Validation of an analytical expression for the absorbed dose from a spherical beta source geometry and its application to micrometastatic radionuclide therapy

The purpose of this study was to validate an analytical expression for the absorbed-dose calculation from the spherical source of beta-emitting radion uclides and to apply it to micrometastases treated with radiolabeled monocl onal antibodies. The self-absorbed fractions from I-131 and P-32 uniform sp herical sources were calculated using the analytical expression introduced by P. K. Leichner (J. Nucl. Med., 35: 1721-1729, 1994). The calculated abso rbed fractions were compared with previously reported values and were found to be in reasonable agreement, with a maximum difference of 15% for smalle r masses and a long-range beta emitter. The expression was subsequently app lied to estimate the absorbed dose within spheroid models with nonuniform p enetration of radiolabeled antibody. The corresponding absorbed dose for I- 131 was compared with reported micro-thermoluminescence dosimeter measureme nts and found to be in good agreement. This work has independently substant iated the methodology outlined by Leichner and may be reliably incorporated into new software developments for radionuclide dosimetry treatment planni ng.