VALIDATION OF A DOSE-POINT KERNEL CONVOLUTION TECHNIQUE FOR INTERNAL DOSIMETRY

The objective of this study was to validate a dose-point kernel convol ution technique that provides a three-dimensional (3D) distribution of absorbed dose from a 3D distribution of the radionuclide I-131. A dos e-point kernel for the penetrating radiations was calculated by a Mont e Carlo simulation and cast in a 3D rectangular matrix. This matrix wa s convolved with the 3D activity map furnished by quantitative single- photon-emission computed tomography (SPECT) to provide a 3D distributi on of absorbed dose. The convolution calculation was performed using a 3D fast Fourier transform (FFT) technique, which takes less than 40 s for a 128 x 128 x 16 matrix on an Intel 486 DX2 (66 MHz) personal com puter. The calculated photon absorbed dose was compared with values me asured by thermoluminescent dosimeters (TLDs) inserted along the diame ter of a 22 cm diameter annular source of I-131. The mean and standard deviation of the percentage difference between the measurements and t he calculations were equal to -1% and 3.6%, respectively. This convolu tion method was also used to calculate the 3D dose distribution in an Alderson abdominal phantom containing a liver, a spleen, and a spheric al tumour volume loaded with various concentrations of I-131. By avera ging the dose calculated throughout the liver, spleen, and tumour the dose-point kernel approach was compared with values derived using the MIRD formalism, and found to agree to better than 15%.