Post-radioembolization yttrium-90 PET/CT - part 1: diagnostic reporting

Background: Yttrium-90 (Y-90) positron emission tomography with integrated computed tomography (PET/CT) represents a technological leap from Y-90 bremsstrahlung single-photon emission computed tomography with integrated computed tomography (SPECT/CT) by coincidence imaging of low abundance internal pair production. Encouraged by favorable early experiences, we implemented post-radioembolization Y-90 PET/CT as an adjunct to Y-90 bremsstrahlung SPECT/CT in diagnostic reporting. Methods: This is a retrospective review of all paired Y-90 PET/CT and Y-90 bremsstrahlung SPECT/CT scans over a 1-year period. We compared image resolution, ability to confirm technical success, detection of non-target activity, and providing conclusive information about Y-90 activity within targeted tumor vascular thrombosis. Y-90 resin microspheres were used. Y-90 PET/CT was performed on a conventional time-of-flight lutetium-yttrium-oxyorthosilicate scanner with minor modifications to acquisition and reconstruction parameters. Specific findings on Y-90 PET/CT were corroborated by Y-90 bremsstrahlung SPECT/CT, Tc-99m macroaggregated albumin SPECT/CT, follow-up diagnostic imaging or review of clinical records. Results: Diagnostic reporting recommendations were developed from our collective experience across 44 paired scans. Emphasis on the continuity of care improved overall diagnostic accuracy and reporting confidence of the operator. With proper technique, the presence of background noise did not pose a problem for diagnostic reporting. A counter-intuitive but effective technique of detecting non-target activity is proposed, based on the pattern of activity and its relation to underlying anatomy, instead of its visual intensity. In a sub-analysis of 23 patients with a median follow-up of 5.4 months, Y-90 PET/CT consistently outperformed Y-90 bremsstrahlung SPECT/CT in all aspects of qualitative analysis, including assessment for non-target activity and tumor vascular thrombosis. Parts of viscera closely adjacent to the liver remain challenging for non-target activity detection, compounded by a tendency for mis-registration. Conclusions: Adherence to proper diagnostic reporting technique and emphasis on continuity of care are vital to the clinical utility of post-radioembolization Y-90 PET/CT. Y-90 PET/CT is superior to Y-90 bremsstrahlung SPECT/CT for the assessment of target and non-target activity.