Quantification of I-131 Activity from Gamma Camera Images of Thyroid Cancer Patients

Sammanfattning: BACKGROUND: Radioiodine (131I) has been used in thyroid cancer treatments for many years, and has demonstrated good results with regards to disease management. Radioiodine therapy is however partially empirical, as not very much is known concerning radiation dosimetry. At Skåne University Hospital, the number of treatments has been growing steadily over the past couple of years, resulting in an increased demand for dosimetry. The aim of this thesis was to develop a method for patient-specific 131I activity quantifications, and thereby provide a foundation for patient-specific dosimetry at Skåne University Hospital. METHOD: Planar measurements were performed to evaluate the degradation from penetration and scatter, and assess the effectiveness of two different window based scatter corrections (namely a DEW and a TEW correction). Calibration factors were established for planar and SPECT acquisitions, and the NEMA Body Phantom was used to study partial volume effects. Lastly, an anthropomorphic phantom study was carried through to test the quantification method and thereby assess potential quantification errors in patient imaging. RESULTS: For spherical structures, quantifications errors of up to 40% were obtained for planar images, while SPECT quantification errors were generally below 10%. For non-spherical structures, the error was significantly increased. In the planar studies of scatter and penetration, a TEW scatter correction was shown to be most effective. However, in quantifications, the scatter correction appeared to have little impact on the accuracy of the results. Instead, the quantification error seem to majorly depend on size and shape of the lesion, as well as the overall attenuation properties of the examined object. CONCLUSIONS: With the developed method, accurate SPECT quantifications are feasible. However, a varying patient geometry can potentially induce significant errors, as a window based scatter correction alone cannot compensate for large variations in the level of attenuation. Hence, the accuracy of the calibration is crucial. Object shape effects appear to be significant, as applying a sphere-based recovery coefficient to a non-spherical structure will not recover all of the activity, and errors remain large. Planar quantifications are generally associated with large errors, and accurate planar quantifications would be difficult to achieve in patient imaging. Additional work is needed to improve the accuracy of the method. Nonspherical recovery coefficient needs to be established, and effects of variations in patient geometry and 131I uptake should be examined more thoroughly. Furthermore, the reconstruction protocol should be optimized. In order for dosimetry to be performed, methods of mass determination would need to be explored. Also, methods of determining the effective half-life would need assessment.