SU‐F‐J‐148: A Collapsed Cone Algorithm Can Be Used for Quality Assurance for Monaco Treatment Plans for the MR‐Linac

Abstract

Purpose:Treatment plans for the MR‐linac, calculated in Monaco v5.19, include direct simulation of the effects of the 1.5T B0‐field. We tested the feasibility of using a collapsed‐cone (CC) algorithm in Oncentra, which does not account for effects of the B0‐field, as a fast online, independent 3D check of dose calculations.Methods:Treatment plans for six patients were generated in Monaco with a 6 MV FFF beam and the B0‐field. All plans were recalculated with a CC model of the same beam. Plans for the same patients were also generated in Monaco without the B0‐field. The mean dose (Dmean) and doses to 10% (D10%) and 90% (D90%) of the volume were determined, as percentages of the prescribed dose, for target volumes and OARs in each calculated dose distribution. Student's t‐tests between paired parameters from Monaco plans and corresponding CC calculations were performed.Results:Figure 1 shows an example of the difference between dose distributions calculated in Monaco, with the B0‐field, and the CC algorithm. Figure 2 shows distributions of (absolute) difference between parameters for Monaco plans, with the B0‐field, and CC calculations. The Dmean and D90% values for the CTVs and PTVs were significantly different, but differences in dose distributions arose predominantly at the edges of the target volumes. Inclusion of the B0‐field had little effect on agreement of the Dmean values, as illustrated by Figure 3, nor on agreement of the D10% and D90% values.Conclusion:Dose distributions recalculated with a CC algorithm show good agreement with those calculated with Monaco, for plans both with and without the B0‐field, indicating that the CC algorithm could be used to check online treatment planning for the MRlinac. Agreement for a wider range of treatment sites, and the feasibility of using the γ‐test as a simple pass/fail criterion, will be investigated.