An Estimate Of Y-90 Dosimetry For Bremsstrahlung Spect/Ct Imaging

124 Objectives: The aim of our study was to develop an estimation of mean absorbed-dose to the liver usingbremsstrahlung SPECT/CT(bSPECT/CT) in patients undergoing radioembolization with Y-90 microspheres. Methods: In our previous study comparing Y-90 dosimetry obtained usingbSPECT/CT vs PET/CT, we found that there was a large difference between the mean doses to the liver. In most cases the mean dose obtained using bSPECT/CT was only one-third of that obtained using PET/CT. However, there was a high linear correlation between the doses, presenting an opportunity for quantitative assessment using bSPECT/CT Y-90 imaging, provided appropriate scaling and image enhancement. After selective internal radiation therapy (SIRT) with Y-90 microspheres, 36 patients were immediately imaged on a dual-head Infinia SPECT/CT gamma camera (GE Medical Systems, Milwaukee, WI), with MEGP collimator. After two to three hours of recovery, patients were then taken for PET/CT imaging (mCT, Siemens Medical) in the adjacent building. The data were reconstructed using the vendor’s reconstruction software and transferred to a common platform where the bSPECT/CT and PET/CT images were used to calculate Y-90 dosimetry using MIM 6.6 software (MIM Software Inc.). Local deposition method with known activity of Y-90 was used for dosimetry calculations. Images from 25 of the patients, randomly selected, were used to calculate the correlation of mean liver doses obtained from bSPECT/CT vs. PET/CT. For the remaining 11 patients, the calculated correlation was used to estimate doses obtained from bSPECT/CT, which were then compared to the doses obtained from PET/CT, considered the gold standard for quantitative analysis. Results: From the 25 selected patients, the calculated linear correlation between bSPECT/CT and PET/CT Y-90 doses was high ( r^2 =0.99), with a slope of 2.97 and an intercept of -3.43. This linear fit was used to calculate the bSPECT/CT doses for the remaining 11 patients ( Fig.1). For these patients, the mean whole-liver dose obtained from bSPECT/CT fitted data vs that obtained from PET/CT were 49.0 Gy and 51.2 Gy, respectively. The greatest difference in the mean whole-liver dose was a bSPECT/CT derived estimation that came out to 13% of the respective PET/CT value. Conclusions: Quantitative bremsstrahlungimaging is difficult due to scatter, septal penetration, the continuous nature of the bremsstrahlungenergy spectrum, and inefficient bremsstrahlungproduction. However, it is possible to calculate adequate estimates of whole-liver dosimetry from bSPECT/CT Y-90 imaging that is calibrated using its correlation with post-therapy PET/CT Y-90 images.