Benchmarking the mARC performance – treatment time and dosimetric linearity

Abstract

PurposeThe mARC technique is a hybrid rotational IMRT modality operating in “burst mode”. While it is generally assumed that it will be slower than VMAT, the real limits of operation have not been defined so far. We here present the first systematic study of the technical limits on mARC treatment. MethodsThe following scenarios are considered: 18, 30, 36 or 45 arclets per rotation (spacing between 20° and 8°), flat and flattening-filter-free (FFF) energy, arclet width 4° or 2°, from 1 MU/arclet to 1000 MU/plan. All scenarios are irradiated, treatment times are measured and treatment parameters reported. Dose linearity was assessed by point dose measurements of the 18 arclet plans with 1-30 MU per arclet. ResultsMinimum treatment times (no MLC movement, few MUs) depend strongly on the number of arclets per rotation (1minute for 18 arclets to 1:50min for 45 arclets), and rise linearly with MU/arclets after a given cut-off value depending on scenario, arclet width and available maximum dose rate. MLC movement adds up to 2minutes of treatment time, but generally less (ca. 45seconds in realistic plans).The rules by which irradiation parameters are selected by the firmware can be partly discovered. The choice of dose rate is most clearly defined. For the flat 6 MV energy, the highest available dose rate (300 MU/min) is always applied. For FFF 7 MV dose rate is reduced for arclets with few MUs, so that an arclet is irradiated in no less than 0.3 s. Only for the case of 1 MU/arclet can this constraint not be met (the technical limit on the dose rate if 500 MU/min for FFF 7 MV). In this case, dosimetric linearity is reduced. In all other instances, deviations from linearity at low MU remain below 2%. ConclusionsTreatment times of down to 90seconds are technically achievable for treatment with FFF beams using up to 36 arclets per rotation (arclet spacing every 10°) for up to 900 MU/plan, comparable to VMAT treatment times. The values provided here are meant to serve as a reference for the design of mARC plans (choice of arclets spacing etc.) and as minimum times against which the performance of different treatment planning systems can be evaluated.