Detailed analysis of dose difference in using water as tissue-equivalent material in Cf brachytherapy

Abstract

Aim

The purpose of this study is to analyse how small variations in the elemental composition of soft tissue lead to differences in dose distributions from a ²⁵²Cf brachytherapy source and to determine the error percentage in using water as a tissue-equivalent material.

Background

Water is normally used as a tissue-equivalent phantom material in radiotherapy dosimetry.

Materials and methods

Neutron energy spectra, neutron and gamma-ray dose rate distributions were calculated for a ²⁵²Cf AT source located at the center of a spherical phantom filled with various types of tissue compositions: adipose, brain, muscle, International Commission on Radiation Units and Measurements (ICRU) report No. 44 9-component soft tissue and water, using Monte Carlo simulation.

Results

The obtained results showed differences between total dose rates in various tissues relative to water varying between zero and 4.94%. The contributions of neutron and total gamma ray doses to these differences are, on average, 81% and 19%, respectively. It was found that the dose differences between various soft tissues and water depend not only on the soft tissue composition, but also on the beam type emitted from the ²⁵²Cf source and the distance from the source.

Conclusion

Assuming water as a tissue-equivalent material, although leads to overestimation of dose rate (except in the case of adipose tissue), is acceptable and suitable for use in ²⁵²Cf brachytherapy treatment planning systems based on the recommendation by the ICRU that the uncertainties in dose delivery in radiotherapy should be lower than 5%.