Dosimetric considerations for moldable silicone composites used in radiotherapy applications

Abstract

Due to their many favorable characteristics, moldable silicone (MS) composites have gained popularity in medicine and recently, in radiotherapy applications. We investigate the dosimetric properties of silicones in radiotherapy beams and determine their suitability as water substitutes for constructing boluses and phantoms. Two types of silicones were assessed ( $\rho$ = 1.04 g/cm³ and $\rho$ = 1.07 g/cm³ ). Various dosimetric properties were characterized, including the relative electron density, the relative mean mass energy-absorption coefficient, and the relative mean mass restricted stopping power. Silicone slabs with thickness of 1.5 cm and 5.0 cm were molded to mimic a bolus setup and a phantom setup, respectively. Measurements were conducted for Co-60 and 6 MV photon beams, and 6 MeV electron beams. The doses at 1.5 cm and 5.0 cm depths in MS were measured with solid water (SW) backscatter material (D_MS-SW ), and with a full MS setup (D_MS-MS ), then compared with doses at the same depths in a full SW setup (D_SW-SW ). Relative doses were reported as D_MS-SW /D_MS-SW and D_MS-MS /D_SW-SW . Experimental results were verified using Monaco treatment planning system dose calculations and Monte Carlo EGSnrc simulations. Film measurements showed varying dose ratios according to MS and beam types. For photon beams, the bolus setup D_MS-SW /D_SW-SW exhibited a 5% relative dose reduction. The dose for 6 MV beams was reduced by nearly 2% in a full MS setup. Up to 2% dose increase in both scenarios was observed for electron beams. Compared with dose in SW, an interface of MS-SW can cause relatively high differences. We conclude that it is important to characterize a particular silicone's properties in a given beam quality prior to clinical use. Because silicone compositions vary between manufacturers and differ from water/SW, accurate dosimetry using these materials requires consideration of the reported differences.

Keywords: anthropomorphic; bolus; deformable; phantom; radiotherapy; silicone.

FIGURE 1

The molding process for the silicone slabs included using a custom‐built acrylic open faced cuboid container, which had an optional Markus IC dummy insert that can be added at the base to form a slot for IC placement. The molded E10 and E50 silicone slabs are shown on the right‐hand image, and were 1.5 cm and 5.0 cm thick

FIGURE 2

Pictoral representations of the experimental setups used for each beam type. Photon beams were measured at depths of 1.5 cm and 5.0 cm: setups (1)–(6) as shown in (a) and (b). Electron beams were measured using 1.5 cm depth slabs: setups (1), (2), and (3) as shown in (a). IC measurements were conducted in the lower MS slabs that were made to fit the IC flush against its surface. The measurement points (at the interfaces) are identified with the x marker in the illustrations shown in (a) and (b) and evaluated dose ratios are shown in grey boxes. Measurements are compared to Monaco TPS calculations for 6 MV and to EGSnrc Monte Carlo simulations for Co‐60 and 6 MV at the same depths. An example of one of the setups used for measurements in the Co‐60 beam is provided in (c), in which the sides of the acrylic mold were used as a frame to maintain the silicone slabs in an upright position for a lateral beam orientation. An example of one of the setups used for measurements in the 6 MV beam, using the Markus IC, is provided in (d). Measurements for 6 MeV beams were conducted with a 10 × 10 cm² electron applicator in a similar setup to that shown in (d)

FIGURE 3

Monte Carlo (MC, DOSXYZnrc) simulation results showing relative dose values around the interface of two phantom slabs when different configurations of material placements were used: top and bottom phantoms slabs are solid water (SW–SW), the top slab is molded silicone and the bottom slab is solid water (MS–SW), or the top and bottom slabs are molded silicone (MS–MS). Results are shown using a Co‐60 photon beam with a 1.5 cm top phantom slab thickness (a) and a 5.0 cm top phantom slab thickness (b), as well as for a 6 MV photon beam with a top phantom slab thickness of 1.5 cm (c), and a top phantom slab thickness of 5.0 cm (d). In all cases, the dose is presented relative to the dose at 100 cm source‐to‐axis distance (SAD) for the SW–SW setup at each respective depth and beam energy. The field size and SAD for all simulations were 10 × 10 cm², and 100 cm, respectively, and all simulations yielded values with uncertainties below 0.3%. Dose ratios from film measurements made with silicone E10 and E50 types are also shown for comparison and are labeled as (film‐E10) or (film‐E50), for film measurements in each type of silicone material (E10 or E50)