Dynamic Electron Arc Radiotherapy (DEAR): A New Conformal Electron Therapy Technique

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Rodrigues, Anna Elisabeth


Wu, Qiuwen

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Electron beam therapy represents an underutilized area in radiation therapy. While electron radiation therapy has existed for many decades and electron beams with multiple energies are available on linear accelerators – the most common device to deliver radiation therapy – efforts to advance the field have been slow. In contrast, photon beam therapy has seen rapid advancements in the past decade, and has become the main modality for radiation therapy treatment.

This doctoral research project comprises the development of a novel treatment modality, dynamic electron arc radiotherapy (DEAR) that seeks to address challenges to clinical implementation of electron beam therapy by providing a technique that may be able to treat specific patient subsets with better outcomes than current techniques. This research not only focused on the development of DEAR, but also aimed to improve upon and introduce new tools and techniques that could translate to current clinical electron beam therapy practice.

The concept of DEAR is presented. DEAR represents a new conformal electron therapy technique with synchronized couch motion. DEAR utilizes the combination of gantry rotation, couch motion, and dose rate modulation to achieve desirable dose distributions in patient. The electron applicator is kept to minimize scatter and maintain narrow penumbra. The couch motion is synchronized with the gantry rotation to avoid collision between patient and the electron cone.

First, the feasibility of DEAR delivery was investigated and the potential of DEAR was demonstrated to improve dose distributions on simple cylindrical phantoms. DEAR was delivered on Varian’s TrueBeam linac in Research Mode. In conjunction with the recorded trajectory log files, mechanical motion accuracies and dose rate modulation precision were analyzed. Experimental and calculated dose distributions were investigated for a few selected energies (6 MeV and 9 MeV) and cut-out sizes (1x10 cm2 and 3x10 cm2 for a 15x15 cm2 applicator). Our findings show that DEAR delivery is feasible and has the potential to deliver radiation dose with high precision (RMSE of <0.1 MU, <0.1° gantry, and <0.1 cm couch positions) and good dose rate precision (1.6 MU/min). Dose homogeneity within ±2 % in large and curved targets can be achieved while comparable penumbra to a standard electron beam on a flat surface can be maintained. Further, DEAR does not require fabrication of patient-specific shields, which has hindered the widespread use of electron arc therapy. These benefits make DEAR a promising technique for conformal radiotherapy of superficial tumors.

Next, an accurate dose calculation framework for DEAR was developed since current commercial dose calculation systems cannot handle the dynamic nature of the DEAR. Comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam linacs against measurement data were assessed. In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3×3, 4×4, 5×5, 6×6, 10×10, 15×15, 20×20, and 25×25 cm2 were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R100, R50, Rp, and Rp+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors. Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo were generally within 2% or 1 mm. The largest discrepancies were observed for depths within 5 mm from the phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R100, R50, Rp, and Rp+ were within 1 mm, 1 mm, and 2%, respectively. Simulated and measured orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement. The presented Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam linacs for electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3×3 to 25×25 cm2 were studied and results were compared to the measurement data with excellent agreement.

DEAR uses the superposition of many small fields for its delivery, as such accurate planning requires the knowledge of accurate small field dosimetry. Prior research has shown that previous versions of the clinically used eMC dose calculation algorithm (Varian Medical Systems, Inc., Palo Alto, CA) cannot accurately calculate small static electron fields, leading to discrepancies in the dose distributions and output. Further, the clinical treatment planning system, Eclipse, currently does not support the planning of dynamic electron radiation therapy. Therefore, the aforementioned validation was extended to small fields and compared to dose calculations from the treatment planning system.

Subsequently, small field optimization was explored. Monte Carlo simulations were performed using validated Varian TrueBeam phase space files for electron beam energies of 6, 9, 12, and 16 MeV and square (1x1, 2x2, 3x3, 4x4, and 5x5 cm2) and circular (1, 2, 3, 4, and 5 cm diameter) fields. Resulting dose distributions (kernels) were used for subsequent calculations. The following analyses were performed: (1) Comparison of composite square fields and reference 10x10 cm2 dose distributions and (2) Scanning beam deliveries for square and circular fields realized as the convolution of kernels and scanning pattern. Preliminary beam weight and pattern optimization were also performed. Two linear scans of 10 cm with/without overlap were modeled. Comparison metrics included depth and orthogonal profiles at dmax. (1) Composite fields regained reference depth dose profiles for most energies and fields within 5%. Smaller kernels and higher energies increased dose in the build-up and Bremsstrahlung region (30%, 16 MeV and 1x1 cm2), while reference dmax was maintained for all energies and composite fields. Smaller kernels (<2x2 cm2) maintained penumbra and field size within 0.2 cm, and flatness within 2 and 4% in the cross-plane and in-plane direction, respectively. Deterioration of penumbra for larger kernels (5x5 cm2) was observed. Balancing desirable dosimetry and efficiencies suggests that smaller kernels should be used at the target edges and larger kernels in the center of the target. (2) Beam weight optimization improves cross-plane penumbra (0.2 cm) and increases the field size (0.4 cm) on average. In-plane penumbra and field size remain unchanged. Overlap depends on kernel size and optimal overlap results in flatness ±2%. Dynamic electron beam therapy in virtual scanning mode is feasible by employing small fields to achieve desired dose distributions and acceptable efficiencies.

Further, tools to generally improve upon limitations in Monte Carlo simulations for electron beams were investigated. The phase space file contains a finite number of particle histories and can have very large file size, yet still contains inherent statistical noises. A characterization of the phase space file was investigated to overcome its inherent limitations. To characterize the phase space file, distributions for energy, position, and direction of all particles types were analyzed as piece-wise parameterized functions of radius. Subsequently, a pseudo phase space file was generated based on this characterization. Validation was assessed by directly comparing the original and pseudo phase space file, and by comparing the resulting dose distributions from Monte Carlo simulations using both phase space files. Monte Carlo simulations were run for energies 6, 9, 12, and 16 MeV and all standard field sizes 6x6, 10x10, 15x15, 20x20, and 25x25 cm2. Percent depth dose and orthogonal profiles at depths R100, R50, and Rp were evaluated. Histograms of the original and pseudo phase space file agree very well with correlation coefficients greater than 0.98 for all particle attributes. Dosimetric comparison between original and pseudo dose distributions yielded agreement within 2%/1mm for PDDs and profiles at all depths for all field sizes 6x6, 10x10, 15x15, 20x20, and 25x25 cm2 and energies 6, 9, 12, and 16 MeV. Phase space files were found to be successfully characterized by piece-wise distributions for energy, position, and direction as parameterized functions of radius and polar angle. This facilitates generation of sufficient particles at any statistical precisions.

Additionally, new hardware for improved DEAR capability was investigated. Few leaf electron collimators (FLEC) or electron MLCs (eMLC) are highly desirable for dynamic electron beam therapies as they produce multiple apertures within a single delivery to achieve conformal dose distributions. However, their clinical implementation has been challenging. Alternatively, multiple small apertures in a single cut-out with variable jaw sizes could be utilized in a single dynamic delivery. A Monte Carlo simulation study was performed to investigate the dosimetric characteristics of such an arrangement. Investigated quantities included: Energy (6 and 16 MeV), jaw size (1x1 to 22x22 cm2; centered to aperture), applicator/cut-out (15x15 cm2), aperture (1x1, 2x2, 3x3, and 4x4 cm2), and aperture placement (on/off central axis). Three configurations were assessed: (a) single aperture on-axis, (b) single aperture off-axis, and (c) multiple apertures. Reference was configuration (a) with the standard jaw size. Aperture placement and jaw size were optimized to maintain reference dosimetry and minimize leakage through unused apertures to <5%. Comparison metrics included depth dose and orthogonal profiles. Configuration (a) and (b): Jaw openings were reduced to 10x10 cm2 without affecting dosimetry (gamma 2%/1mm) regardless of on- or off-axis placement. For smaller jaw sizes, reduced surface (<2%, 5% for 1x1 cm2 aperture) and increased Bremsstrahlung (<2%, 10% for 1x1 cm2 aperture) dose was observed. Configuration (c): Optimal aperture placement was in the corners (order: 1x1, 4x4, 2x2, 3x3 cm2 for quadrants I, II, III, and IV) and jaw size were 2x2, 2x2, 3x3, and 7x7 cm2 and 7x7, 7x7, 10x10, and 10x10 cm2 for apertures: 1x1, 2x2, 3x3, 4x4 cm2 and energies 6 and 16 MeV, respectively. Asymmetric leakage was found from upper and lower jaws. Leakage was generally within 5% with a maximum of 10% observed for the 1x1 cm2 aperture irradiation. Multiple apertures in a single cut-out with variable jaw size can be used in a single dynamic delivery, thus providing a practical alternative to FLEC or eMLC.

Based on all the results from this project, DEAR has been found to be a feasible technique and demonstrates the potential to improve electron therapy.





Rodrigues, Anna Elisabeth (2015). Dynamic Electron Arc Radiotherapy (DEAR): A New Conformal Electron Therapy Technique. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/10489.


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