Voxeldose:: A computer program for 3-D dose calculation in therapeutic nuclear medicine

A computer program, VoxelDose, was developed to calculate patient specific 3-D-dose maps at the voxel level. The 3-D dose map is derived in three steps: i) The SPECT acquisitions are reconstructed using a filtered back projection method, with correction for attenuation and scatter; ii) the 3-D cumulated activity map is generated by integrating the SPECT data; and iii) a 3-D dose map is computed by convolution (using the Fourier Transform) of the cumulated activity map and corresponding MIRD voxel S values. To validate the VoxelDose software, a Liqui-Phil(TM) abdominal phantom with four simulated organ inserts and one spherical tumor (radius 4.2 cm) was filled with known activity concentrations of In-111. Four cylindrical calibration tubes (from 3.7 to 102 kBq/mL) were placed on the phantom. Thermoluminescent mini-dosimeters (mini-TLDs) were positioned on the surface of the organ inserts. Percent differences between the known and measured activity concentrations were determined to be 12.1 (tumor), 1.8 (spleen), 1.4, 8.1 (right and left kidneys), and 38.2% (liver), leading to percent differences between the calculated and TLD measured doses of 41, 16, 3, 5, and 62%. Large differences between the measured and calculated dose in the tumor and the liver may be attributed to several reasons, such as the difficulty in precisely associating the position of the TLD to a voxel and limits of the quantification method (mainly the scatter correction and partial volume effect). Further investigations should be performed to better understand the impact of each effect on the results and to improve absolute quantification. For all other organs, activity concentration measurements and dose calculations agree well with the known activity concentrations.