy (MERT) has been demonstrated to be an ideal treatment for lesions close to the surface in terms of dose coverage and sparing of underlying critical organs and normal tissues. The purpose of this study was to obtain the parameters and devise strategies for segment matching which are necessary for efficient MERT treatment planning. Methods: In this study we used EGS4nrc/BEAMnrc and MCSim code for Monte Carlo calculations. The Computational Environment for Radiotherapy research (CERR) tool was used for viewing and comparing plans. A water phantom, delineated with stepped target idealized for two segments with sizes from 2 to 6 cm and electron beam energies of 6, 9, 12, 16 and 20 MeV were used for the calculations. The variation in dose distribution with different energies of abutte electron segments and change in segment size were studied. In order to obtain homogenous dose at the segment match, various gaps and smoothing between segments were introduced. Finally, a salivary gland MERT plan was executed using the segment match optimization scheme. Results: As the energy difference between two consecutive segments increased, dose uniformity at the segment junction decreased. For small segments (< 4 cm) optimization can be improved by introducing a gap (1–2 mm) between segments. However, for larger segments, smoothing was found to be more beneficial than gapping the segments. The salivary gland case was successful in terms of dose coverage and homogeneity aided by the segment matching techniques described. Conclusions: We conclude that use of bigger segments, and smaller differences in energy for abutting segments, should be prioritized for MERT planning. This study provides an optimization regimen of segment matching for MERT treatment planning with gaps/smoothing, depending on segment size and energy in order to provide better dose coverage and expedite the forward planning process. Varian Master Research Agreement

MO‐G‐213AB‐05: Beam Simulation and Measurement for Energy‐Intensity Modulated Electron Radiotherapy (MERT) with a Computer‐Controlled Electron Multileaf Collimator Device

Purpose: To verify an add‐on computer‐controlled multileaf collimator (eMLC) device on a Varian linac capable of delivering accurate dose for energy‐intensity modulated electron radiotherapy (MERT). Methods: The eMLC has 27 pairs of tungsten leaves (tongue and groove design to reduce intraleaf leakage)with 0.56cm width and 2cm thickness, providing a field size as large as 15 cm × 15 cm defined at 94cm SSD. Measurements were done to determine the appropriate jaw setting for an eMLC shaped field, mainly to reduce the leaf leakage outside the eMLC shaped field. The phase space data were acquired by Monte Carlo (MC) simulations for electron beams of energies 6, 9, 12 and 15 MeV, respectively and used as an input source in MC dose calculations in a phantom. MC calculated PDDs and dose profiles were compared with measurements for large fields (e.g. 10 cm × 10 cm) and small fields (e.g. 3.4 cm × 3.4 cm). The eMLC leakage for various energies was measured both in‐air and in phantom (at dmax) as a ratio of doses with the eMLC closed and completely open. Results: With the jaw position at 0.5 cm beyond the edge of the eMLC shaped field, it was showed to best eliminate the interleaf leakage, especially for high energies, e.g. 15 MeV. The average leaf leakage ranged from 0.3% (6 MeV) to 2.3% (15 MeV), which were consistent with lower in‐phantom values than in‐air values. MC calculated PDDs and dose profiles generally agreed with measurements to within 2mm/2%. Conclusions: This eMLC device is capable of delivering energy and intensity modulated electron beams accurately with acceptable leaf leakage for advanced MERT treatment.

MO‐G‐213AB‐04: Investigation of Photon MLC to Deliver Energy and Intensity Modulated Electron Beams for Advanced Radiotherapy

Purpose: To implement energy and intensity modulated electron radiotherapy (MERT) using an existing photon multileaf collimator (pMLC) in a modern medical accelerator. Methods: Both Siemens Artiste and Varian Trilogy accelerators have been investigated for MERT using the existing pMLC in the service mode. The limitations of SSD for Varian linac were examined. A Monte Carlo (MC)‐based inverse treatment planning code was used for plan optimization and dose calculation. Water scanning data including PDDs and dose profiles at various depths were acquired for various fields shaped by pMLC for 6, 9, 12 and 15MeV electron energies of both Artiste and Varian linacs and compared with MC calculations. Results: Artiste can treat patient with an SSD down to 61 cm to reduce the field penumbra caused by electron in‐air scatters. This is comparable to the Siemens Primus accelerator that has an SSD about 60cm. The electron beam characteristics are acceptable in terms of flatness and penumbra as compared with those collimated by electron applicators. Varian Trilogy has to use a longer SSD (about 85 to 90cm) due to the current restriction of the table height. Comparisons between measured PDDs and dose profiles, and MC calculations for various energies for pMLC shaped fields of 3 × 3 cm2and 10 × 10 cm2 showed agreement to within 2%/2mm. Conclusions: With pMLC shaped fields, MC dose calculations agree with measured PDDs and dose profiles at various depths, which warrant the dose accuracy for MERT delivered by pMLC using MC‐based dose calculation planning systems. With large SSD on Trilogy, dose profiles deteriorate to some extent as compared with those on Artiste with SSD = 61cm in terms of penumbral width. However the impact of this penumbra difference on the ultimate MERT plan using energy and intensity modulated electron beams is yet to be investigated.

WE‐C‐BRB‐04: Fast and Accurate Hybrid Source Model for Modulated Electron Radiotherapy

Purpose: To develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). Method: An algorithm has been developed for creating phase‐ space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre‐calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction, a particle is rejected if it is expected to suffer a leaf‐hit. A leaf‐hit counter is used to calculate the output of scatter particles based on the pre‐calculated scatter probabilities. To account for multiple coulomb scattering in air a MC‐corrected version of the Fermi‐Eyges scattering theory was implemented. Results: Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off‐axis fields. The model agreed with full MC dose distributions within 3% in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6% in all cases. The model was 16–22 times faster in generating a phase‐space file than a full MC simulation with the BEAMnrc code. Conclusions: Fast, dynamic electron beam calculations open up the possibility for real time delivery of MERT in the clinic and renew interest in electron beam 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 Rp 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 cm2 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 manufacturer's 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.

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-401: The Development of Intensity and Energy Modulated Electron Radiotherapy; An Alternative to Photon Volumetric Modulated Arc Therapy

Purpose: This work explains the development of an advanced treatment planning system for the generation of intensity and energy modulated electron radiotherapy (MERT) plans. The quality of MERT plans for the treatment of tumour bed boost in breast cancer is compared to direct electrons (DE) and volumetric modulated photon arc therapy (MAT). Method and Materials: The MERT treatment planning and delivery system at McGill University consists of MMCTP, an inverse optimization toolkit and the few leaf electron collimator. The stepwise planning process consists of: 1) generating a series of field openings, 2) Monte Carlo dose calculation for each field, 3) planning constraints, 4) iterative direct‐aperture optimization. For evaluation purposes, fourteen patients with breast cancer treated by lumpectomy and requiring post‐operative whole breast radiotherapy with tumour bed boost were planned using conventional DE, MAT and MERT. The planning goal was to deliver 10 Gy to at least 95% of the target volume. Dosimetry parameters for all techniques were compared. Results: Target coverage and homogeneity was best for MERT (D98=9.77 Gy, D2=11.03 Gy) followed by MAT (D98=9.56 Gy, D2=11.07 Gy) and DE (D98=9.81 Gy, D2=11.52 Gy). Relative to the DE plans, the MERT plans predicted a reduction of 35% in mean breast dose (p<0.05), 54% in mean lungdose (p<0.05) and 46% in mean body dose (p<0.05). Relative to the MAT plans, the MERT plans predicted a reduction of 24%, 36% and 39% in mean breast dose,heartdose and body dose respectively (p<0.05). Conclusions: MERT was a considerable improvement in dosimetry over DE. In some cases, there was a dosimetric advantage in using MERT over MAT for increased target conformity and low‐dose sparing of healthy tissue. Based on the favorable comparisons shown in this work it is reasonable to suggest that MERT could play a more significant role in breast radiotherapy.

SU-E-T-402: Modulated Electron Radiotherapy Dose Delivery Using Photon Multileaf Collimators

Purpose: Different designs of multileaf collimators (MLC) have been investigated for dose delivery for energy and intensity modulated electron radiotherapy (MERT). This work aims at verifying MERT dose delivery accuracy on clinical accelerators, Siemens Artiste and Varian Clinac iX, equipped with conventional photon multileaf collimators (pMLC). Method and Materials:Expensive measurements and Monte Carlo (MC) simulations were performed to acquire the electron beam data for Monte Carlo-based MERT treatment planning. The calculated percentage depth dose (PDD) and dose profiles were benchmarked with measurements. Differences of electron beam characteristics for fields shaped by the Artiste and Clinac iX pMLC were examined, demonstrating their strengths and weaknesses in MERT dose delivery. Results: MC simulated PDDs and dose profiles for electron energies, 6, 9, 12, 15 (Artiste) and 16 (Clinac iX) MeV with fields shaped by either applicators or MLCs agreed well (2%/2mm) with ion chamber measurements. For pMLC-shaped fields, no significant changes in PDDs were observed as SSD varied from 61 cm to 85 cm while the flatness and penumbra of dose profiles showed obvious deterioration as SSD increased. In comparison, the Clinac iX pMLC resulted in flatter electron beams with smaller penumbra than those collimated the Artiste pMLC most likely due to the differences of the pMLC locations. Conclusions: Both Siemens Artiste and Varian Clinac iX are capable of delivering energy and intensity modulated electron dose distributions efficiently using their existing pMLC. The electron beam properties shaped by pMLCs were better suited for MERT at smaller SSDs (with smaller gaps between the pMLC and the patient surface). For the same SSD, the Clinac iX pMLC manifested superior beam flatness and penumbra compared to the Artiste pMLC.

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

SU‐C‐BRA‐04: Experimental Feasibility Study on the Use of Scattering Foil Free Beams for Modulated Electron Radiotherapy

Purpose: This work aims to investigate the feasibility of scattering foil free beams for use in energy modulated electron therapy. The main significance of this investigation is the potential reduction in photon contamination within the Bremsstrahlung tail region and its corresponding reduction in dose to healthy tissue. Method and Materials: A Varian (Varian Medical, Inc., Palo Alto, CA) 21EX linear accelerator was used in conjunction with an existing, custom‐built electron collimator to produce 8×8 cm2 and 2×2 cm2 fields at isocenter. The scattering foil was removed from the beamline by removing the port plug from one of the extra ports and manually rotating the empty port into the beamline. PDDs and profiles were acquired with and without the scattering foil. Measurement data was used to commission a Monte Carlo model of the accelerator and the model was incorporated into an in‐house designed inverse treatment planning system. Plans for each beamline were generated on a solid water test phantom consisting of a target and two organs at risk. DVHˈs were then compared for each plan. Results: PDD curves showed a dose reduction factor between 7.4 and 12.2 within the Bremsstrahlung tail region with the scattering foil removed. For quasi‐identical target coverage, removal of the scattering foil was able to produce superior organ at risk sparing in the optimized plans in the test phantom.Conclusion: This work has shown that by removing the scattering foil from the beamline in energy modulated electron therapy the component of Bremsstrahlung photons produced in the accelerator head is significantly reduced allowing superior plans that spare healthy tissue. The practical aspects of the dosimetry of these beams using the internal transmission ion chamber in the accelerator is a topic of further study.

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.

SU-GG-T-104: Modulated Electron Radiotherapy for Bilateral Chest Wall Treatment

Purpose: To evaluate modulated electron radiotherapy (MERT) planning for delivery with the photon multicollimator (xMLC) for treatment of bilateral post-mastectomy chest wall treatment. Current techniques are complex, time consuming, and deficient in terms of target coverage and organ at risk (OAR) sparing. Conventional techniques require blocks and energy modifying bolus, while IMRT is volatile due to patient motion and provides unnecessary dose baths. Materials and Methods: An in-house Monte-Carlo-based MERT planning system has been developed for planning bilateral chest wall (MERT-biCW) treatment aided by previously developed in-house MERT optimization tools. The MERT-biCW planning was performed in three steps: 1) determine the number of portal fields needed to cover a PTV and partition the PTV into sub-PTVs for corresponding portals; 2) each sub-PTVs is then planned with MERT individually incorporating multiple energy assigned segments within each portal; 3) optimizing all portals to achieve the final plan. Several new features have been added to our MERT planning tools including; automatic selection of the best incident angle for each portal, whereby the distal surface of the tumor will be reached by selected energy and minimizing the air gap to <5 cm to maintain dose conformity. An algorithm has been developed to obtain optimal segments to facilitate target coverage and OAR sparing. Results: We typically covered the PTV with four portals for biCW. For a challenging case, we achieved acceptable PTV coverage with 80% isotope line covered by the prescription dose, and significantly reduced doses to OAR compared with conventional therapy. The V20 of left lung was reduced from 45.5 Gy to 36.9 Gy. V20 of heart was reduced from 18 to 5 Gy. Conclusions: MERT-biCW with its well-defined ranges and sharp fall-off is a promising alternative to conventional or IMRT post-mastectomy chest wall (CW) irradiation. Research is sponsored by Varian Grant.