Pre-treatment dosimetry inY-SIRT: Is it possible to optimise SPECT reconstruction parameters and calculation methods for accurate dosimetry?

Purpose: The aim of this study was (a) to optimise theTc-SPECT reconstruction parameters for the pre-treatment dosimetry ofY-selective internal radiation therapy (SIRT) and (b) to compare the accuracy of clinical dosimetry methods with full Monte-Carlo dosimetry (fMCD) performed with Gate.

Methods: To optimise the reconstruction parameters, two hundred reconstructions with different parameters were performed on a NEMA phantom, varying the number of iterations, subsets, and post-filtering. The accuracy of the dosimetric methods was then investigated using an anthropomorphic phantom. Absorbed dose maps were generated using (1) the Partition Model (PM), (2) the Dose Voxel Kernel (DVK) convolution, and (3) the Local Deposition Method (LDM) with known activity restricted to the whole phantom (WP) or to the liver and lungs (LL). The dose to the lungs was calculated using the "multiple DVK" and "multiple LDM" methods.

Results: Optimal OSEM reconstruction parameters were found to depend on object size and dosimetric criterion chosen (D or DVH-derived metric). The D of all three dosimetric methods was close (≤ 10%) to the D of fMCD simulations when considering large segmented volumes (whole liver, normal liver). In contrast, the D to the small volume (∅=31) was systemically underestimated (12%-25%). For lungs, the "multiple DVK" and "multiple LDM" methods yielded a D within 20% for the WP method and within 10% for the LL method.

Conclusions: All three methods showed a substantial degradation of the dose-volume histograms (DVHs) compared to fMCD simulations. The DVK and LDM methods performed almost equally well, with the "multiple DVK" method being more accurate in the lungs.