Ice as a water-equivalent solid medium for brachytherapy dosimetric measurements.

Precise positioning of source and dosimeters is essential in the experimental determination of dosimetric characteristics of brachytherapy sources. Various near-water equivalent solid phantoms have been used to achieve the necessary precision in the positioning. However, the uncertainties in their chemical compositions may lead to non-negligible uncertainties in the determined doses. It is proposed here that ice may be used as an alternative to the conventional solid phantoms, since its chemical composition is identical to water while the positioning advantage associated with solid phantoms is retained. In this work, the feasibility of using ice as a solid phantom for brachytherapy dosimetry is investigated. Ice-to-water conversion factors are calculated at distances of 0.2-10 cm from the source, for six high- and low-energy photon-emitting brachytherapy sources and mono-energetic photons between 10 keV to 2.0 MeV. Practical issues and challenges associated with measuring dose in an ice phantom are discussed.