Characterization of yttrium-90 PET/CT as a tool to evaluate initial dosimetry and efficacy of yttrium-90 radioembolization.

Objectives Yttrium-90 intra-arterial radioembolization is an effective therapy in patients with both unresectable hepatocellular carcinoma and metastatic disease to the liver, demonstrated to prolong survival. Recently, PET-CT has emerged as a promising technique for investigation of yttrium-90 dosimetry within the targeted neoplasm. The yttrium-90 decay to zirconium-90, a nearly pure β- particle emission, has been well described. Traditionally, post-procedural yttrium-90 microsphere distribution is imaged using SPECT through detection of indirect Bremsstrahlung radiation. However, a small percentage of yttrium-90 decay occurs through internal production of a β+/β- from the 0+/0+ transition state. Although a proportionally very small event, this positron emission can be captured with PET imaging. Recent advancements in post-processing software have allowed for quantification of detailed dose distribution. However, a variety of technical factors pertaining to image acquisition, postprocessing and dosimetric analysis potentially influence the reported dosimetric profile.

Methods During initial investigations, a phantom is used with clinically appropriate dose deposited within the phantom liver. PET-CT is acquired of the phantom and data is post processed using a systematic variety of different reconstruction algorithms. Constructed data is then analyzed with MIM processing software (MIM Software Inc, Cleveland, OH) to validate volumetric and dosimetric analysis of the phantom. In a small cohort of patients receiving yttrium-90 radioembolization, PET-CT is then performed immediately following therapy. Volumetric regional analysis of tumor, liver and all imaged intra-abdominal solid organs is obtained using phantom validated methods. Results are compared with pre-procedure FDG PET-CT, correlated with a variety of other available imaging modalities and followed with post procedure FDG PET-CT, in order to assess for tumor recurrence and establish tumoricidal dosimetry.

Results Initial phantom measurements document the wide influences and artificial variation that different PET-CT post processing techniques may potentially have upon acquired data. This work details the different strengths and weaknesses of each post-processing technique and elucidates methods to achieve the greatest accuracy in dosimetric analysis. Clinical data illustrates tumor imaging with multimodality comparison (illustrating correlation of PET-CT images with CT, MRI and ultrasonography). Pre and postprocedural imaging as well as clinical assessment are compared with yttrium-90 radioembolization dosimetric profile of each clinical case. Cases of both tumor recurrence and tumoricidal dosimetry are discussed. Furthermore, biodsitribution of yttrium-90 and assessment of dose heterogeneity is assessed. Experimental metrics are compared with more traditional measurements including tumor volume, and SUVmax/mean.

Conclusions This translational research provides a critical initial analysis of yttrium-90 PET-CT techniques using phantom measurements to establish validated methods for image acquisition, post processing and dosimetric analysis. Validated methods are then directly applied to a clinical case series to demonstrate the potential value of detailed volumetric dosimetry in evaluating treatment response and potentially predicting tumor recurrence.