Abstract
124
Objectives: The aim of our study was to develop an estimation of mean absorbed-dose to the liver using bremsstrahlung SPECT/CT (bSPECT/CT) in patients undergoing radioembolization with Y-90 microspheres. Methods: In our previous study comparing Y-90 dosimetry obtained using bSPECT/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 bremsstrahlung imaging is difficult due to scatter, septal penetration, the continuous nature of the bremsstrahlung energy spectrum, and inefficient bremsstrahlung production. 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.