Intensity Modulated Electron Beams Research Articles

Intensity Modulation in Electron FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Combining methods of conformal dose delivery with ultra-high dose rate (UHDR) beams is advantageous for realizing the benefits of the FLASH effect in clinical treatments. We hypothesize that deployment of intensity modulation in passive electron FLASH radiotherapy through treatment plan optimization is feasible. <h3>Materials/Methods</h3> A beam model of an electron FLASH irradiator (delivering ∼300Gy/s at the isocenter) in the decimal ElectronRT treatment planning system (TPS) was validated with film measured lateral and percent-depth-dose (PDD) profiles. Plans were developed comparing collimated open fields and intensity modulated electron beams achieved with cylindrical passive metal compensators that vary in spacing and size. Homogeneity and conformity were quantified for plans developed in a water phantom and anonymized patient cases considering constraints in prescribed dose to the target volume and minimizing dose to organs-at-risk (OAR). <h3>Results</h3> The film measured profiles and TPS beam model agreed on average to within 1% and 2% for lateral profile and PDD, respectively. For a large 15 × 15 cm² field in a water phantom the intensity modulation improved the beam flatness (∼30% to <5%) and penumbra (35 mm to 15mm) at 3 cm depth while retaining UHDR in the treatment field with a 38% reduction in central axis output while still retaining the UHDR conditions (∼180Gy/sec). The PDD exhibited <2% difference along the central axis and minimal changes to symmetry, shift, and practical range. In the rib metastasis case, the unmodulated and modulated plans treat the tumor volume with a homogeneity index (HI), improvement by 0.2. In the facial orbital plan, the conformity index improved by 8% with an improvement in HI by 0.05 and comparable dose to OARs. <h3>Conclusion</h3> Intensity modulation of electrons FLASH is achievable and improves homogeneity of prescribed dose with proper treatment constraints while reducing hot spots for clinical plans. Depending on the treatment case intensity modulation can also generate superior dose distributions while retaining the UHDR conditions to best exploit the FLASH effect. Future study will focus on demonstrating dosimetric benefits quantified by tumor control probability and normal tissue complication probability models in a larger number of relevant cases.

Planning and delivery of intensity modulated bolus electron conformal therapy.

PurposeBolus electron conformal therapy (BECT) is a clinically useful, well‐documented, and available technology. The addition of intensity modulation (IM) to BECT reduces volumes of high dose and dose spread in the planning target volume (PTV). This paper demonstrates new techniques for a process that should be suitable for planning and delivering IM‐BECT using passive radiotherapy intensity modulation for electrons (PRIME) devices.MethodsThe IM‐BECT planning and delivery process is an addition to the BECT process that includes intensity modulator design, fabrication, and quality assurance. The intensity modulator (PRIME device) is a hexagonal matrix of small island blocks (tungsten pins of varying diameter) placed inside the patient beam‐defining collimator (cutout). Its design process determines a desirable intensity‐modulated electron beam during the planning process, then determines the island block configuration to deliver that intensity distribution (segmentation). The intensity modulator is fabricated and quality assurance performed at the factory (.decimal, LLC, Sanford, FL). Clinical quality assurance consists of measuring a fluence distribution in a plane perpendicular to the beam in a water or water‐equivalent phantom. This IM‐BECT process is described and demonstrated for two sites, postmastectomy chest wall and temple. Dose plans, intensity distributions, fabricated intensity modulators, and quality assurance results are presented.ResultsIM‐BECT plans showed improved D90‐10 over BECT plans, 6.4% versus 7.3% and 8.4% versus 11.0% for the postmastectomy chest wall and temple, respectively. Their intensity modulators utilized 61 (single diameter) and 246 (five diameters) tungsten pins, respectively. Dose comparisons for clinical quality assurance showed that for doses greater than 10%, measured agreed with calculated dose within 3% or 0.3 cm distance‐to‐agreement (DTA) for 99.9% and 100% of points, respectively.ConclusionThese results demonstrated the feasibility of translating IM‐BECT to the clinic using the techniques presented for treatment planning, intensity modulator design and fabrication, and quality assurance processes.

SU‐F‐T‐81: Treating Nose Skin Using Energy and Intensity Modulated Electron Beams with Monte Carlo Based Dose Calculation

Purpose:Treating nose skin with an electron beam is of a substantial challenge due to uneven nose surfaces and tissue heterogeneity, and consequently could have a great uncertainty of dose accuracy on the target. This work explored the method using Monte Carlo (MC)‐based energy and intensity modulated electron radiotherapy (MERT), which would be delivered with a photon MLC in a standard medical linac (Artiste).Methods:The traditional treatment on the nose skin involves the usage of a bolus, often with a single energy electron beam. This work avoided using the bolus, and utilized mixed energies of electron beams. An in‐house developed Monte Carlo (MC)‐based dose calculation/optimization planning system was employed for treatment planning. Phase space data (6, 9, 12 and 15 MeV) were used as an input source for MC dose calculations for the linac. To reduce the scatter‐caused penumbra, a short SSD (61 cm) was used. A clinical case of the nose skin, which was previously treated with a single 9 MeV electron beam, was replanned with the MERT method. The resultant dose distributions were compared with the plan previously clinically used. The dose volume histogram of the MERT plan is calculated to examine the coverage of the planning target volume (PTV) and critical structure doses.Results:The target coverage and conformality in the MERT plan are improved as compared to the conventional plan. The MERT can provide more sufficient target coverage and less normal tissue dose underneath the nose skin.Conclusion:Compared to the conventional treatment technique, using MERT for the nose skin treatment has shown the dosimetric advantages in the PTV coverage and conformality. In addition, this technique eliminates the necessity of the cutout and bolus, which makes the treatment more efficient and accurate.

SU‐E‐T‐632: Preliminary Study On Treating Nose Skin Using Energy and Intensity Modulated Electron Beams with Monte Carlo Based Dose Calculations

Purpose:Uneven nose surfaces and air cavities underneath and the use of bolus present complexity and dose uncertainty when using a single electron energy beam to plan treatments of nose skin with a pencil beam‐based planning system. This work demonstrates more accurate dose calculation and more optimal planning using energy and intensity modulated electron radiotherapy (MERT) delivered with a pMLC.Methods:An in‐house developed Monte Carlo (MC)‐based dose calculation/optimization planning system was employed for treatment planning. Phase space data (6, 9, 12 and 15 MeV) were used as an input source for MC dose calculations for the linac. To reduce the scatter‐caused penumbra, a short SSD (61 cm) was used. Our previous work demonstrates good agreement in percentage depth dose and off‐axis dose between calculations and film measurement for various field sizes. A MERT plan was generated for treating the nose skin using a patient geometry and a dose volume histogram (DVH) was obtained. The work also shows the comparison of 2D dose distributions between a clinically used conventional single electron energy plan and the MERT plan.Results:The MERT plan resulted in improved target dose coverage as compared to the conventional plan, which demonstrated a target dose deficit at the field edge. The conventional plan showed higher dose normal tissue irradiation underneath the nose skin while the MERT plan resulted in improved conformity and thus reduces normal tissue dose.Conclusion:This preliminary work illustrates that MC‐based MERT planning is a promising technique in treating nose skin, not only providing more accurate dose calculation, but also offering an improved target dose coverage and conformity. In addition, this technique may eliminate the necessity of bolus, which often produces dose delivery uncertainty due to the air gaps that may exist between the bolus and skin.

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.

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.

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.

Intensity- and energy-modulated electron radiotherapy by means of an xMLC for head and neck shallow tumors

The purpose of this paper is to assess the feasibility of delivering intensity- and energy-modulated electron radiation treatment (MERT) by a photon multileaf collimator (xMLC) and to evaluate the improvements obtained in shallow head and neck (HN) tumors. Four HN patient cases covering different clinical situations were planned by MERT, which used an in-house treatment planning system that utilized Monte Carlo dose calculation. The cases included one oronasal, two parotid and one middle ear tumors. The resulting dose-volume histograms were compared with those obtained from conventional photon and electron treatment techniques in our clinic, which included IMRT, electron beam and mixed beams, most of them using fixed-thickness bolus. Experimental verification was performed with plane-parallel ionization chambers for absolute dose verification, and a PTW ionization chamber array and radiochromic film for relative dosimetry. A MC-based treatment planning system for target with compromised volumes in depth and laterally has been validated. A quality assurance protocol for individual MERT plans was launched. Relative MC dose distributions showed a high agreement with film measurements and absolute ion chamber dose measurements performed at a reference point agreed with MC calculations within 2% in all cases. Clinically acceptable PTV coverage and organ-at-risk sparing were achieved by using the proposed MERT approach. MERT treatment plans, based on delivery of intensity-modulated electron beam using the xMLC, for superficial head and neck tumors, demonstrated comparable or improved PTV dose homogeneity with significantly lower dose to normal tissues. The clinical implementation of this technique will be able to offer a viable alternative for the treatment of shallow head and neck tumors.

Electron conformal radiotherapy using bolus and intensity modulation

Purpose: Conformal electron beam therapy can be delivered using shaped bolus, which varies the penetration of the electrons across the incident beam so that the 90% isodose surface conforms to the distal surface of the planning target volume (PTV). Previous use of this modality has shown that the irregular proximal surface of the bolus causes the dose heterogeneity in the PTV to increase from 10%, the typical dose spread of a flat-water surface to approximately 20%. The present work evaluates the ability to restore dose homogeneity by varying the incident electron intensity.Methods and Materials: Three patients, one each with chest wall, thorax, and head-and-neck cancer, were planned using electron conformal therapy with bolus, with and without intensity modulation. Resulting dose distributions and dose-volume histograms were compared with non-intensity-modulated bolus plans.Results: In all cases, the ΔD90%−10% for the PTV was reduced; for example, for the head-and-neck case, the ΔD90%−10% for the PTV was reduced from 14.9% to 9.2%. This reduction in dose spread is a direct result of intensity modulation.Conclusions: The results showed that intensity-modulated electron beams could significantly improve the dose homogeneity in the PTV for patients treated with electron conformal therapy using shaped bolus.

High energy electron beams in intensity modulated radiation therapy

The aims of the thesis were the development of the phase space evolution (PSE) model for electron beam treatment planning, investigation of techniques to improve electron beam dose distributions, and investigation of the added value of high energy (⩽50 MeV) electron beams in intensity modulated radiotherapy. The PSE model is capable of calculating dose distributions by taking into account density variations in full three dimensions and is accurate and fast compared to Fermi–Eyges based pencil beam models or Monte Carlo models. A relatively simple model that requires only measurements as input is adequate to describe the initial electron beam. The PSE model can also be used for the calculation of pencil beam kernels for optimization of intensity modulated electron beam treatments. Techniques, such as intensity modulation, energy modulation, and mixing with photons are suggested to improve dose distributions resulting from electron beam irradiations from a single direction. It is shown that, by the combination of intensity modulated electron and photon beams from one direction, dose distributions can have steep dose falloffs similar to unmodulated electron beams, and narrow penumbras similar to photon beams. The added value of high energy electron beams was investigated in a comparative treatment planning study using optimized intensity modulated electron and photon beams. For the studied clinical cases use of intensity modulated photon beams gives significant improvements over conventional treatments. Inclusion of electron beams could only marginally improve the expected treatment outcome, although dose volume histograms of organs at risk show improvements, especially at lower dose levels (e.g., &lt;40 Gy) in the treatments with electrons.

Energy- and intensity-modulated electron beams forradiotherapy

This work investigates the feasibility of optimizing energy- andintensity-modulated electron beams for radiation therapy. A multileafcollimator (MLC) specially designed for modulated electron radiotherapy (MERT)was investigated both experimentally and by Monte Carlo simulations. Aninverse-planning system based on Monte Carlo dose calculations was developedto optimize electron beam energy and intensity to achieve dose conformity fortarget volumes near the surface. The results showed that an MLC with 5 mm leafwidths could produce complex field shapes for MERT. Electron intra- andinter-leaf leakage had negligible effects on the dose distributions deliveredwith the MLC, even at shallow depths. Focused leaf ends reduced the electronscattering contributions to the dose compared with straight leaf ends. Asanticipated, moving the MLC position toward the patient surface reduced thepenumbra significantly. There were significant differences in the beamletdistributions calculated by an analytic 3-D pencil beam algorithm and theMonte Carlo method. The Monte Carlo calculated beamlet distributions wereessential to the accuracy of the MERT dose distribution in cases involvinglarge air gaps, oblique incidence and heterogeneous treatment targets (at thetissue-bone and bone-lung interfaces). To demonstrate the potential of MERTfor target dose coverage and normal tissue sparing for treatment ofsuperficial targets, treatment plans for a hypothetical treatment werecompared using photon beams and MERT.

Mixing intensity modulated electron and photon beams: combining a steep dose fall-off at depth with sharp and depth-independent penumbras and flat beam profiles

For application in radiotherapy, intensity modulated high-energy electron and photon beams were mixed to create dose distributions that feature: (a) a steep dose fall-off at larger depths, similar to pure electron beams, (b) flat beam profiles and sharp and depth-independent beam penumbras, as in photon beams, and (c) a selectable skin dose that is lower than for pure electron beams. To determine the required electron and photon beam fluence profiles, an inverse treatment planning algorithm was used. Mixed beams were realized at a MM50 racetrack microtron (Scanditronix Medical AB, Sweden), and evaluated by the dose distributions measured in a water phantom. The multileaf collimator of the MM50 was used in a static mode to shape overlapping electron beam segments, and the dynamic multileaf collimation mode was used to realize the intensity modulated photon beam profiles. Examples of mixed beams were generated at electron energies of up to 40 MeV. The intensity modulated electron beam component consists of two overlapping concentric fields with optimized field sizes, yielding broad, fairly depth-independent overall beam penumbras. The matched intensity modulated photon beam component has high fluence peaks at the field edges to sharpen this penumbra. The combination of the electron and the photon beams yields dose distributions with the characteristics (a)-(c) mentioned above.

37 Energy- and intensity-modulated electron beams for treatment of breast cancer

37 Energy- and intensity-modulated electron beams for treatment of breast cancer

Intensity modulation with electrons: calculations, measurements and clinical applications

Intensity modulation of electron beams is one step towards truly conformal therapy. This can be realized with the MM50 racetrack microtron that utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the monitor chambers in the treatment head are segmented in both x- and y-directions it is possible to verify the fluence distribution to the patient at any time during the treatment. Intensity modulated electron beams have been measured with film and a plane parallel chamber and compared with calculations. The calculations were based on a pencil beam method. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan patterns. By convolving this distribution with a Gaussian pencil beam, which has propagated from the MLC to the isocentre, a fluence distribution at isocentre level was obtained. The agreement between calculations and measurements was within 2% in dose or 1 mm in distance in the penumbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment planning system and are used for manual optimization. A clinical example (prostate) of such an application is presented and compared with a standard irradiation technique.

Electron-surface interaction and metallization of the CaF2(111)-surface studied by photothermal techniques

Abstract The interaction of low energy electrons with the surface of alkaline-earth halides results in a variety of microscopical physical phenomena commonly described in terms of defect formation and diffusion, surface metallization and desorption of neutral- and charged particles. These processes are accompanied by local changes in the electronic and geometrical structure of bulk and surface and result in a variation of macroscopically measurable parameters like modulated optical reflectance and a deformation of the crystal lattice. In this paper it will be shown that photothermal analysis, that so far has mostly been used for the determination of optical and thermophysical properties of materials, is also capable of measuring defect related nonthermal phenomena apparent during electron irradiation of insulator surfaces. Experiments were performed with an intensity modulated electron beam of typically 1 μA at 1 keV focused into a spot of 1 mm2 on the (111)-surface of a polished CaF2 single crystal under ultra-high vacuum conditions. Measurements revealed that modulated reflectance is sensitive to changes in optical properties induced by electron irradiation induced defects at low electron dosages. At a dosage level where metallization starts, a dramatic change in the modulated reflectance signal was observed indicating changes in electronic structure due to metal clustering at the surface. The photothermal displacement technique has been utilized to monitor surface deformations induced by electron bombardment. It was found that results cannot be explained by a thermo-elastic expansion model that works well for metal surfaces. Therefore, this technique can be used for the measurement of nonthermal contributions to lattice expansion resulting from volume changes of created defects.

Investigation of Si-Fe transformer sheets by scanning electron acoustic microscopy (SEAM)

Scanning electron acoustic microscopy (SEAM) is a fairly new technique in the microcharacterization of materials in which an intensity-modulated electron beam is used to generate an acoustic signal within the probed specimen location. These signals can be used for location and identification of many material parameters. Detection of the sound at the modulation frequency,f, results in so-called linear images, whereas the use of signals at even harmonics of this frequency (especially:2f) yields images as a consequence of nonlinear interactions. These nonlinear techniques were first introduced to the microscopy of semiconductors and have proven now to be very advantageous for the investigation of metals, ln this paper both the linear and the nonlinear technique are applied to the characterization of Si-Fe-transformer steel sheets. It can be shown that the linear mode exhibits only the grain structure of the material, whereas the nonlinear mode reveals the magnetic domain structure of the sheets. Thus by one experiment it is possible to immage grains and domains separately. Furthermore, this is possible even without any specimen preparation,especially without removal or polishing of the ceramic coating.

Measurement of minority carrier lifetime in GaAs and GaAs 1−xP x with an intensity-modulated electron beam

A new method of measuring short minority carrier lifetimes with a Scanning Electron Microscope (SEM) is reported. Main features are high spatial resolution (3 ωm in GaAs at 30 keV energy of beam electrons) and the insensitivity to surface recombination by use of a Schottky-contact. The shortest measurable lifetime is approx. 100 ps depending on sample conditions. The minority carrier lifetime is evaluated from the phase rotation between the measured ac-EBIC and the ac-component of the intensity modulated electron beam. The diffusion length required for the analysis is measured by the collection efficiency of the Schottky-contact, similar to the method of Wu and Wittry. Application of the method is demonstrated with a GaAs sample and a GaAs 1− x P x sample. EBIC micrographs (phase and amplitude) show local flucuations of lifetime and diffusion length.

Photoconduction in CdS Crystals Generated by a Modulated Electron Beam

AbstractA modulated photocurrent in CdS crystals is obtained by irradiation with an intensity modulated electron beam. The experimental set‐up for the irradiation is described. From the frequency dependence of the ac photocurrent a relaxation time τR is defined. τR decreases with increasing irradiation intensity B. At complete filling of the traps it reaches the value of the free electron lifetime τ. From the dependence τR(B) the approximate depth and the concentration of the traps, which are involved in the relaxation processes, are determined to Ec − EH = 0.215 eV and H = 8.5 × 1014 cm−3.