DOSIMETRY OF I-125 SOURCES IN A LOW-DENSITY MATERIAL USING SCALING

One may apply O'Connor's scaling theorem to dose measurements with bra chytherapy sources in order to overcome the difficulties associated wi th the need for high spatial accuracy. This possibility has been evalu ated by Measuring the dose distribUtion around I-125 sources in a low- density styrofoam phantom and comparing it with the dose distributions in water and solid water. Some generalization of the scaling theorem is proposed to allow for the minor differences in atomic composition b etween styrofoam and water, and the distances are scaled according to the ratio of the linear attenuation coefficients, instead of the physi cal densities, of the two media. The validity of this application of t he scaling theorem has also been tested using Monte Carlo calculations . The results indicate that the scaling of the styrofoam measurements to water is a useful approximation in brachytherapy dosimetry.