An estimate of 90Y dosimetry for bremsstrahlung SPECT/CT imaging in liver therapy with 90Y microspheres

Objective: Bremsstrahlung SPECT/CT (bSPECT/CT) is one of the most common methods for post-therapy imaging in Y-90 microspheres selective internal radiation therapy (SIRT) of liver cancers. Here, we are proposing a simple approach using bSPECT/CT to estimate mean absorbed dose to the liver in patients undergoing treatment with Y-90 microspheres. Materials and 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 absorbed dose values to the whole-liver. However, there was a high linear correlation between the doses, which presented an opportunity for quantitative assessment using bSPECT/CT Y-90 imaging. In this study, after treatment with Y-90 microspheres, 43 patients were immediately imaged on a dual-head Infinia SPECT/CT gamma camera and on a mCT PET/CT system. 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 18 patients, the calculated correlation was used to estimate doses obtained from bSPECT/CT, and these estimations 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 mean absorbed doses in whole liver was high ((r) over cap (2) = 0.97), with a slope of 2.80 and an intercept of -0.63. This linear fit was used to calculate the bSPECT/CT doses for the remaining 18 patients. For these patients, the mean whole-liver dose obtained from bSPECT/CT fitted data vs that obtained from PET/CT were 50.59 Gy and 50.81 Gy, respectively. The average dose difference was 0.2 +/- 5.4 Gy (range - 18.2%-13.0%). The repeatability coefficient was 10.5 (20.8 % of the mean). Conclusion: Although quantitative bremsstrahlung imaging is difficult, it is possible to calculate adequate estimates of whole-liver dosimetry from bSPECT/CT imaging that is calibrated using its correlation with posttherapy PET/CT Y-90 images.