Room: Track 2
Electron-return (ERE) and electron-streaming effects (ESE) are radiation transport phenomena unique to MR-guided radiotherapy. Limited information exists describing modeling accuracy of these effects in commercial treatment planning systems (TPS). This work evaluates the Monaco TPS’s capabilities to simulate the ERE and ESE.
For ERE measurements, 14 cm of 40x40 cm² solid water (SW) slabs were stacked and centered at machine isocenter. GAFchromic films placed at 0, 0.5, and 1 cm depth from the top surface were irradiated with 2x2, 5x5, and 10x10 cm² posterior-anterior (PA) beams. For ESE measurements, a cylindrical SW phantom is positioned at isocenter with the long axis of the phantom perpendicular to the bore axis. A 5 cm thick SW slab was placed inferior to the cylindrical phantom such that it was outside the radiation field. PA beams of 5x5 and 10x10 cm² irradiated the cylindrical phantom to produce electrons streams which impacted the film mounted on the out-of-field phantom surface. Film calibration and dosimetry was performed in the Elekta Unity’s magnetic field. Corresponding Monaco simulations of the phantom set-up were performed, and dose profiles were extracted and compared with the film measurements.
ERE and ESE dose profile shapes between film and Monaco simulation were in good agreement. The 5x5 cm² results are presented and are representative of the other field sizes. The DTA pass rate was at worst 93% for all depths for the ERE and 29% for the ESE. Monaco dose was at most 5% low for all ERE depth measurements and 29% less than the film for the ESE measurement.
Monaco can simulate the ERE within experimental and simulation uncertainty. However, ESE results differed notably from experimental dose magnitudes. This work provides insight into the limitations and uncertainties associated with in and out-of-field dose calculations in Monaco.