Development of a SPECT-based three-dimensional treatment planning system for radioimmunotherapy

Two major obstacles in the development of improved methods for more accurate dose estimates for radioimmunotherapy have been the difficulty in obtaining an accurate patient-specific three-dimensional activity map in vivo and calculating the resulting absorbed dose. We propose a method for three- dimensional internal dosimetry that integrates the three-dimensional activity map from SPECT with a dose-point kernel convolution technique to provide the three-dimensional distribution of absorbed dose. Methods: Accurate activity quantitation was achieved with appropriate methods. The count density map from SPECT images was converted into an activity concentration map with a calibration phantom approach. This map was then convolved with an ¹³¹I dose-point kernel and three-dimensional fast Fourier transform to yield three-dimensional distribution of absorbed dose, which was then processed to provide the absorbed 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 thermoluminescent dosimeter measurements to be within 8%. With standard organs and configuration, 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 parameters for treatment optimization. Absorbed dose distribution from any plane can be graphically displayed in various ways.