Dose optimization in nuclear medicine using mathematical simulation

The aim of this work was to determine the dose received by a patient who benefited from a bone scintigraphy and obtain data concerning the acquisition of the images, with the goal reducing the administered activity for such a procedure. In order to determine such doses, it is necessary to make use of a biokinetic model concerning the specific radiopharmaceutical (in our case, 99mTc-MDP) Building up a model or validating a pre-existing one requires in vivo and in vitro measurements of the activities present in the organisms, as determined at different times after intake. Our in vitro measurements were obtained by means of urine samples from 20 patients. Our in vivo data were obtained directly from irradiated human bone, by means of a scintillation camera and a tibia / fibula phantom (65 patients). We studied the following parameters, concerning a conventional bone scintigraphy (125 patients): injected activity, lapsed time between injection and acquisition, number of counts at the first scanning, age and body mass index of the patient, target/back ground ratio and the quality of the image. The in vitro data were comprised between 9.25 and 118 Bq, as related to the times after of 15 and 3000 minutes. The in vivo data ranged from 398 Bq and 2.5 MBq at 125 and 2570 minutes after in take. The effective dose was 6.8 x 10-3 mSv/MBq. We validated the biokinet model as published by the ICRP (Publication n. 53), according to the local conditions. Our data indicate that it is possible to reduce the injected activity to patients benefiting from a bone scintigraphy, without a worsening in the quality of the image. This would reduce the dose delivered to the patient, as well as the exposition of the related workers to radiation coming from such patients. (author)