Purpose: Due to their favourable characteristics, moldable silicones (MS) have gained popularity in medicine and recently, in radiotherapy. We investigate the dosimetric properties of MS in high-energy radiotherapy photon and electron beams and determine their suitability as water substitutes for constructing radiotherapy boluses and phantoms.
Methods: Two MS types were used (ρ=1.04 and 1.07 g/cc). Various dosimetric properties were characterized (e.g., Z₍eff₎, relative electron density, mass-energy absorption coefficients). For each type, two MS slabs were molded with a base of 16 x 16 cm², and 1.5 cm and 5.0 cm thicknesses. Two additional 5.0 cm thick slabs were molded to fit a Markus ionization chamber (IC) flush to the surface. For each MS type, the doses at 1.5 cm and 5.0 cm depths were measured with 5.0 cm Solid Water (SW) backscatter material (MS-SW), and with a full MS setup (MS-MS), then compared with doses at the same depths in a full SW setup (SW-SW). Relative doses were reported as MS-SW/SW-SW and MS-MS/SW-SW. Measurements were repeated using film. Experimental results were verified using Monaco Treatment Planning System (TPS) dose calculations and DOSXYZnrc Monte Carlo (MC) simulations.
Results: Dose ratios varied according to MS and beam types. For photon beams, a bolus setup caused ~5% dose reduction (MS-SW/SW-SW). Using MS alone resulted in ~2% dose reduction (MS-MS/SW-SW). For electrons, up to 2% dose increase was observed for MS-SW and MS-MS relative to a full SW setup (SW-SW).
Conclusion: MS compositions differ from water/SW's and vary between manufacturers. Accurate dosimetry using these materials requires consideration of these differences. Compared with dose in SW, an interface of MS-SW can cause relatively high differences. Using MS alone for phantom construction provides a more tissue-equivalent setup. However, it is important to characterize a particular MS’s properties for a given beam quality prior to clinical use.