Purpose: To evaluate differences in fraction of particles shunting to the lungs and possible factors contributing to these differences across patients undergoing Tc-99m MAA mapping prior to Selective Internal Radiation Therapy (SIRT) with Y-90 microspheres. Patients and Methods: Retrospective analysis of data for 130 patients with hepatocellular carcinoma (HCC) or liver metastases, who underwent arterial 90Y radioembolization over a 6-year period at University of Washington Medical Center. All patients who received treatment had undergone prior arterial Tc-99m MAA mapping to predict distribution of radioembolization particles. Overall 141 Tc-99m MAA injections and 199 90Y treatments were performed in these 130 patients. Three patients were denied Y-90 treatment following Tc-99m MAA mapping due to abnormally high pulmonary shunt fraction. Pulmonary shunt fractions were compared between patients with HCC and those with metastatic disease to the liver. Further comparison of shunt fraction was made between patients who had had mapping of the entire liver, versus of a single lobe. The relationship between the pulmonary shunt fraction and the volume of liver injected for mapping was also evaluated. Results: Based on the retrospective analysis of data from 141 Tc-99m MAA liver mapping in 130 patients, unilobar mapping had been performed more commonly in the HCC group. Unilobar mapping prior to bi-lobar SIRT had also become more common over time during the 6-year period reviewed. This could be attributed to interventional radiologists gaining more confidence over time in the predictive value of unilobar mapping prior to bi-lobar treatment. Although qualitatively, the HCC and the non-HCC groups had similar distribution of shunt fractions, relatively large shunt fractions were slightly more common in the HCC group. In diffuse injections, most of the shunt fractions were concentrated between 0 and 5%, but with a long tail to the right. In unilobar injections, most were concentrated between 0 and 10%. There was a strong indication of a difference in pulmonary shunt fraction between the diffuse and unilobar groups, the latter being higher, but only in the subset of patients with HCC. Conclusions: The average shunt fraction is overall higher in HCC patients than in patients with hepatic metastases, likely due to underlying cirrhosis as well as intra-tumoral arteriovenous shunting. Contrary to expectations, shunt fractions tend to be higher when only one lobe is injected versus when the entire liver is being mapped.