Departmental Oral Examination (Thesis Title: SPECT/CT Quantification of Lu-177 for Dosimetry in Radionuclide Therapy Treatments of Neuroendocrine Tumors)

Speaker: 
CARLOS URIBE
Event Date and Time: 
Fri, 2015-11-06 09:00 - 11:00
Location: 
VGH Research Pavilion - Room 700
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

ABSTRACT:
Peptide receptor radionuclide therapy (PRRT) has shown promising results in the treatment of neuroendocrine tumors. These tumors over-express somatostatin receptors that allow us to label somatostatin analogues with 177Lu to deliver dose to the tumor.  However, currently 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 patients being under-treated while others over-treated. The aim of this thesis was to develop a simple protocol for 177Lu activity quantification for patients undergoing PRRT with the purpose of performing personalized dose assessments. Several physics phenomena that influence image quantification were investigated.

As electrons emitted in the decay of 177Lu result in creation of Bremsstrahlung, Monte-Carlo simulations were performed to investigate this effect on image quantification of 177Lu. 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. An experiment to measure the camera deadtime was performed by adding activity into a bottle placed in a cylindrical phantom. Plots for observed count rate as a function of the true count rate were made, and the deadtime was calculated based on the paralyzable model. The protocol was applied to four patient data, and OLINDA and voxelized dosimetry calculations were used to create dose volume histograms for the kidneys. Lastly, a graphical user interface to allow for the quantitative reconstruction of the data obtained using any of the manufacturers was developed.

Our results suggest that Bremsstrahlung contributions to the detected energy spectrum in imaging studies of 177Lu have no degrading effects in image quantification. Our protocol recovers the activity in kidneys to within 10%. The deadtime correction based on paralyzable model was accurate for the count rates measured. The deadtime corrections should be performed based on scatter corrected photopeak window instead of the full spectrum. Lastly, the dose delivered to the kidneys in patient data was lower than the suggested dose per session in order to reach current toxicity limits.

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