Abstract
1845
Objectives Radioembolization of 90Y microspheres is an emerging therapy for unresectable liver cancer. Post-treatment imaging is performed to evaluate the delivery of the microspheres and also as a basis for dosimetry. PET/CT has been suggested for this purpose by exploiting a small internal pair production decay branch of 90Y. Little is known, however, about the accuracy of 90Y PET images or how errors in the reported activity affect the dosimetry. This study describes a first effort to investigate these concerns by imaging a phantom prepared with a well-known concentration of 90Y which was traceable to national radioactivity standards.
Methods A Jaszczak cylinder was gravimetrically filled with a uniform concentration of 90YCl3. Aliquots of the phantom solution were obtained for primary radioactivity measurements via TDCR liquid scintillation counting. Images were acquired with a Philips Gemini TF PET/CT and analyzed for uniformity and accuracy of the reported activity. Voxel dose maps were generated by using the PET images as the source term in Monte Carlo dose calculations.
Results The calibrated phantom activity had a combined standard uncertainty of 0.4% and was 82.1 kBq/mL at the midpoint of scanning. The PET images for a 12 hr scan were quite noisy with a voxel variability of 21%. Despite this, on average, we recovered 95.4% of the known activity, demonstrating that accurate quantitation of 90Y is possible with a long scan and no partial volume effects. As expected, image-based dosimetry underestimated the true voxel dose and the dose maps were smoother than the PET images because of the long range of 90Y betas.
Conclusions This is the first time that image-based dosimetry with 90Y has been performed with traceable activity. Future experiments will look at a more realistic scenario involving a heterogeneous distribution of 90Y microspheres.
Research Support Dr. Mille was supported by a National Research Council Research Associateship Award at the National Institute of Standards and Technology.