Caren van Roekel

186 Chapter 7 exclusively. All registration results were checked visually (RB). Minor manual adjustments were allowed, but only based on CT and never on nuclear imaging. A 1 cm dilation of the tumor and liver contours was used, to account for breathing movement, errors in registration and resolution differences. The counts in the dilated contours were used for activity calculation, but the volume of the non-dilated VOIs was used for absorbed dose calculation. The quantitative Monte Carlo-based SPECT reconstruction used in this study yields voxels that contain absolute activity (in units of MBq). The absorbed dose (Gy) in each voxel was subsequently calculated using the local deposition model, which posits that -at the resolution of SPECT- all dose is deposited within the voxel of origin. The average doses in parenchymal tissue and tumors was calculated using the transferred delineations, as described previously (Figure 1). FIGURE 1. Example of tumor delineation and absorbed-dose estimation. Using the liver contour, the low-dose CT of the PET was matched to the low-dose CT of the SPECT (A). The tumors were automatically defined using a threshold. Liver- and tumor contours were transferred from the PET/CT to the SPECT/CT and absorbed doses were calculated (B).