Date Published: June 19, 2019
Publisher: Public Library of Science
Author(s): Lawrie Skinner, Benjamin P. Fahimian, Amy S. Yu, Gayle E. Woloschak.
Electron radiotherapy is a labor-intensive treatment option that is complicated by the need for field shaping blocks. These blocks are typically made from casting Cerrobend alloys containing lead and cadmium. This is a highly toxic process with limited precision. This work aims to provide streamlined and more precise electron radiotherapy by 3D using printing techniques.
The 3D printed electron cutout consists of plastic shells filled with 2 mm diameter tungsten ball bearings. Five clinical Cerrobend defined field were compared to the planned fields by measuring the light field edge when mounted in the electron applicator on a linear accelerator. The dose transmitted through the 3D printed and Cerrobend cutouts was measured using an IC profiler ion chamber array with 6 MeV and 16 MeV beams. Dose profiles from the treatment planning system were also compared to the measured dose profiles. Centering and full width half maximum (FWHM) metrics were taken directly from the profiler software.
The transmission of a 16MeV beam through a 12 mm thick layer of tungsten ball bearings agreed within 1% of a 15 mm thick Cerrobend block (measured with an ion chamber array). The radiation fields shaped by ball bearing filled 3D printed cutout were centered within 0.4 mm of the planned outline, whereas the Cerrobend cutout fields had shift errors of 1–3 mm, and shape errors of 0.5–2 mm. The average shift of Cerrobend cutouts was 2.3 mm compared to the planned fields (n = 5). Beam penumbra of the 3D printed cutouts was found to be equivalent to the 15 mm thick Cerrobend cutout. The beam profiles agreed within 1.2% across the whole 30 cm profile widths.
This study demonstrates that with a proper quality assurance procedure, 3D-printed cutouts can provide more accurate electron radiotherapy with reduced toxicity compared to traditional Cerrobend methods.
It has been long recognized that custom field shaping blocks used for electron beam therapy complicate and slow down the treatment procedure. Although low cost, production of blocks cast from the low melting point alloys require manual labor and handling of toxic materials. In addition, transferring the outline from the treatment planning system (TPS) to the cutting tools introduces field shape and placement uncertainties of several millimeters. As an example of this error, Fig 1A shows the light field edge from a typical clinical Cerrobend insert compared to the planned outline. The 3D printed cutout methods in this study represent one way to reduce these uncertainties. While many current electron treatments do not require increased accuracy, developments in combination with modern dose calculations and modulating bolus [2–4] open up the potential for more advanced therapies, such as mixed beam therapy, which offer increased distal sparing compared to traditional intensity modulated, or volumetric arc radiation therapy photon plans [5, 6].
Dose profiles of 6 MeV and 16 MeV electron beams delivered through the 5.5 cm circle of Cerrobend and 3D printed cutout are shown in Fig 3 (10 x 10 cm2 insert). The planned, Cerrobend, and 3D printed dose profiles (80% to 20% penumbra widths) agree within 0.4 mm. The comparisons between the FWHM, centering, and off-axis dose of the planned, Cerrobend and 3D printed cutouts are listed in Table 2. The 5.5 cm cutout circle was defined at 95 cm SSD such that it produces field sizes of 5.8 cm in diameter at 100 cm SSD. The FWHM for 3D printed circle, and the planned dose profile agreed within 1 mm. The centering of the Cerrobend circle was found to be up to 0.7 mm off center, compared to 0.1 mm and 0.3 mm for the 3D printed circle at 6 MeV and 16 MeV, respectively. The 12 mm deep volume is calculated to be 159.7 cm3. The tungsten ball bearings were weighed to be 1652 g which is giving a density of 10.34 g/cm3. This is in close accordance with the expected density from random sphere packing theory (17.5 g/cm3*0.6 = 10.5 g/cm3). The dose under the block was up to 0.9% higher for the 3D printed cutout at 16 MeV than that for the planned dose or Cerrobend cutout (Fig 3 and Table 2).
A simple and non-toxic solution for electron field shaping using 3D printing and tungsten ball bearings is presented. By replacing Bismuth-lead alloy block casting, this method reduces manual labor and removes toxic materials from the clinic. Safety is also improved through printing of patient’s information and custom codes directly into the 3D printed part. This method allows accurate field shaping using standard applicators. The all-digital workflow ensures accuracy and reproducibility of the inserts.
In this work 3D printed designs for electron cutout have been demonstrated to accurately reproduce the dose profiles compared to that of Cerrobend cutouts. This method removes toxic material from the clinic, reduces manual labor, and provides improved reproduction of the field placement and field shape compared to Cerrobend. One caveat is that for higher energies the thickness of the cutout may need to be increased. Given the current rapid rate of development of 3D printing, it is expected that these technologies will be dramatically improved in the coming years, giving yet more convenience, speed, and precision. This increased precision, in concert with other recent developments, such as modulating electron bolus, opens up new opportunities for advancing electron radiotherapy.