Another study by the group from New York Proton Center (ProBeam) assessed the feasibility of FLASH RT using single-energy transmission PBS plans of 4.5 Gy × 15 fractions for hepatocellular carcinoma patients. Using between 2 and 5 fields, the minimum MU/spot between 100 and 400, and minimum spot time (MST) of 0.5ms and 2 ms, for the FLASH threshold of V40Gy/s, this study found that hypofractionation with smaller fractional doses (4.5 Gy/fraction) is challenging for FLASH planning. Using fewer fields can allow higher minimum MU/spot, resulting in higher OAR FLASH dose rate coverages while achieving similar plan quality compared to plans with more fields.