Caren van Roekel

185 Dose-effect relationships of holmium-166 radioembolization in colorectal cancer MATERIALS AND METHODS Patients This was a retrospective analysis of CRC patients who were treated with 166 Ho- radioembolization in the HEPAR II (NCT01612325 (6)) and the SIM (NCT02208804 (8)). Before study inclusion, all patients provided written informed consent. The institution’s Medical Ethics Committee approved both studies. The CRC patients of the HEPAR II study were already part of a preliminary mixed tumor-type cohort analysis and were also included in this CRC-only analysis (11). Treatment Procedures During work-up, laboratory and clinical examinations were performed and patients underwent multiphasic liver CT and 18 FDG-PET/CT at a median of 16 days before treatment (range 6-42). Pre-treatment activity calculation was performed using a method similar to the medical internal radiation dosimetry (MIRD) method (12). The injected activity (IA) to reach an average absorbed dose of 60 Gy in the target volume was calculated as (7): = ( − 0,2) + ( + ) ( ) = ( ( ) ∗ ( )) 50 ( ) = ( ) ( ) ( ) ( ) ⁄ ( ) = ( ) ∗ ([ ∗ ( )] + ( )) 49,670 ∗ (1 − ℎ ) ( ) = ℎ ( ) ∗ 3780 ( ) ( ) = ℎ ( ) ∗ 3780 ( ) ( ) = ( ( ) ∗ ( )) 50 ( ) ( ) = ( ( ) ∗ ( )) 16 ( ) ( ) = ( ) ∗ ([ ∗ ( )] + ℎ ℎ ( )) 49.67 ∗ (1 − ℎ ) The IA was not adjusted for lung shunt fraction, in line with the instructions for use for Quiremspheres. Since the abundance of gamma photons invokes detector dead-time, patients underwent a quantitative 166 Ho-SPECT/CT to assess the therapeutic dose distribution three to five days after treatment. The threshold used for tumor delineation was defined per patient, based on twice the mean aortic blood pool SUV corrected for lean body mass. Using this patient-relative threshold and the volume restriction of 5 mL, tumors were automatically defined. This way, only regions with metabolic activity, significantly exceeding the background activity of the liver, were defined. The threshold used to delineate tumors at follow-up was defined again on the 3-month 18 FDG PET/CT. A rigid registration (using Elastix software (13)) of the CT scans of the PET and SPECT acquisitions was used to transfer the PET-based tumor- and liver contours to the corresponding 166 Ho-SPECT reconstructions. The previously manually contoured livers acted as a mask to focus registration on this region 7