Evaluation of water-mimicking solid phantom materials for use in HDR and LDR brachytherapy dosimetry

In modern HDR or LDR brachytherapy with photon emitters, fast checks of the dose profiles generated in water or a water-equivalent phantom have to be available in the interest of patient safety. However, the commercially available brachytherapy photon sources cover a wide range of photon emission spectra, and the range of the in-phantom photon spectrum is further widened by Compton scattering, so that the achievement of water-mimicking properties of such phantoms involves high requirements on their atomic composition. In order to classify the degree of water equivalence of the numerous commercially available solid water-mimicking phantom materials and the energy ranges of their applicability, the radial profiles of the absorbed dose to water, D _w, have been calculated using Monte Carlo simulations in these materials and in water phantoms of the same dimensions. This study includes the HDR therapy sources Nucletron Flexisource Co-60 HDR (⁶⁰Co), Eckert und Ziegler BEBIG GmbH CSM-11 (¹³⁷Cs), Implant Sciences Corporation HDR Yb-169 Source 4140 (¹⁶⁹Yb) as well as the LDR therapy sources IsoRay Inc. Proxcelan CS-1 (¹³¹Cs), IsoAid Advantage I-125 IAI-125A (¹²⁵I), and IsoAid Advantage Pd-103 IAPd-103A (¹⁰³Pd). Thereby our previous comparison between phantom materials and water surrounding a Varian GammaMed Plus HDR therapy ¹⁹²Ir source (Schoenfeld et al 2015) has been complemented. Simulations were performed in cylindrical phantoms consisting of either water or the materials RW1, RW3, Solid Water, HE Solid Water, Virtual Water, Plastic Water DT, Plastic Water LR, Original Plastic Water (2015), Plastic Water (1995), Blue Water, polyethylene, polystyrene and PMMA. While for ¹⁹²Ir, ¹³⁷Cs and ⁶⁰Co most phantom materials can be regarded as water equivalent, for ¹⁶⁹Yb the materials Plastic Water LR, Plastic Water DT and RW1 appear as water equivalent. For the low-energy sources ¹⁰⁶Pd, ¹³¹Cs and ¹²⁵I, only Plastic Water LR can be classified as water equivalent.