Absorbed dose ratios for repeated therapy of neuroblastoma with I-131 mIBG

Patients undergoing targeted radionuclide therapy (TRT) may receive a series of two or more treatment administrations at varying intervals. However, the level of activity administered and the frequency of administration can vary widely from centre to centre for the same therapy. Tumour dosimetry is seldom employed to determine the optimum treatment plan mainly due to the potential inaccuracies of image quantification. In this work 3D dose distributions obtained from repeated therapies have been registered to enable the dose ratios to be determined. These ratios are independent of errors in image quantification and, since the same target volume can be transferred from one distribution to the next, independent of inconsistencies in outlining these volumes. These techniques have initially been applied to ten sets of I-131 mIBG therapy scan data from five patients, each undergoing two therapies. It was found that where a similar level of activity was administered for the second therapy, a similar tumour dose was delivered, and in two cases where a higher level of activity was administered for the second treatment, a correspondingly higher absorbed dose was delivered. This justifies an approach of administering activities based on individual patient kinetics rather than administering standard activities to all patients.