DEVELOPMENT OF A SPECT-BASED 3-DIMENSIONAL TREATMENT PLANNING SYSTEM FOR RADIOIMMUNOTHERAPY

Two major obstacles in the development of improved methods for more ac curate dose estimates for radioimmunotherapy have been the difficulty in obtaining an accurate patient-specific three-dimensional activity m ap in vivo and calculating the resulting absorbed dose. We propose a m ethod for three-dimensional internal dosimetry that integrates the thr ee-dimensional activity map from SPECT with a dose-point kernel convol ution technique to provide the three-dimensional distribution of absor bed dose. Methods: Accurate activity quantitation was achieved with ap propriate methods. The count density map from SPECT images was convert ed into an activity concentration map with a calibration phantom appro ach. This map was then convolved with an I-131 dose-point kernel and t hree-dimensional fast Fourier transform to yield three-dimensional dis tribution of absorbed dose, which was then processed to provide the ab sorbed dose distribution in regions of interest. Results: The accuracy of quantitative SPECT was validated to be within 16%. The calculated penetrating radiation absorbed dose was verified with thermoluminescen t dosimeter measurements to be within 8%. With standard organs and con figuration, the method calculated absorbed dose in good agreement with the MIRD formalism (less than 14%). Conclusion: This method overcomes the limitations of planar imaging techniques and the current routine implementation of the MIRD formalism. The results can be processed to provide the absorbed dose distribution in regions of interest and para meters for treatment optimization. Absorbed dose distribution from any plane can be graphically displayed in various ways.