Page 30 - 89Zr-Immuno-PET:Towards a Clinical Tool to Guide Antibody-based Therapy in Cancer

Chapter 2
same (patient and bed) position during subsequent studies, by using rigid head immobilization devices (e.g. radiotherapy mask) and belts around the patient. To ensure correct positioning of the VOIs, the elevation of the bed was registered for all scans and in case of differences in patient or bed position, scans were aligned using in home-developed software program. Furthermore, in case of remaining dislocation of VOIs, all VOIs were relocated in one step to keep the internal relations intact. For relocation, the alignment of lungs VOIs and the contour of the head and neck were verified using reconstructed transmission scans (for each subsequent study) as well. However, if still necessary, VOIs were ungrouped and individual VOIs were relocated to improve the position. After this VOI positioning procedure all VOIs were saved and projected onto the emission scans. The mean uptake in Bq/cm3 was then derived using these final VOIs.
The tumor was defined on 144 h p.i. scan using a tool, which semi- automatically defined a 3D 50% (to max) isocontour around the tumor, and next the mean uptake in Bq/cm3 was calculated. The mean tumor uptake was determined for the latest acquired PET study only, as correct repositioning of the tumor VOI onto earlier scans could not be reliably visually verified due to lack of signal from the tumor.
All VOIs set on organs and tumors, were evaluated by 2 experienced nuclear physicians.
Internal radiation dosimetry
The internal radiation dosimetry for the adult human was evaluated through the normalized cumulated activities for each patient provided as input to the OLINDA/ EXM 1.0 code (23).
Residence times were calculated for liver, kidneys, lungs, spleen, and the remainder of the body, entering the percentage of the injected dose at each time point for each patient in OLINDA/EXM 1.0 and fitting these data using a mono- exponential function. The residence times for the thyroid were calculated using trapezoidal integration. The remaining area under the curve from the end of data collection until infinity was determined by considering physical decay only. The residence time of the remainder of the body was defined as: 113 - sum of residence times of source organs. A total residence time for the entire body of 113 h is obtained in case of no biological clearance. Red marrow dose was estimated using sampled blood clearance data (24,25). The red marrow concentration was assumed to be 30% of the blood activity concentration (conversion factor = 0.3). In addition, the dose from the remainder of the body onto red marrow was considered as well.
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