Very High-energy Electron Beam Research Articles

Design of a 100-MeV compact VHEE beam line in Tsinghua University

A 100-MeV Compact Electron Accelerator design has been proposed for Very High Energy Electron (VHEE) radiotherapy research at Tsinghua University. The microwave source for this system is a 50 MW X-band klystron, paired with a pulse compressor featuring a correction cavity chain. During high-power test, the system achieved a flat-top power gain three times the input. The acceleration system consists of three main components: a backward traveling-wave buncher that bunches and accelerates electrons from a thermionic cathode gun to 8 MeV, followed by two 72-cell X-band constant-gradient traveling-wave accelerating structures, which further increase the electron energy to 100 MeV with a gradient of 80 MV/m. The total length of the system is 1.8 m, and its design is detailed in this paper.

Very High-Energy Electron Therapy Toward Clinical Implementation.

The use of very high energy electron (VHEE) beams, with energies between 50 and 400 MeV, has drawn considerable interest in radiotherapy due to their deep tissue penetration, sharp beam edges, and low sensitivity to tissue density. VHEE beams can be precisely steered with magnetic components, positioning VHEE therapy as a cost-effective option between photon and proton therapies. However, the clinical implementation of VHEE therapy (VHEET) requires advances in several areas: developing compact, stable, and efficient accelerators; creating sophisticated treatment planning software; and establishing clinically validated protocols. In addition, the perspective of VHEE to access ultra-high dose-rate regime presents a promising avenue for the practical integration of FLASH radiotherapy of deep tumors and metastases with VHEET (FLASH-VHEET), enhancing normal tissue sparing while maintaining the inherent dosimetric advantages of VHEET. However, FLASH-VHEET systems require validation of time-dependent dose parameters, thus introducing additional technological challenges. Here, we discuss recent progress in VHEET research, focusing on both conventional and FLASH modalities, and covering key aspects including dosimetric properties, radioprotection, accelerator technology, beam focusing, radiobiological effects, and clinical outcomes. Furthermore, we comprehensively analyze initial VHEET in silico studies on coverage across various tumor sites.

Optimizing focused very-high-energy electron beams for radiation therapy based on Monte Carlo simulation

A TOPAS-based optimization program has been developed to precisely concentrate the dose of focused very-high-energy electron (VHEE) beams on deep-seated targets. This is accomplished by optimizing the magnetic gradients, positions, and number of quadrupole magnets within TOPAS. Using only three quadrupole magnets, the program focuses 250 MeV VHEE beams to achieve a maximum dose position deeper than 17 cm, while maintaining entrance and exit doses within 25% and limiting the lateral dimensions to ≤ 1 cm at the maximum dose location. The linear relationship between the magnetic gradient of the last quadrupole magnet and the maximum dose position enables dose location adjustments through gradient variation. Multiple positions were validated in TOPAS with errors within 1%. The spread-out electron peak (SOEP) is achieved by combining two VHEE beams with different maximum dose positions using the differential evolution method, covering a target depth of 12–17 cm and attaining a dose flatness better than 99%. This pioneering program imposes constraints on entrance dose, exit dose, maximum dose position, and the lateral dimensions of dose deposition at the maximum dose position within phantom. This program may be a promising tool in the applications of focused VHEE in highly conformal treatment plans based on TOPAS.

Very high-energy electron therapy as light-particle alternative to transmission proton FLASH therapy – An evaluation of dosimetric performances

PurposeClinical translation of FLASH-radiotherapy (RT) to deep-seated tumours is still a technological challenge. One proposed solution consists of using ultra-high dose rate transmission proton (TP) beams of about 200–250 MeV to irradiate the tumour with the flat entrance of the proton depth-dose profile. This work evaluates the dosimetric performance of very high-energy electron (VHEE)-based RT (50–250 MeV) as a potential alternative to TP-based RT for the clinical transfer of the FLASH effect. MethodsBasic physics characteristics of VHEE and TP beams were compared utilizing Monte Carlo simulations in water. A VHEE-enabled research treatment planning system was used to evaluate the plan quality achievable with VHEE beams of different energies, compared to 250 MeV TP beams for a glioblastoma, an oesophagus, and a prostate cancer case. ResultsLike TP, VHEE above 100 MeV can treat targets with roughly flat (within ± 20 %) depth-dose distributions. The achievable dosimetric target conformity and adjacent organs-at-risk (OAR) sparing is consequently driven for both modalities by their lateral beam penumbrae. Electron beams of 400[500] MeV match the penumbra of 200[250] MeV TP beams and penumbra is increased for lower electron energies. For the investigated patient cases, VHEE plans with energies of 150 MeV and above achieved a dosimetric plan quality comparable to that of 250 MeV TP plans. For the glioblastoma and the oesophagus case, although having a decreased conformity, even 100 MeV VHEE plans provided a similar target coverage and OAR sparing compared to TP. ConclusionsVHEE-based FLASH-RT using sufficiently high beam energies may provide a lighter-particle alternative to TP-based FLASH-RT with comparable dosimetric plan quality.

Time dynamics of the dose deposited by relativistic ultra-short electron beams

Ultra-short electron beams are used as ultra-fast radiation source for radiobiology experiments aiming at very high energy electron beams (VHEE) radiotherapy with very high dose rates. Laser plasma accelerators are capable of producing electron beams as short as 1 fs and with tunable energy from few MeV up to multi-GeV with compact footprint. This makes them an attractive source for applications in different fields, where the ultra-short (fs) duration plays an important role. The time dynamics of the dose deposited by electron beams with energies in the range 50–250 MeV have been studied and the results are presented here. The results set a quantitative limit to the maximum dose rate at which the electron beams can impart dose.

New Advantage in Stereotactic Treatment of Lung and Pancreatic Cancer. Performance of Ultra-High Energy Electron (VHEE) Therapy Adjuvanted to the FLASH Effect: Clinical Implications and Treatment Plans Analysis

New Advantage in Stereotactic Treatment of Lung and Pancreatic Cancer. Performance of Ultra-High Energy Electron (VHEE) Therapy Adjuvanted to the FLASH Effect: Clinical Implications and Treatment Plans Analysis

Secondary radiation dose modeling in passive scattering and pencil beam scanning very high energy electron (VHEE) radiation therapy.

Electrons with kinetic energy up to a few hundred MeV, also called very high energy electrons (VHEE), are currently considered a promising technique for the future of radiation therapy (RT) and in particular ultra-high dose rate (UHDR) therapy. However, the feasibility of a clinical application is still being debated and VHEE therapy remains an active area of research for which the optimal conformal technique is also yet to be determined. In this work, we will apply two existing formalisms based on analytical Gaussian multiple-Coulomb scattering theory and Monte Carlo (MC) simulations to study and compare the electron and bremsstrahlung photon dose distributions arising from two beam delivery systems (passive scattering with or without a collimator or active scanning). We therefore tested the application of analytical and MC models to VHEE beams and assessed their performance and parameterization in the energy range of 6-200MeV. The optimized electron beam fluence, the bremsstrahlung, an estimation of central-axis and off-axis x-ray dose at the practical range and neutron contributions to the total dose, along with an extended parameterization for the photon dose model were developed, together with a comparison between double scattering (DS) and pencil beam scanning (PBS) techniques. MC simulations were performed with the TOPAS/Geant4 toolkit to verify the dose distributions predicted by the analytical calculations. The results for the clinical energy range (between 6 and 20MeV) as well as for higher energies (VHEE range between 20 and 200MeV) and for two treatment field sizes (5×5 and 10×10 cm2 ) are reported, showing a reasonable agreement with MC simulations with mean differences below 2.1%. The relative contributions of photons generated in the medium or by the scattering system along the central-axis (up to 50% of the total dose) are also illustrated, along with their relative variations with electron energy. The fast analytical models parametrized in this study allow an estimation of the amount of photons produced behind the practical range by a DS system with an accuracy lower than 3%, providing important information for the eventual design of a VHEE system. The results of this work could support future research on VHEE radiotherapy.

Beam optics study for a potential VHEE beam delivery system

VHEE (Very High Energy Electron) therapy can be superior to conventional radiotherapy for the treatment of deep seated tumours, whilst not necessarily requiring the space and cost of proton or heavy ion facilities. Developments in high gradient RF technology have allowed electrons to be accelerated to VHEE energies in a compact space, meaning that treatment could be possible with a shorter linac. A crucial component of VHEE treatment is the transfer of the beam from accelerator to patient. This is required to magnify the beam to cover the transverse extent of the tumour, whilst ensuring a uniform beam distribution. Two principle methodologies for the design of a compact transfer line are presented. The first of these is based upon a quadrupole lattice and optical magnification of beam size. A minimisation algorithm is used to enforce certain criteria on the beam distribution at the patient, defining the lattice through an automated routine. Separately, a dual scattering-foil based system is also presented, which uses similar algorithms for the optimisation of the foil geometry in order to achieve the desired beam shape at the patient location.

Determination of beam quality correction factors for the Roos plane-parallel ionisation chamber exposed to very high energy electron (VHEE) beams using Geant4

Detailed characterisation of the Roos secondary standard plane-parallel ionisation chamber has been conducted in a novel 200 MeV Very High Energy Electron (VHEE) beam with reference to the standard 12 MeV electron calibration beam used in our experimental work. Stopping-power-ratios and perturbation factors have been determined for both beams and used to calculated the beam quality correction factor using the Geant4 general purpose MC code. These factors have been calculated for a variety of charged particle transport parameters available in Geant4 which were found to pass the Fano cavity test. Stopping-power-ratios for the 12 MeV electron calibration beam quality were found to agree within uncertainties to that quoted by current dosimetry protocols. Perturbation factors were found to vary by up-to 4% for the calibration beam depending on the parameter configuration, compared with only 0.8% for the VHEE beam. Beam quality correction factors were found to describe an approximately 10% lower dose than would be originally calculated if a beam quality correction were not accounted for. Moreover, results presented here largely resolve unphysical chamber measurements, such as collection efficiencies greater than 100%, and assist in the accurate determination of absorbed dose and ion recombination in secondary standard ionisation chambers.

Deep Seated Tumour Treatments With Electrons of High Energy Delivered at FLASH Rates: The Example of Prostate Cancer.

Different therapies are adopted for the treatment of deep seated tumours in combination or as an alternative to surgical removal or chemotherapy: radiotherapy with photons (RT), particle therapy (PT) with protons or even heavier ions like 12C, are now available in clinical centres. In addition to these irradiation modalities, the use of Very High Energy Electron (VHEE) beams (100–200 MeV) has been suggested in the past, but the diffusion of that technique was delayed due to the needed space and budget, with respect to standard photon devices. These disadvantages were not paired by an increased therapeutic efficacy, at least when comparing to proton or carbon ion beams. In this contribution we investigate how recent developments in electron beam therapy could reshape the treatments of deep seated tumours. In this respect we carefully explored the application of VHEE beams to the prostate cancer, a well-known and studied example of deep seated tumour currently treated with high efficacy both using RT and PT. The VHEE Treatment Planning System was obtained by means of an accurate Monte Carlo (MC) simulation of the electrons interactions with the patient body. A simple model of the FLASH effect (healthy tissues sparing at ultra-high dose rates), has been introduced and the results have been compared with conventional RT. The study demonstrates that VHEE beams, even in absence of a significant FLASH effect and with a reduced energy range (70–130 MeV) with respect to implementations already explored in literature, could be a good alternative to standard RT, even in the framework of technological developments that are nowadays affordable.

Focused VHEE (very high energy electron) beams and dose delivery for radiotherapy applications

This paper presents the first demonstration of deeply penetrating dose delivery using focused very high energy electron (VHEE) beams using quadrupole magnets in Monte Carlo simulations. We show that the focal point is readily modified by linearly changing the quadrupole magnet strength only. We also present a weighted sum of focused electron beams to form a spread-out electron peak (SOEP) over a target region. This has a significantly reduced entrance dose compared to a proton-based spread-out Bragg peak (SOBP). Very high energy electron (VHEE) beams are an exciting prospect in external beam radiotherapy. VHEEs are less sensitive to inhomogeneities than proton and photon beams, have a deep dose reach and could potentially be used to deliver FLASH radiotherapy. The dose distributions of unfocused VHEE produce high entrance and exit doses compared to other radiotherapy modalities unless focusing is employed, and in this case the entrance dose is considerably improved over existing radiations. We have investigated both symmetric and asymmetric focusing as well as focusing with a range of beam energies.

Characteristics of very high-energy electron beams for the irradiation of deep-seated targets.

Driven by advances in accelerator technology and the potential of exploiting the FLASH effect for the treatment of deep-seated targets (>5cm), there is an active interest in the construction of devices to deliver very high-energy electron (VHEE) beams for radiation therapy. The application of novel VHEE devices, however, requires an assessment of the tradeoffs between the different beam parameter choices including beam energies, beam divergences, and maximal field sizes. This study systematically examines the dosimetric beam properties of VHEE beams, determining their clinical usefulness while marking their limits of applications for different beam configurations. We performed Monte Carlo simulations of the dose distributions of electron beams for different energies (25-250MeV), source-to-surface distances (SSD) (50cm, 100cm, parallel), and field sizes (2cm2 ×2cm2 to 15cm2 ×15cm2 ) in water using a research version of the RayStation treatment planning system (RaySearch Labs 9A IONPG). The beam was simulated using a monoenergetic point source and perfect collimation. Central axis percentage depth dose (PDD) and transverse dose profiles at multiple depths were evaluated and compared to those of MV photon beams. Profile characteristics including therapeutic range (TR) at 90%, proximal fall-off (PFO) at 90%, lateral penumbra (LP) at 90%-10%, and field width (FW) at 90% were obtained. Very high-energy electrons beams with SSD 100cm and parallel beams (infinite SSD) exhibit a linear to near-linear increase of TR as a function of energy in the simulated energy range and reach values well beyond the typical depths of lesions encountered in clinics (<20cm). Their TR show a marked field size dependence only for field sizes <10cm2 ×10cm2 . For VHEE beams with SSD 50cm, TR are largely reduced (4-8cm). For beam energies >150MeV with large SSD (>100cm), for many configurations, there is no substantial difference in PDD when adding an opposed beam. This may potentially reduce the number of VHEE beams needed for treatment by a factor of two compared to a treatment using lower energies and lower SSD. In order to cover deep-seated targets homogeneously, VHEE devices with a parallel beam must provide a maximum field size up to several centimeters larger than the tumor size. For the investigated diverging beams, there is not such a significant field width reduction with depth for larger fields as it is compensated by divergence. Penumbrae of VHEE beams are smaller than those of clinical MV photon beams for lower depths (<5cm) but increase quickly for larger depths. There is only a relatively small dependence of penumbra on the SSD of the beam. The findings presented in this study assess the performance of VHEE beams and offer a first estimate of treatment indications and tradeoffs for a given design of a VHEE device. SSD >100cm results in clinically more favorable PDD. Beam energies of 100MeV and above are needed to cover common tumors (5-15cm in-depth) conformally. Higher energies provide an additional benefit specifically for small and deep-seated lesions due to their reduced lateral penumbrae.

An experimental study of focused very high energy electron beams for radiotherapy

Very high energy electron (VHEE) beams have been proposed as an alternative radiotherapy modality to megavoltage photons; they penetrate deeply without significant scattering in inhomogeneous tissue because of their high relativistic inertia. However, the depth dose distribution of a single, collimated VHEE beam is quasi-uniform, which can lead to healthy tissue being overexposed. This can be largely overcome by focusing the VHEE beam to a small spot. Here, we present experiments to demonstrate focusing as a means of concentrating dose into small volumetric elements inside a target. We find good agreement between measured dose distributions and Monte Carlo simulations. Focused radiation beams could be used to precisely target tumours or hypoxic regions of a tumour, which would enhance the efficacy of radiotherapy. The development of new accelerator technologies may provide future compact systems for delivering these focused beams to tumours, a concept that can also be extended to X-rays and hadrons.

Influence of heterogeneous media on Very High Energy Electron (VHEE) dose penetration and a Monte Carlo-based comparison with existing radiotherapy modalities

Influence of heterogeneous media on Very High Energy Electron (VHEE) dose penetration and a Monte Carlo-based comparison with existing radiotherapy modalities

Very high-energy electron (VHEE) beams in radiation therapy; Treatment plan comparison between VHEE, VMAT, and PPBS.

The aim of this study was to evaluate the performance of very high-energy electron beams (VHEE) in comparison to clinically derived treatment plans generated with volumetric modulated arc therapy (VMAT) and proton pencil beam scanning (PPBS) technology. We developed a custom optimization script that could be applied automatically across modalities to eliminate operator bias during IMRT optimization. Four clinical cases were selected (prostate cancer, lung cancer, pediatric brain tumor, and head and neck cancer (HNC)). The VHEE beams were calculated in the EGSnrc/DOSXYZnrc Monte Carlo code for 100 and 200MeV beams. Treatment plans with VHEE, VMAT, and PPBS were optimized in a research version of RayStation using an in-house developed script to minimize operator bias between the different techniques. The in-house developed script generated similar or superior plans to the clinically used plans. In the comparisons between the modalities, the integral dose was lowest for the PPBS-generated plans in all cases. For the prostate case, the 200MeV VHEE plan showed reduced integral dose and reduced organ at risk (OAR) dose compared to the VMAT plan. For all other cases, both the 100 and the 200MeV VHEE plans were superior to the VMAT plans, and the VHEE plans showed better conformity and lower spinal cord dose in the pediatric brain case and lower brain stem dose in the HNC case when compared to the PPBS plan. The automated optimization developed in this study generated similar or superior plans as compared to the clinically used plan and represents an unbiased approach to compare treatment plans generated for different modalities. In the present study, we also show that VHEE plans are similar or superior to VMAT plans with reduced mean OAR dose and increased target conformity for a variety of clinical cases, and VHEE plans can even achieve reductions in OAR doses compared to PPBS plans for shallow targets. With increased VHEE energy, better conformity and even higher reductions in mean OAR doses are achieved. On the whole, VHEE was intermediate between photon VMAT and PPBS for OAR sparing.

Sci—Thur PM: Planning &amp; Delivery — 02: Treatment planning workflow for very high‐energy electron beam radiotherapy

Purpose:To develop treatment planning workflow for rapid radiotherapy delivered with very‐high energy electron (VHEE) scanning beam.Methods:VHEE radiotherapy treatment planning was performed by linking Monte Carlo (MC) dose calculations with inverse optimization in a research version of RayStation. In order to study a number of treatment parameters, a Matlab graphical user interface (GUI) for calculation of VHEE beamlet dose was developed. Through the GUI, EGSnrc MC simulations were run for a number of beam energies, number of beams, beamlet spot and grid sizes, and machine bore sizes. VHEE plans for a pediatric patient with a 4.3 cm3 brain target optimized with spot‐scanning algorithm in RayStation were compared to the clinically delivered 6 MV VMAT plan.Results and Discussion:VHEE beam energy had the largest effect on the quality of dose distributions. For the same target dose, the mean doses to critical organs decreased by 10–15% when planned with 100 MeV compared to 60 MeV. VHEE plans calculated with 36 beams outperformed plans calculated with 13 and 17 beams. While beamlet spacing and bore size had a small effect on VHEE dose distributions, 0.1‐3mm beamlet sizes resulted in identical dose distributions. Critical organ doses were by up to 70% lower in the best VHEE plan compared to the clinical 6 MV VMAT plan.Conclusions:We have developed a GUI for MC beamlet generation for treatment planning of VHEE radiotherapy. We have demonstrated that pediatric VHEE plans resulted in significant critical organ dose sparing compared to the clinical VMAT plan.

MO‐H‐19A‐01: FEATURED PRESENTATION – Treatment Planning Tool for Radiotherapy with Very High‐Energy Electron Beams

Purpose:To develop a tool for treatment planning optimization for fast radiotherapy delivered with very high‐energy electron beams (VHEE) and to compare VHEE plans to state‐of‐the‐art plans for challenging pelvis and H' N cases.Methods:Treatment planning for radiotherapy delivered with VHEE scanning pencil beams was performed by integrating EGSnrc Monte Carlo (MC) dose calculations with spot scanning optimization run in a research version of RayStation. A Matlab GUI for MC beamlet generation was developed, in which treatment parameters such as the pencil beam size and spacing, energy and number of beams can be selected. Treatment planning study for H' N and pelvis cases was performed and the effect of treatment parameters on the delivered dose distributions was evaluated and compared to the clinical treatment plans. The pelvis case with a 691cm3 PTV was treated with 2‐arc 15MV VMAT and the H' N case with four PTVs with total volume of 531cm3 was treated with 4‐arc 6MV VMAT.Results:Most studied VHEE plans outperformed VMAT plans. The best pelvis 80MeV VHEE plan with 25 beams resulted in 12% body dose sparing and 8% sparing to the bowel and right femur compared to the VMAT plan. The 100MeV plan was superior to the 150MeV plan. Mixing 100 and 150MeV improved dose sparing to the bladder by 7% compared to either plan. Plans with 16 and 36 beams did not significantly affect the dose distributions compared to 25 beam plans. The best H' N 100MeV VHEE plan decreased mean doses to the brainstem, chiasm, and both globes by 10‐42% compared to the VMAT plan.Conclusion:The pelvis and H' N cases suggested that sixteen 100MeV beams might be sufficient specifications of a novel VHEE treatment machine. However, optimum machine parameters will be determined with the presented VHEE treatment‐planning tool for a large number of clinical cases.BW Loo and P Maxim received research support from RaySearch Laboratories. E Hynning and B Hardemark are employees of RaySearch Laboratories.

SU‐E‐T‐274: Radiation Therapy with Very High‐Energy Electron (VHEE) Beams in the Presence of Metal Implants

Purpose:To evaluate the effect of metal implants on treatment plans for radiation therapy with very high‐energy electron (VHEE) beams.Methods:The DOSXYZnrc/BEAMnrc Monte Carlo (MC) codes were used to simulate 50–150MeV VHEE beam dose deposition and its effects on steel and titanium (Ti) heterogeneities in a water phantom. Heterogeneities of thicknesses ranging from 0.5cm to 2cm were placed at 10cm depth. MC was also used to calculate electron and photon spectra generated by the VHEE beams' interaction with metal heterogeneities. The original VMAT patient dose calculation was planned in Eclipse. Patient dose calculations with MC‐generated beamlets were planned using a Matlab GUI and research version of RayStation. VHEE MC treatment planning was performed on water‐only geometry and water with segmented prostheses (steel and Ti) geometries with 100MeV and 150MeV beams.Results:100MeV PDD 5cm behind steel/Ti heterogeneity was 51% less than in the water‐only phantom. For some cases, dose enhancement lateral to the borders of the phantom increased the dose by up to 22% in steel and 18% in Ti heterogeneities. The dose immediately behind steel heterogeneity decreased by an average of 6%, although for 150MeV, the steel heterogeneity created a 23% increase in dose directly behind it. The average dose immediately behind Ti heterogeneities increased 10%. The prostate VHEE plans resulted in mean dose decrease to the bowel (20%), bladder (7%), and the urethra (5%) compared to the 15MV VMAT plan. The average dose to the body with prosthetic implants was 5% higher than to the body without implants.Conclusion:Based on MC simulations, metallic implants introduce dose perturbations to VHEE beams from lateral scatter and backscatter. However, when performing clinical planning on a prostate case, the use of multiple beams and inverse planning still produces VHEE plans that are dosimetrically superior to photon VMAT plans.BW Loo and P Maxim received research support from RaySearch laboratories; B Hardemark and E Hynning are employees of RaySearch.

SU-D-19A-06: The Effect of Beam Parameters On Very High-Energy Electron Radiotherapy: A Planning Study

Purpose: We evaluated the effect of very high-energy electron (VHEE) beam parameters on the planning of a lung cancer case by means of Monte Carlo simulations. Methods: We simulated VHEE radiotherapy plans using the EGSnrc/BEAMnrc-DOSXYZnrc code. We selected a lung cancer case that was treated with 6MV photon VMAT to be planned with VHEE. We studied the effect of beam energy (80 MeV, 100 MeV, and 120 MeV), number of equidistant beams (16 or 32), and beamlets sizes (3 mm, 5 mm or 7 mm) on PTV coverage, sparing of organs at risk (OARs) and dose conformity. Inverse-planning optimization was performed in a research version of RayStation (RaySearch Laboratories AB) using identical objective functions and constraints for all VHEE plans. Results: Similar PTV coverage and dose conformity was achieved by all the VHEE plans. The 100 MeV and 120 MeV VHEE plans were equivalent amongst them and were superior to the 80 MeV plan in terms of OARs sparing. The effect of using 16 or 32 equidistant beams was a mean difference in average dose of 2.4% (0%–7.7%) between the two plans. The use of 3 mm beamlet size systematically reduced the dose to all the OARs. Based on these results we selected the 100MeV-16beams-3mm-beamlet-size plan to compare it against VMAT. The selected VHEE plan was more conformal than VMAT and improved OAR sparing (heart and trachea received 125% and 177% lower dose, respectively) especially in the low-dose region. Conclusion: We determined the VHEE beam parameters that maximized the OAR dose sparing and dose conformity of the actually delivered VMAT plan of a lung cancer case. The selected parameters could be used for the planning of other treatment sites with similar size, shape, and location. For larger targets, a larger beamlet size might be used without significantly increasing the dose. B Palma: None. M Bazalova: None. B Hardemark: Employee, RaySearch Americas. E Hynning: Employee, RaySearch Americas. B Qu: None. B Loo Jr.: Research support, RaySearch, Varian. P Maxim: Research support, RaySearch, Varian.

WE‐C‐108‐01: JUNIOR INVESTIGATOR WINNER — Towards Radiation Therapy with Very High‐Energy Electron Beams

Purpose: To experimentally validate Monte Carlo (MC) simulations for dose calculations with very high‐energy electron (VHEE) beams in the presence of heterogeneities and to use MC simulations for treatment planning optimization of ultra‐fast radiation therapy with VHEE beams. Methods: Polystyrene phantom with heterogeneities made of nine materials of various thicknesses was irradiated at a VHEE beam line at SLAC National Accelerator Laboratory. Films were used to measure the dose delivered to the phantom by 60–110MeV VHEE beams of 0.6–2.2mm in size. A MC model of the beam line was built and validated against the experimental data based on percentage depth‐dose (PDD) curves. The MC validated code was used for VHEE radiotherapy planning of three patients. More specifically, MC beamlets were exported into a modified version of RayStation for spot‐scanning optimization. The quality of VHEE dose distributions as a function of electron beam energy, beamlet size and spacing, source‐to‐isocenter distance, and number of beams was studied. Results: MC calculated PDD curves for heterogeneous phantoms were in agreement with the experimental PDD curves obtained at the VHEE beam line for all beam energies and beam sizes. VHEE treatment planning optimization study for a pediatric patient brain tumor revealed that significant normal tissue sparing was achieved in 60–100MeV VHEE plans compared to the state‐of‐the‐art 6MV VMAT plan. The mean dose to the cochleae was by 56–72% lower in the VHEE plans. Out of the studied parameters, electron beam energy had the largest effect on the quality of VHEE treatment plans. In order to generate a VHEE plan matching a VMAT plan for a large patient, VHEE beams of 100–150MeV are needed. Conclusion: We have validated VHEE beam MC dose calculations and we have developed a MC/RayStation treatment planning optimization tool for evaluation of the potential to rapidly treat cancer with VHEE beams. This work has been supported by the 2012 AAPM Research Seed Grant.