Equivalent phantom sizes and shapes for brachytherapy dosimetric studies of 192Ir and 137Cs

The impact of phantom size and shape in brachytherapy dosimetry was assessed using Monte Carlo methods in liquid water for 192Ir and 137Cs point sources. This is needed since differences in published dosimetry data, both measurements and simulations, employ a variety of phantom sizes and shapes which can cause dose differences exceeding 30% near the phantom periphery. Spheres of radius, Rsph, 10-40 cm were examined to determine the equivalent spherical phantom size to a variety of cylinder and cube sizes, Rcyl and Rcube, respectively. These sizes ranged from 10 to 30 cm. The equivalent Rsph for a given size cylinder or cube was determined using a figure of merit (FOM) function to minimize differences between radial dose functions, g(r). Using the FOM approach, a linear fit (R2 > 0.99) was obtained for the equivalent Rsph for a given size cylinder or cube. The equivalent phantom for a cylinder, of 40 cm diameter and length 40 cm, is a sphere of 21 cm in radius and the equivalent phantom for a cube of 30 cm on each side is a sphere of 17.5 in radius. When normalizing all results to r=1 cm for g(r) comparisons of phantom shape, the absolute dose rates were equivalent within 0.1% for Rsph > or =10 cm for both 192Ir and 137Cs. Correlation factors to permit comparisons of unbounded g(r) data for r < or =15 cm in 20 published datasets resulted in agreement generally within 2%. Residual differences with four datasets were attributed to methodological uncertainties in the published references.