Abstract
Purpose: It is well‐known that fluctuation in chemical compositions can lead to significant errors in the dosimetry of low‐energy brachytherapy sources using the traditional “water‐equivalent” solid phantoms. The aim of this work was to investigate the feasibility of using ice as a consistent water equivalent solid medium for low‐energy brachytherapydosimetry.Methodod and Materials: The MCNP Monte Carlo code was used to compute the solid‐phantom‐to‐water conversion factor for ice (Rice) and for SolidWater™ (RSW) irradiated with mono‐energetic photons of 10 keV to 2.0 MeV and photons of six brachytherapy sources at various physical distances from the source. The RSW for different chemical compositions observed in SolidWater™ was also calculated. The feasibility of making precise ice phantoms was explored theoretically from an engineering point of view. Results: The uncertainty associated with the chemical composition of the SolidWater™ phantom can cause large errors in RSW for photons emitted by 103 Pd source: 9% at 1 cm, 47% at 5 cm and 72% at 10 cm radial distance while ice would be free from such errors. However, due to its lower physical density compared to liquid water, the Rice was found to depend on both the photon energy and distance from the source. At the distance of 1 cm used in reference dosimetry, Rice varied from 0.890 at 15 keV to 1.015 at 50 keV. A practical approach for making ice slabs with pre‐designed molds of brachytherapy sources and dosimeters was proposed. Conclusions: A comprehensive set of Rice has been calculated for mono‐energetic photons and for photons emitted by existing brachytherapy sources at various measurement depths. Using Rice with an ice phantom would eliminate errors resulted from chemical composition fluctuations in traditional SolidWater™ phantom while retaining the positioning advantages of a solid phantom. Experimental measurements using ice phantom are being planned.
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