Defining ablative radiotherapy in patients with hepatocellular carcinoma: An expert consensus.

485 Background: External beam radiotherapy (EBRT) is a highly effective treatment in select patients with hepatocellular carcinoma (HCC). However, there are many types of EBRT described in the literature with no formal definition of what constitutes “ablative.” Thus, we convened a group of international experts to provide consensus on the parameters that define ablative EBRT in HCC. Methods: A Steering Committee was convened to generate a series of Key Criteria (KC) that could be used to define ablative EBRT for HCC. KC were based on factors related to dose, fractionation, radiobiology, target identification, and delivery technique. An international panel of experts participated in a modified Delphi (mDelphi) process to independently answer questions for each KC. Respondents were given 30 days to respond in round 1 of the mDelphi and 14 days to respond in round 2. A threshold of ≥ 70% was used to define consensus for answers to each KC. Results: Invitations were sent to 40 individuals and 35 (88%) returned responses. In the first round of the mDelphi, 3 of 7 KC reached consensus. In the second round, 100% of participants returned responses and consensus was reached in 3 of the remaining 4 KC. Based on this mDelphi analysis, there was expert agreement that ablative EBRT for HCC should be defined as 1) a BED10 ≥ 80 Gy, 2) daily imaging and multi-phasic contrast used for target delineation should be used, 3) treatment breaks (e.g., for adaptive EBRT) are allowed, but the total treatment time should be ≤ 6 weeks, 4) the equivalent dose when treating with protons should use a conversion factor of 1.1, and 5) there is no single conversion factor for carbon ions. In one KC, which questioned the expert’s opinion on the α/β ratio of HCC, consensus was not achieved. Conclusions: Using an mDelphi method assessing expert opinion, we provide the first consensus definition of ablative EBRT for HCC. Empiric data are required to define the α/β of HCC.