Combining dosimetry for targeted radionuclide and external beam therapies using the biologically effective dose.

It is not uncommon for a patient to receive both external beam and targeted radionuclide therapy during the course of a cancer treatment. The total dose received by the tumor and by normal tissues will therefore be subject to the contributions of both treatment modalities. However, the two treatments are generally applied independently of one another, with little attention paid to the combined effect. With the availability of patient-specific three-dimensional dosimetry for radionuclide therapies, it is pertinent now to consider the combined effect of the two treatments, and to investigate how dosimetry for this situation may be carried out. Methodology has been developed to allow a combination of dose information from the two types of therapy. The biologically effective dose (BED) has been employed to address the issue of inequivalence of biological effect of the two therapies. Dose distributions have been represented as distributions of BED, and the net effect resulting from the combination of these two therapies demonstrated through a combination of BED maps. Examples are presented of cases in which this analysis of a combined therapy provides a more favorable treatment than either therapy alone. For one patient the ratio of the mean spinal cord dose to the mean CTV dose was calculated for both an external beam therapy alone and for a combined therapy and was found to be 0.40 and 0.16, respectively.