Retrospective SPECT/CT Dosimetry Following Transarterial Radioembolization

Abstract Background: Transarterial Radioembolization (TARE) effectively treats unresectable primary and metastatic liver tumors through local injection of Yttrium-90 (90Y) beta particle emitting microspheres. These microspheres implant around the tumor, damaging tumorous cells while sparing healthy liver tissue. Current dosimetry models are highly simplistic and based patient characteristics such as body surface area and fail to consider many important factors. There is a large need for an imaged based dosimetry post-TARE which would improve treatment safety and efficacy. Current post-TARE imaging is 90Y bremsstrahlung SPECT/CT and we study the use of these images for post-TARE dosimetry. Methods: Retrospective image review of 10 patients having a Philips HealthcareTM SPECT/CT following TARE SIR-Spheres® implantation. Emission series with attenuation correction were resampled to 3mm resolution and used to create image based dose distributions. Dose distributions and analysis were performed in MIM Software SurePlanTM utilizing SurePlanTM Local Deposition Method (LDM) and our own dose convolution method (WFBH). We sought to implement a patient specific background subtraction technique prior to dose calculation to make these noisy bremsstrahlung SPECT images suitable for post-TARE dosimetry calculations. Results: On average the percentage of mean background counts to maximum count in the image across all patients was 9.4 ± 4.9% with a maximum of 17.6% and minimum of 2.3%. Absolute dose increased and profile line width decreased as background subtraction value increased. The average value of the LDM and WFBH dose methods were statistically the same. As background subtraction value increased, we found the DVH curves to become unrealistic and distorted.Conclusion: Background subtraction on bremsstrahlung SPECT image had a large effect on post-TARE dosimetry. The background contour we defined provides a systematic estimate to the activity background that accounts for the scanner and patient conditions at the time of the image study and is easily implemented using commercially available software. We found using the mean count in the background contour as a constant subtraction across the entire image gave the most realistic dose distributions. Comparison of dosimetry from background subtracted SPECT images to image based dosimetry obtained via 90Y PET images will be the subject of our next analysis.