Dosimetric verification of a Monte Carlo electron beam model for an add-on eMLC

Design and evaluation of electron beam energy degraders for breast boost irradiation

Background

For breast cancer patients who require electron boost energies between 6 and 9 MeV, an energy degraders (ED) in the 9 MeV beamline was specially designed and manufactured to increase the skin dose of 6 MeV and to reduce the penetration depth of 9 MeV beams.

Methods

We used Monte Carlo (MC) techniques as a guide in the design of ED for use with linear accelerators. In order to satisfy percent depth dose (PDD) characteristics and dose profile uniformity in water, the shape and thickness of Lucite® ED in the 9 MeV beamline was iteratively optimized and then manufactured. The ED geometry consists of a truncated cone attached on top of a plane plate, with total central thickness of 1.0 cm. The ED was placed on the lower most scraper of the electron applicator. The PDDs, profiles, and output factors were measured in water to validate the MC-based design.

Results

Skin doses with the EDs increased by 8–9 %, compared to those of the 9 MeV beam. The outputs with the EDs were 0.882 and 0.972 for 10 × 10 and 15 × 15 cm² cones, respectively, as compared to that of a conventional 9 MeV beam for a 10 × 10 cm² cone. The X-ray contamination remained less than 1.5 %. In-vivo measurements were also performed for three breast boost patients and showed close agreement with expected values.

Conclusions

The optimally designed ED in the 9 MeV beamline provides breast conserving patients with a new energy option of 7 MeV for boost of the shallow tumor bed. It would be an alternative to bolus and thus eliminate inconvenience and concern about the daily variation of bolus setup.

Validation of Varian TrueBeam electron phase-spaces for Monte Carlo simulation of MLC-shaped fields

PURPOSE

This work evaluates Varian's electron phase-space sources for Monte Carlo simulation of the TrueBeam for modulated electron radiation therapy (MERT) and combined, modulated photon and electron radiation therapy (MPERT) where fields are shaped by the photon multileaf collimator (MLC) and delivered at 70 cm SSD.

METHODS

Monte Carlo simulations performed with EGSnrc-based BEAMnrc/DOSXYZnrc and penelope-based PRIMO are compared against diode measurements for 5 × 5, 10 × 10, and 20 × 20 cm(2) MLC-shaped fields delivered with 6, 12, and 20 MeV electrons at 70 cm SSD (jaws set to 40 × 40 cm(2)). Depth dose curves and profiles are examined. In addition, EGSnrc-based simulations of relative output as a function of MLC-field size and jaw-position are compared against ion chamber measurements for MLC-shaped fields between 3 × 3 and 25 × 25 cm(2) and jaw positions that range from the MLC-field size to 40 × 40 cm(2).

RESULTS

Percent depth dose curves generated by BEAMnrc/DOSXYZnrc and PRIMO agree with measurement within 2%, 2 mm except for PRIMO's 12 MeV, 20 × 20 cm(2) field where 90% of dose points agree within 2%, 2 mm. Without the distance to agreement, differences between measurement and simulation are as large as 7.3%. Characterization of simulated dose parameters such as FWHM, penumbra width and depths of 90%, 80%, 50%, and 20% dose agree within 2 mm of measurement for all fields except for the FWHM of the 6 MeV, 20 × 20 cm(2) field which falls within 2 mm distance to agreement. Differences between simulation and measurement exist in the profile shoulders and penumbra tails, in particular for 10 × 10 and 20 × 20 cm(2) fields of 20 MeV electrons, where both sets of simulated data fall short of measurement by as much as 3.5%. BEAMnrc/DOSXYZnrc simulated outputs agree with measurement within 2.3% except for 6 MeV MLC-shaped fields. Discrepancies here are as great as 5.5%.

CONCLUSIONS

TrueBeam electron phase-spaces available from Varian have been implemented in two distinct Monte Carlo simulation packages to produce dose distributions and outputs that largely reflect measurement. Differences exist in the profile shoulders and penumbra tails for the 20 MeV phase-space off-axis and in the outputs for the 6 MeV phase-space.

Simultaneous production of mixed electron--photon beam in a medical LINAC: A feasibility study

PURPOSE

The electron or photon beams might be used for treatment of tumors. Each beam has its own advantage and disadvantages. Combo beam can increase the advantages. No investigation has been performed for producing simultaneous mixed electron and photon beam. In current study a device has been added to the Medical Linac to produce a mixed photon-electron beam.

METHODS

Firstly a Varian 2300CD head was simulated by MCNP Monte Carlo Code. Two sets of perforated lead sheets with 1 and 2 mm thickness and 0.2, 0.3, and 0.5 cm punches then placed at the top of the applicator holder tray. This layer produces bremsstrahlung x-ray upon impinging fraction electrons on it. The remaining fraction of electrons passes through the holes. The simulation was performed for 10 × 10, 6 × 6, and 4 × 4 cm(2) field size.

RESULTS

For 10 × 10 cm(2) field size, among the punched targets, the largest penumbra difference between the depth of 1 and 7 cm was 72%. This difference for photon and electron beams were 31% and 325% respectively. A maximum of 39% photon percentage was produced by 2 mm target with 0.2 cm holes diameter layer. The minimum surface dose value was 4% lesser than pure electron beam. For small fields, unlike the pure electron beam, the PDD, penumbra, and flatness variations were negligible.

CONCLUSIONS

The advantages of mixing the electron and photon beam is reduction of pure electron's penumbra dependency with the depth, especially for small fields, also decreasing of dramatic changes of PDD curve with irradiation field size.

A source model for modulated electron radiation therapy using dynamic jaw movements

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.

Comparison of conventional inserts and an add-on electron MLC for chest wall irradiation of left-sided breast cancer

BACKGROUND

Collimation of irregularly shaped clinical electron beams is currently based on electron inserts made of low melting point alloys. The present investigation compares a conventional electron applicator with insert and add-on eMLC-based dose distributions in the postoperative chest wall irradiation of left-sided breast cancer.

MATERIAL AND METHODS

Voxel Monte Carlo++ (VMC++) calculated dose distributions related to electron fields were compared with 10 left-sided breast cancer patients after radical mastectomy. The prescription dose was 50 Gy at a build-up maximum. The same dose was prescribed for the ipsilateral axillary, parasternal and supraclavicular lymph nodes that were treated with photons and calculated with a pencil beam algorithm. The insert beams were shaped with 1.5 cm thick Wood's metal electron inserts in an electron applicator of a Varian 2100 C/D linac. Doses for the eMLC-shaped beams were calculated for an eMLC prototype with 2 cm thick and 5 mm wide steel leaves. The same collimator-to-surface distance (CSD) of 5.8 cm was used for both collimators.

RESULTS

The mean PTV dose was slightly higher for the eMLC plans (50.7 vs 49.5 Gy, p<0.001, respectively). The maximum doses assessed by D5% for the eMLC and insert were 60.9 and 59.1 Gy (p<0.001). The difference was due to the slightly higher doses near the field edges for the eMLC. The left lung V20 volumes were 34.5% and 34.0% (p<0.001). There was only a marginal difference in heart doses.

DISCUSSION

Despite a slight increase of maximum dose in PTV the add-on electron MLC for chest wall irradiation results in practically no differences in dose distributions compared with the present insert-based collimation.

Enhancement of electron-beam surface dose with an electron multi-leaf collimator (eMLC): a feasibility study

Use of a water-equivalent bolus in electron-beam radiotherapy is sometimes impractical and non-hygienic. Therefore, the feasibility of applying adjacent narrow beams for producing high surface dose electron beams without a bolus was investigated. Depth dose curves and profiles in water were calculated and measured for 6 and 9 MeV electron-beam segments (width 0.3-1.5 cm, length 10 cm) for source-to-surface distances (SSD) 102 and 105 cm. Segment shaping was performed with an add-on electron multi-leaf collimator prototype attached to the Varian 2100 C/D linac. Dose calculations were performed with the Voxel Monte Carlo++ algorithm. Resulting dose distributions in typical clinical cases were compared with the bolus technique. With a composite segmental field with 1.0 cm wide segments the surface dose was over 90% of the depth dose maximum for both energies. The build-up area practically disappeared with a 0.5 cm wide single beam. This led to decrease in the therapeutic range for composite fields with segment widths smaller than 1.0 cm. The new technique yielded similar surface doses as the bolus technique. The photon contamination was 4% with a 9 x 10 cm(2) field (1.0 cm wide segments) compared to 1% for the respective open field with 9 MeV with a bolus. The calculated dose agreed within 2 mm and 3% of the measured dose in 93.7% and 85.2% of the voxels. Adjacent narrow eMLC beams with a 1.0 cm width are suitable to produce electron fields with high surface dose. Despite a slight nonuniformity in the surface profiles in the lateral part of the field at SSD 102 cm, surface dose and target coverage are comparable with the bolus technique.