[Principles of dosimetric planning in contact therapy with highly active Cf-252 sources]

In clinical practice, some dosimetric problems of contact therapy with CF-252 are not yet sufficiently clarified. One possibility to solve these problems is to apply the conception of total photon-equivalent energy doses. This conception serves to calculate clinical guidance values of the RBE of the Cf-252 neutron constituent for radiogenic late effects in dependence on the total dose rate of the mixed radiation. The correlations indicated are valid for dose rates of 0.05 to 4 Gy/h and 4 to 1.25.10(4) Gy/h. On the basis of these functional correlations and taking into consideration the critical normal tissues of rectum and urinary bladder, the minimum RBE values of the tumors are indicated which allow a therapeutic effect of more than 1 in case of application of Cf-252. The formulas for the RBE dependence of the Cf-252 neutron constituent on the total dose rate permit to take into account any modifications of this value caused by varying source distances in contact therapy. It is concluded from these investigations that in oncologic patients, better results are to be expected from a treatment with high-energy Cf-252 sources, if the other treatment conditions are identical.