Final PhD Oral Examination (Thesis Title: ““SPECT/CT Quantification of 177Lu for Dosimetry in Radionuclide Therapy Treatments of Neuroendocrine Tumors””)

Event Date and Time: 
Wed, 2016-01-20 12:30 - 14:30
Room 200, Graduate Student Centre
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 

Peptide receptor radionuclide therapy (PRRT) shows promising results in treatment of neuroendocrine tumors (NETs). This treatment is based on fact that NETs over-express somatostatin receptors, therefore somatostatin analogues labeled with radioisotopes, such as 177Lu, deliver dose directly to the tumor. Currently, however, every patient receives the same amount of radioactivity of approximately 7400 MBq per treatment cycle. With this “one dose fits all” approach, differences between patients are not taken into account resulting in some subjects being under-treated while others over-treated. The aim of this study was to develop a simple protocol for activity quantification for patients undergoing PRRT. Quantitative images of activity distribution and information about its biokinetics are necessary for personalized dose assessments. Physics phenomena that influence image quantification were investigated.
As electrons emitted by 177Lu create Bremsstrahlung emissions, Monte-Carlo simulations were performed to investigate its effect on image quantification. Phantom experiments with different attenuation and scatter conditions were performed to test quantification accuracy and evaluate performance of several segmentation methods. Images were reconstructed using the OSEM algorithm and two scatter correction methods were compared. Additionally, camera deadtime was measured by adding activity into a bottle placed in a water-filled cylindrical phantom. Observed count rate vs. true count rate were plotted, and the deadtime was calculated based on the paralyzable model. The protocol was applied to four patient data; OLINDA and voxelized dosimetry calculations were used to create dose volume histograms for the kidneys. Lastly, a graphical user interface that allows for easy change of reconstruction parameters and quantitative reconstruction of the data obtained from different cameras was developed.
Our results suggest that Bremsstrahlung contributions to the detected energy spectrum have no degrading effects on image quantification. Activity in volumes >100mL was recovered with errors <10% of the true values. Analysis of deadtime based on paralyzable model demonstrated. that deadtime corrections should be performed based on scatter corrected photopeak window instead of the full spectrum. Lastly, our estimates of the dose delivered to the kidneys in patient data was substantially lower than the limit suggested by current guidelines.

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