Caren van Roekel

168 Chapter 6 The 166 Ho-microspheres activity distribution following treatment was estimated with quantitative SPECT/CT reconstructions using a quantitative fast Monte Carlo- based reconstructor (UMCS), which has been previously validated for 166 Ho (11). The PET-based tumor and liver outlines were transferred to the corresponding 166 Ho-SPECT reconstructions, using a rigid registration of the CT scans of the PET and SPECT acquisitions (12). The liver contours served as a mask to focus the registration on the liver region only. The liver and tumor outlines were subsequently dilated with 1 cm, to minimize difference due to resolution, (respiratory) motion and local registration errors. The tumor doses were estimated using the activity in these dilated masks and the mass of the original contour. The parenchymal dose was calculated in the same fashion, after subtracting the dilated tumor masks from the liver mask. The dose was assumed to be fully absorbed within each volume of origin (local deposition model) (13). For the three-month follow-up scans, the tumors were automatically defined in ROVER, using the method described above. The change in TLG was used to determine the metabolic tumor response. The baseline and follow-up images were assessed side by side to ensure the same tumors were identified. Merged tumors on follow-up imaging were regarded as one tumor at baseline. In those cases, a weighted average of the absorbed dosewas calculated, correcting for tumor volume. Metabolic tumor response was grouped in categories according to the PERCIST criteria (10). Complete metabolic response (CR) was achieved if there was a 100% reduction in TLG, partial metabolic response (PR) when there was a decrease of at least 45%, progressive metabolic response (PD) was characterized by an increase of at least 75%, stable disease (STBD) was defined as an increase of less than 75% and a decrease of less than 45%. Furthermore, these categories were grouped according to objective response (CR + PR) and non-response (STBD + PD). Statistical Analysis The relation between tumor absorbed dose and response were assessed both at the level of individual tumors (local response) as well as at the patient level, in which case the patients were grouped according to PERCIST based