Scaling Neutron Absorbed Dose Distributions from One Medium to Another

Abstract

Central axis depth dose (CADD} and off-axis absorbed dose ratio (OAR) measurements were made in water, muscle and whole skeletal bone TE-solutions, mineral oil and and glycerin with a clinical neutron therapy beam. These measurements show that, for a given neutron beam quality and field size, there is a universal CADD distribution at infinity if the depth in the phantom is expressed in terms of appropriate scaling lengths. These are essentially the kerma-weighted neutron mean free paths in the media. The method used in ICRU #26 to scale the CADD by the ratio of the densities is shown to give incorrect results. The OAR's measured in different media at depths proportional to the respective mean free paths were also found to be independent of the media to a good approximation. Therefore, neutron beam CADD's and OAR's may be measured in either TE-solution (USA practice) or water (European practice), and having determined the respective scaling lengths, all measurements may be scaled from one medium to any other. It is recommended that for general treatment planning purposes, scaling be made to TE-muscle value represents muscle TE-solution of density with a density of 1.04 g cm-3 , since this and other soft tissues better than-3 1.07 g cm. For such a transformation, relative measurements made in water are found to require very TM-1145 2 small corrections. Hence, it is further recommended that relative CADD and OAR measurements be performed in universality and convenience. Finally, water because of its a table of calculated scaling lengths is given for various neutron energy spectra and for various tissues and materials of practical importance in neutron dosimetry.