Validation of a dose point kernal convolution technique for iodine-131 internal dosimetry

The objective of this study was to validate a dose point kernel convolution technique, which provides a 3-D distribution of absorbed dose from a 3-D distribution of activity of I-131. Traditionally, S-factors provided by the MIRD committee have been used to provide one average absorbed dose to each organ of interest. Inaccuracy in the MIRD method is due to assumptions of idealized or standard sizes and shapes of organs in all patients, unavailability of S-factors for tumors, and assumption of uniform activity distribution. MIRD S-factors provide no information about the heterogeneous distribution of absorbed dose in organ of interest. These deficiencies can be resolved by the dose point kernel convolution method. A dose point kernel for I-131 can be derived from a Monte Carlo simulation (EGS-4 computer code). The dose point kernel was then transformed from spherical coordinates into a 3-D rectangular matrix. This matrix was convolved with the 3-D activity map to yield a 3-D distribution of absorbed dose. The 3-D absorbed dose map was then processed to provide absorbed dose distributions within regions of interest, dose volume histograms, and isodose distributions. The convolution calculation was performed using a 3-D fast Fourier transform (FFT) technique, which takes less thanmore » 40 seconds to process a 128 x 128 x 16 matrix on an Intel 496 DX2 (66 MHz) PC. The calculation of absorbed dose due to penetrating radiation was benchmarked with TLD measurement in physical phantom. The results showed good agreement with the sample mean and standard deviation of % error equal to -0.6% and 3.69%, respectively. An Alderson abdominal phantom with spleen, liver and an 88-ml spherical tumor was used to compare the average dose estimate by the proposed method to the MIRD formalism. The percentage differences of the average absorbed dose rates calculated by the two methods were 9.1% for liver, 13.7% for spleen, and 0.9% for the tumor.« less