A DOSIMETRIC MODEL FOR INTRATHECAL TREATMENT WITH I-131 AND (67)GA

Purpose: Calculations were performed of absorbed dose distributions of the beta-emitter I-131 and the Auger emitter Ga-67 for intrathecal ad ministration. Methods and Materials: The proposed dosimetric model acc ounts for the macroscopic distribution of the activity, by means of a Medical Internal Radiation Dose Committee approach, and for the micros copic distribution of activity, by means of a point kernel technique. This point kernel approach was used in combination with a distance his togram technique, to study in more detail the absorbed dose distributi on in the cerebro-spinal fluid, in the surface of the central nervous system, and in tumor sites. We simulated decreased uptake, as well as highly selective uptake in free-floating tumor cells and in meningeal lesions (1-16 cells thick). Results: In case of limited access to lesi ons adherent to the pia mater, the beta-emitter I-131 provides crossfi re from the CSF, resulting in a higher absorbed dose (Gy/MBq) in these lesions as compared with the Auger emitter Ga-67. In case of increasi ng radionuclide uptake, the increment of the absorbed dose in the adhe rent lesions and the free floating cells from Ga-67 is considerable be cause of the local deposition of energy by this radionuclide. Conclusi ons: The model might be useful to select the optimal emission characte ristics of radionuclides applicable for intrathecal therapy, which is demonstrated in a comparison of the Auger emitter Ga-67 and the beta-e mitter I-131.