Application of radiobiological dosimetry to radionuclide directed therapy.

The standard linear quadratic model, which has been used to assess the radiobiological damage to tissue by external beam fractionated radiotherapy, has been extended to encompass a general continuous time varying dose rate protocol such as radionuclide therapy. If the radionuclide clearance from the tissue is purely exponential, the effect is readily calculated. Otherwise, the effect can be evaluated by numerical integration if the dose rate time-1 profile is known. It can be shown that if the maximum percentage initial uptake g-1 uptake in normal or tumour tissue is less than 0.046 or 0.23 for an administered activity of 50 mCi of 90Y or 131I respectively, then the radiation-induced damage will certainly be less than for 2 Gy fraction-1 external beam therapy for the same total dose. Preliminary imaging and knowledge of the radionuclide kinetics using a non-therapeutic dose may be used to calculate the predicted radiation damage to tissues for a particular therapeutic dose provided the tracer and therapy doses have identical kinetics.