Dosimetric characterization of a new surface-conforming electron MLC prototype

The purpose is to reduce normal tissue radiation toxicity for electron therapy through the creation of a surface-conforming electron multileaf collimator (SCEM). The SCEM combines the benefits of skin collimation, electron conformal radiotherapy, and modulated electron radiotherapy. An early concept for the SCEM was constructed. It consists of leaves that protrude towards the patient, allowing the leaves to conform closely to irregular patient surfaces. The leaves are made of acrylic to decrease bremsstrahlung, thereby decreasing the out-of-field dose. Water tank scans were performed with the SCEM in place for various field sizes for all available electron energies (6, 9, 12, and 15 MeV) with a 0.5 cm air gap to the water surface at 100 cm source-to-surface distance (SSD). These measurements were compared with Cerrobend cutouts with the field size-matched at 100 and 110 cm SSD. Output factor measurements were taken in solid water for each energy at dmax for both the cerrobend cutouts and SCEM at 100 cm SSD. Percent depth dose (PDD) curves for the SCEM shifted shallower for all energies and field sizes. The SCEM also produced a higher surface dose relative to Cerrobend cutouts, with the maximum being a 9.8% increase for the 3 cm × 9 cm field at 9 MeV. When compared to the Cerrobend cutouts at 110 cm SSD, the SCEM showed a significant decrease in the penumbra, particularly for lower energies (i.e., 6 and 9 MeV). The SCEM also showed reduced out-of-field dose and lower bremsstrahlung production than the Cerrobend cutouts. The SCEM provides significant improvement in the penumbra and out-of-field dose by allowing collimation close to the skin surface compared to Cerrobend cutouts. However, the added scatter from the SCEM increases shallow PDD values. Future work will focus on reducing this scatter while maintaining the penumbra and out-of-field benefits the SCEM has over conventional collimation.

Nontoxic electron collimators

Purpose

The goal of this work was to develop and test nontoxic electron collimation technologies for clinical use.

Methods

Two novel technologies were investigated: tungsten‐silicone composite and 3D printed electron cutouts. Transmission, dose uniformity, and profiles were measured for the tungsten‐silicone. Surface dose, relative dose output, and field size were measured for the 3D printed cutouts and compared with the standard cerrobend cutouts in current clinical use. Quality assurance tests including mass measurements, Megavoltage (MV) imaging, and drop testing were developed for the 3D printed cutouts as a guide to safe clinical implementation.

Results

Dose profiles of the flexible tungsten‐silicone skin shields had an 80–20 penumbra values of 2–3 mm compared to 7–8 mm for cerrobend. In MV transmission image measurements of the tungsten‐silicone, 80% of the pixels had a transmission value within 2% of the mean. An ∼90% reduction in electron intensity was measured for 6 MeV and a 6.4 mm thickness of tungsten‐silicone and 12.7 mm thickness for 16 MeV. The maximum difference in 3D printed cutout versus cerrobend output, surface dose, and full width at half‐maximum (FWHM) was 1.7%, 1.2%, and 1.5%, respectively, for the 10 cm × 10 cm cutouts.

Conclusions

Both flexible tungsten‐silicone and 3D printed cutouts were found to be feasible for clinical use. The flexible tungsten‐silicone was of adequate density, flexibility, and uniformity to serve as skin shields for electron therapy. The 3D printed cutouts were dosimetrically equivalent to standard cerrobend cutouts and were robust enough for handling in the clinical environment.

Dosimetric evaluation of skin collimation with tungsten rubber for electron radiotherapy: A Monte Carlo study

PURPOSE

Skin collimation provides a sharp penumbra for electron beams, while the effect of bremsstrahlung from shielding materials is a concern. This phantom study was conducted to evaluate the safety and efficacy of a real-time variable shape rubber containing-tungsten (STR) that can be placed on a patient's skin.

METHODS

Electron beam profiles were acquired with the STR placed on a water-equivalent phantom and low melting-point alloy (LMA) placed at the applicator according to commonly used procedures (field sizes: 20- and 40-mm diameters). Depth and lateral dose profiles for 6- and 12-MeV electron beams were obtained by Monte Carlo (MC) simulations and were benchmarked against film measurements. The width of the off-axis distance between 80% and 20% doses (P80-20 ) and the maximum dose were obtained from the lateral dose profiles. Bremsstrahlung emission was analyzed by MC simulations at the depth of maximum dose (R100 ).

RESULTS

The depth dose profiles calculated by the MC simulations were consistently within 2% of the measurements. The P80-20 at R100 for 20- and 40-mm diameters were 4.0 mm vs. 7.6 mm (STR vs. LMA) and 4.5 mm vs. 9.2 mm, respectively, for the 6-MeV electron beam with 7.0-mm-thick STR, and 2.7 mm vs. 5.6 mm and 4.5 mm vs. 7.1 mm, respectively, for the 12-MeV electron beam with 12.0-mm-thick STR. A hotspot was not observed on the lateral dose profiles obtained with the STR at R100 . The bremsstrahlung emission under the region shielded by the STR was comparable to that obtained with the LMA, even though the STR was placed on the surface of the phantom.

CONCLUSIONS

Skin collimator with STR provided superior dosimetric characteristics and comparable bremsstrahlung emission to LMA collimator at the applicator. STR could be a new tool for the safe and efficient delivery of electron radiotherapy.

Evaluation of MVCT images with skin collimation for electron beam treatment planning

This study assessed the potential of using megavoltage CT (MVCT) images taken with high density skin collimation in place for electron beam treatment planning. MVCT images were taken using the TomoTherapy Hi-Art system (TomoTherapy Inc., Madison, WI), and the CT numbers were converted to density by calibrating the Hi-Art system using an electron density phantom. Doses were computed using MVCT images and kVCT images and compared by calculating dose differences in the uniform dose region ( > 90%, excluding buildup region) and calculating distance-to-agreement (DTA) in high dose-gradient regions (penumbra and distal falloff, 90%-10%). For 9 and 16 MeV electron beams of 10 x 10 cm calculated on a homogeneous CIRS Plastic Water (Computerized Imaging Research Systems Inc., Norfolk, VA) phantom without skin collimation, the maximum dose differences were 2.3% and the maximum DTAs were 2.0 mm for both beams. The same phantom was then MVCT scanned nine times with square skin collimators of Cerrobend on its surface - field sizes of 3 x 3, 6 x 6, and 10 x 10 cm and thicknesses of 6, 8, and 10 mm. Using the Philips Pinnacle 3 treatment planning system (Philips Medical Systems, N.A., Bothwell, WA), a treatment plan was created for combinations of electron energies of 6, 9, 12, and 16 MeV and each field size. The same treatment plans were calculated using kVCT images of the phantom with regions-of-interest (ROI) manually drawn to duplicate the sizes, shapes, and density of the skin collimators. With few exceptions, the maximum dose differences exceeded +/-5% and the DTAs exceeded 2 mm. We determined that the dose differences were due to small distortions in the MVCT images created by the high density material and manifested as errors in the phantom CT numbers and in the shape of the skin collimator edges. These results suggest that MVCT images without skin collimation have potential for use in patient electron beam treatment planning. However, the small distortion in images with skin collimation makes them unsuitable for clinical use.