Activity Quantification of Yttrium-90: PET as compared to Bremsstrahlung SPECT

Sammanfattning: Introduction: Cancer treatment is a process with continuous development. The most common treatment, within the field of radiation physics, is external radiation therapy, whereas one of the least common methods is Selective Internal RadioTherapy (SIRT). SIRT is a rather novel method for treating liver carcinoma. Microspheres containing 90Y are infused directly into the liver, via the hepatic artery. Seventeen SIRT-treatments have been performed at Skåne University Hospital (SUS) in Lund since the start in December 2010 until today (June 2012). Due to the work of David Minarik, the activity distribution is today quantified post-treatment in clinical routine with the single photon emission computed tomography camera (SPECT) system. However, recent work performed by Lhommel et. al has shown the ability to use the positron emission tomography (PET) system to create PET-images with 90Y labeled SIR-spheres. Due to the better spatial resolution in the PET-camera, compared to the SPECT-camera, it is of interest to investigate the possibility to use PET instead of SPECT, for activity quantification of SIRT-patients´ liver. A system with a better spatial resolution has the potential to create a more accurate absorbed dose map from the activity distribution within the liver, compared to the SPECT based system used today. The aim of this thesis is to evaluate whether PET can be used for activity quantification after 90Y microsphere treatments. Once the most suitable camera is chosen, absorbed dose calculation can be performed with the outcome from the activity distribution shown on the PET- or SPECT -images of the liver. Materials and methods: For this project, three patients have been treated with SIRT and imaged in both the PET- and SPECT-camera. Furthermore, phantom measurements have been performed both with 18F and 90Y. Results: For all three patients the PET system overestimates the total activity in the liver with about 20%. The SPECT systems result vary between 0% to – 15% compared to the true activity within the liver. However, more corrections are required on the PET-images to be certain that extra counts originating from the crystals within the PET cameras detector, does not contaminate the image with background noise, and thus giving an overestimation in activity quantification of about 20%. Conclusion: The results indicate that it is possible to use PET to determine the activity distribution within the liver for patients treated with SIRT. However, further investigations are needed to determine whether PET should replace SPECT.