The water-equivalence of phantom materials for Sr-90-Y-90 beta particles

Intravascular brachytherapy requires that the dose be specified within mill imeters of the source. High dose gradients near brachytherapy sources requi re that the source-detector distance be accurately known for dosimetry purp oses. Solid phantoms can be designed to accommodate these stringent require ments. This study reports dosimeter readings from Sr-90-Y-90 sources measur ed in water, A150, polystyrene and in an epoxy-based water-equivalent plast ic. Measurements showed that while A150 and the epoxy-based plastic agreed well with water when the surface of the source contacted the detector housi ng, the relative response in the phantoms decreased with increasing depth i n phantom, falling to similar to0.55 those of water at a depth of 5 mm. Rea dings in polystyrene were within 4% of those in water between 1 and 2 mm de pth. However, while polystyrene followed water more closely than the other two materials, at greater depths the relative response in polystyrene to wa ter varied from 0.65 to 1.34. When the density of the materials is accounte d for, the relative response in A150 is nearly constant with increasing are al density. Furthermore, the response in A150 shows the closest agreement w ith that in water of any of the solid materials for higher areal densities. For values below 0.3 g/cm(2), polystyrene shows the closest agreement with water. (C) 2001 American Association of Physicists in Medicine.