Dosimetric validation of intensity-modulated bolus electron conformal therapy planning and delivery using an anthropomorphic cylindrical head phantom.

PURPOSE: This work investigated the dosimetric accuracy of the intensity-modulated bolus electron conformal therapy (IM-BECT) planning and delivery process using the decimal ElectronRT (eRT) treatment planning system. METHODS: An IM-BECT treatment plan was designed using eRT for a cylindrical, anthropomorphic retromolar trigone phantom. Treatment planning involved specification of beam parameters and design of a variable thickness wax bolus and Passive Radiotherapy Intensity Modulator for Electrons (PRIME) device, which was comprised of 33 tungsten island blocks of discrete diameters from 0.158 to 0.223 cm (Intensity Reduction Factors from 0.937 to 0.875, respectively) inside a 10.1 × 6.7 cm2 copper cutout. For comparison of calculation accuracy, a BECT plan was generated by copying the IM-BECT plan and removing the intensity modulation. For both plans, a 16 MeV electron beam was used with 104.7 cm source-to-surface distance to bolus. In-phantom TLD-100 measurements (N = 47) were compared with both eRT planned dose distributions, which used the pencil beam redefinition algorithm with modifications for passive electron intensity modulation (IM-PBRA). Dose difference and distance to agreement (DTA) metrics were computed for each measurement point. RESULTS: Comparison of measured dose distributions with planned dose distributions yielded dose differences (calculated minus measured) characterized by a mean and standard deviation of -0.36% ± 1.64% for the IM-BECT plan, which was similar to -0.36% ± 1.90% for the BECT plan. All dose measurements were within 5% of the planned dose distribution, with both the BECT and IM-BECT measurement sets having 46/47 (97.8%) points within 3% or within 3 mm of the respective treatment plans. CONCLUSIONS: It was found that the IM-BECT treatment plan generated using eRT was sufficiently accurate for clinical use when compared to TLD measurements in a cylindrical, anthropomorphic phantom, and was similarly accurate to the BECT treatment plan in the same phantom.

Factory quality assurance of passive radiotherapy intensity modulators for electrons using kilovoltage x-ray imaging.

PURPOSE: This work developed an x-ray-based method for performing factory quality assurance (QA) of Passive Radiotherapy Intensity Modulators for Electrons (PRIME) device fabrication. This method measures errors in position, diameter, and orientation of cylindrical island blocks on a hexagonal grid that comprises PRIME devices and the impact of such errors on the underlying intensity distribution. METHODS: X-ray images were acquired of six PRIME devices, which modeled three error cases (small random, large random, and systematic errors) for two island block diameters (0.158 and 0.352 cm). Island blocks in each device, 0.6 cm long tungsten cylinders of constant diameter, were spaced 0.6 cm on a hexagonal grid over approximately 8 cm square. Using a 50 kVp x-ray image, each island block projected a racetrack, whose perimeter was fit to a function that allowed determination of its position, diameter, and angular orientation (θ, ϕ). These measured parameters were input into a pencil beam algorithm (PBA) dose calculation performed in water (16 MeV, SSD = 103 cm) for each device. PBA calculated intensity distributions using measured and planned (exact) island block parameters were compared. RESULTS: Θ distributions for the 0.158 and 0.352 cm devices were nearly identical for each error case, with θ values for most island blocks being within 3.2°, 8.5°, and 7.5° for the small random, large random, and systematic error PRIME devices, respectively. Corresponding intensity differences between using measured and planned island block parameters were within 1.0% and 2.8% (small random), 2.2% and 4.8% (large random), and 3.2% and 6.7% (systematic) for the 0.158 and 0.352 cm devices, respectively. CONCLUSION: This approach provides a viable and economical method for factory QA of fabricated PRIME devices by determining errors in their planned intensity distribution from which their quality can be assessed prior to releasing to the customer.

Modeling scatter through sides of island blocks used for intensity-modulated bolus electron conformal therapy.

PURPOSE: Passive Radiotherapy Intensity Modulators for Electrons (PRIME) devices are comprised of cylindrical tungsten island blocks imbedded in a machinable foam slab within the patient's cutout. Intensity-modulated bolus electron conformal therapy (IM-BECT) uses PRIME devices to reduce dose heterogeneity caused by the irregular bolus surface. Heretofore, IM-BECT dose calculations used the pencil beam redefinition algorithm (PBRA) assuming perfect collimation. This study investigates modeling electron scatter into and out the sides of island blocks. METHODS: Dose distributions were measured in a water phantom at 7, 13, and 20 MeV for devices having nominal intensity reduction factors of 1.000 (foam only), 0.937, 0.812, and 0.688, corresponding to nominal island block diameters (dnom ) of 0.158, 0.273, and 0.352 cm, respectively. Pencil beam theory derived an effective diameter (dIS ) to account for in-scattered electrons as a function of dnom and beam energy (Ep,0 ). However, for out-scattered electrons, an effective diameter (dmod ) was estimated by best fitting measured data. RESULTS: In the modulated region (under island blocks, depth < R90 ), modified PBRA-calculated dose distributions showed 2%/2 mm passing rates for dnom  = 0.158, 0.273, and 0.352 cm of (100%, 100%, 100%) at 7 MeV, (100%, 100%, 93.5%) at 13 MeV, and (99.8%, 85.4%, and 71.5%) at 20 MeV. The largest dose differences (≤ 6%) occurred at the highest energy (20 MeV), largest dnom , shallowest depths (≤ 2 cm), and on central axis. CONCLUSIONS: An equation for modeling island block scatter, dmod (dnom , Ep,0 ), has been developed for use in the PBRA, insignificantly impacting calculation time. Although inaccuracy sometimes exceeded our 2%/2 mm criteria, it could be clinically acceptable, as superficial dose differences often fall inside the bolus. Also, patient PRIME devices are expected to have fewer large diameter island blocks than did test devices. Inaccuracies are attributed to out-scattered electrons having energy spectra different than the primary beams.

Evaluation of prototype of improved electron collimation system for Elekta linear accelerators.

PURPOSE: This study evaluated a new electron collimation system design for Elekta 6-20 MeV beams, which should reduce applicator weights by 25%-30%. Such reductions, as great as 3.9 kg for the largest applicator, should result in considerably easier handling by members of the radiotherapy team. METHODS: Prototype 10 × 10 and 20 × 20-cm2 applicators, used to measure weight, in-field flatness, and out-of-field leakage dose, were constructed according to the previously published design with two minor modifications: (a) rather than tungsten, lead was used for trimmer material; and (b) continuous trimmer outer-edge bevel was approximated by three steps. Because of lead plate softness, a 0.32-cm aluminum plate replaced the equivalent lead thickness on the trimmer's downstream surface for structural support. Models of all applicators (6 × 6-25 × 25 cm2 ) with these modifications were inserted into a Monte Carlo (MC) model for dose calculations using 7, 13, and 20 MeV beams. Planar dose distributions were measured and calculated at 1- and 2-cm water depths to evaluate in-field beam flatness and out-of-field leakage dose. RESULTS: Prototype 10 × 10 and 20 × 20-cm2 applicator measurements agreed with calculated weights, in-field flatness, and out-of-field leakage doses for 7, 13, and 20 MeV beams. Also, MC dose calculations showed that all applicators (6 × 6-25 × 25 cm2 ) and 7, 13, and 20 MeV beams met our stringent in-field flatness specifications (±3% major axes; ±4% diagonals) and IEC out-of-field leakage dose specifications. CONCLUSIONS: Our results validated the new electron collimating system design for Elekta 6-20 MeV electron beams, which could serve as basis for a new clinical electron collimating system with significantly reduced applicator weights.

Improved electron collimation system design for Elekta linear accelerators.

Prototype 10 × 10 and 20 × 20-cm2 electron collimators were designed for the Elekta Infinity accelerator (MLCi2 treatment head), with the goal of reducing the trimmer weight of excessively heavy current applicators while maintaining acceptable beam flatness (±3% major axes, ±4% diagonals) and IEC leakage dose. Prototype applicators were designed initially using tungsten trimmers of constant thickness (1% electron transmission) and cross-sections with inner and outer edges positioned at 95% and 2% off-axis ratios (OARs), respectively, cast by the upstream collimating component. Despite redefining applicator size at isocenter (not 5 cm upstream) and reducing the energy range from 4-22 to 6-20 MeV, the designed 10 × 10 and 20 × 20-cm2 applicator trimmers weighed 6.87 and 10.49 kg, respectively, exceeding that of the current applicators (5.52 and 8.36 kg, respectively). Subsequently, five design modifications using analytical and/or Monte Carlo (MC) calculations were applied, reducing trimmer weight while maintaining acceptable in-field flatness and mean leakage dose. Design Modification 1 beveled the outer trimmer edges, taking advantage of only low-energy beams scattering primary electrons sufficiently to reach the outer trimmer edge. Design Modification 2 optimized the upper and middle trimmer distances from isocenter for minimal trimmer weights. Design Modification 3 moved inner trimmer edges inward, reducing trimmer weight. Design Modification 4 determined optimal X-ray jaw positions for each energy. Design Modification 5 adjusted middle and lower trimmer shapes and reduced upper trimmer thickness by 50%. Design Modifications 1→5 reduced trimmer weights from 6.87→5.86→5.52→5.87→5.43→3.73 kg for the 10 × 10-cm2 applicator and 10.49→9.04→8.62→7.73→7.35→5.09 kg for the 20 × 20-cm2 applicator. MC simulations confirmed these final designs produced acceptable in-field flatness and met IEC-specified leakage dose at 7, 13, and 20 MeV. These results allowed collimation system design for 6 × 6-25 × 25-cm2 applicators. Reducing trimmer weights by as much as 4 kg (25 × 25-cm2 applicator) should result in easier applicator handling by the radiotherapy team.

Radiation leakage dose from Elekta electron collimation system.

This study provided baseline data required for a greater project, whose objective was to design a new Elekta electron collimation system having significantly lighter electron applicators with equally low out-of field leakage dose. Specifically, off-axis dose profiles for the electron collimation system of our uniquely configured Elekta Infinity accelerator with the MLCi2 treatment head were measured and calculated for two primary purposes: 1) to evaluate and document the out-of-field leakage dose in the patient plane and 2) to validate the dose distributions calculated using a BEAMnrc Monte Carlo (MC) model for out-of-field dose profiles. Off-axis dose profiles were measured in a water phantom at 100 cm SSD for 1 and 2 cm depths along the in-plane, cross-plane, and both diagonal axes using a cylindrical ionization chamber with the 10 × 10 and 20 × 20 cm2 applicators and 7, 13, and 20 MeV beams. Dose distributions were calculated using a previously developed BEAMnrc MC model of the Elekta Infinity accelerator for the same beam energies and applicator sizes and compared with measurements. Measured results showed that the in-field beam flatness met our acceptance criteria (± 3% on major and ±4% on diagonal axes) and that out-of-field mean and maximum percent leakage doses in the patient plane met acceptance criteria as specified by the International Electrotechnical Commission (IEC). Cross-plane out-of-field dose profiles showed greater leakage dose than in-plane profiles, attributed to the curved edges of the upper X-ray jaws and multileaf collimator. Mean leakage doses increased with beam energy, being 0.93% and 0.85% of maximum central axis dose for the 10 × 10 and 20 × 20 cm2 applicators, respectively, at 20 MeV. MC calculations predicted the measured dose to within 0.1% in most profiles outside the radiation field; however, excluding model-ing of nontrimmer applicator components led to calculations exceeding measured data by as much as 0.2% for some regions along the in-plane axis. Using EGSnrc LATCH bit filtering to separately calculate out-of-field leakage dose components (photon dose, primary electron dose, and electron dose arising from interactions in various collimating components), MC calculations revealed that the primary electron dose in the out-of-field leakage region was small and decreased as beam energy increased. Also, both the photon dose component and electron dose com-ponent resulting from collimator scatter dominated the leakage dose, increasing with increasing beam energy. We concluded that our custom Elekta Infinity with the MLCi2 treatment head met IEC leakage dose criteria in the patient plane. Also, accuracy of our MC model should be sufficient for our use in the design of a new, improved electron collimation system.

Real-time simulator for designing electron dual scattering foil systems.

The purpose of this work was to develop a user friendly, accurate, real-time com- puter simulator to facilitate the design of dual foil scattering systems for electron beams on radiotherapy accelerators. The simulator allows for a relatively quick, initial design that can be refined and verified with subsequent Monte Carlo (MC) calculations and measurements. The simulator also is a powerful educational tool. The simulator consists of an analytical algorithm for calculating electron fluence and X-ray dose and a graphical user interface (GUI) C++ program. The algorithm predicts electron fluence using Fermi-Eyges multiple Coulomb scattering theory with the reduced Gaussian formalism for scattering powers. The simulator also estimates central-axis and off-axis X-ray dose arising from the dual foil system. Once the geometry of the accelerator is specified, the simulator allows the user to continuously vary primary scattering foil material and thickness, secondary scat- tering foil material and Gaussian shape (thickness and sigma), and beam energy. The off-axis electron relative fluence or total dose profile and central-axis X-ray dose contamination are computed and displayed in real time. The simulator was validated by comparison of off-axis electron relative fluence and X-ray percent dose profiles with those calculated using EGSnrc MC. Over the energy range 7-20 MeV, using present foils on an Elekta radiotherapy accelerator, the simulator was able to reproduce MC profiles to within 2% out to 20 cm from the central axis. The central-axis X-ray percent dose predictions matched measured data to within 0.5%. The calculation time was approximately 100 ms using a single Intel 2.93 GHz processor, which allows for real-time variation of foil geometrical parameters using slider bars. This work demonstrates how the user-friendly GUI and real-time nature of the simulator make it an effective educational tool for gaining a better understanding of the effects that various system parameters have on a relative dose profile. This work also demonstrates a method for using the simulator as a design tool for creating custom dual scattering foil systems in the clinical range of beam energies (6-20 MeV).