A dosimetric model for intrathecal treatment with 131I and 67Ga.

ABSTRACT

PURPOSE: Calculations were performed of absorbed dose distributions of the beta-emitter 131I and the Auger emitter 67Ga for intrathecal administration. METHODS AND MATERIALS The proposed dosimetric model accounts for the macroscopic distribution of the activity, by means of a Medical Internal Radiation Dose Committee approach, and for the microscopic distribution of activity, by means of a point kernel technique. This point kernel approach was used in combination with a distance histogram technique, to study in more detail the absorbed dose distribution 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 lesions adherent to the pia mater, the beta-emitter 131I provides crossfire from the CSF, resulting in a higher absorbed dose (Gy/MBq) in these lesions as compared with the Auger emitter 67Ga. In case of increasing radionuclide uptake, the increment of the absorbed dose in the adherent lesions and the free floating cells from 67Ga is considerable because of the local deposition of energy by this radionuclide. CONCLUSIONS: The model might be useful to select the optimal emission characteristics of radionuclides applicable for intrathecal therapy, which is demonstrated in a comparison of the Auger emitter 67Ga and the beta-emitter 131I.