(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To use optically stimulated luminescence dosimeters (OSLDs) to measure skin dose near intracranial tumors and to compare the skin doses between a 1.5T MR-Linac and a conventional linac. The hypothesis is that the MR-Linac skin doses will be higher than for the conventional linac as the magnetic field elicits a high exit surface dose, called the electron return effect.
Methods: Six patients with intracranial tumors were treated on our MR-Linac. These patients had prescription doses of 54 Gy or 60 Gy in 30 fractions. Due to a preventative maintenance day, these patients were treated on conventional linacs for a single fraction with a 6MV beam. For the single fraction on conventional linacs, and for one fraction on the MR-Linac, an OSLD was placed on each patient’s skin located near the PTV. The conventional linac treatments were planned on a convolution-superposition treatment planning system (TPS) and the MR-Linac treatments were planned using a separate Monte Carlo TPS. A dose point located at the OSLD and at the water equivalent depth of the OSLD (0.6 mm) was created in each of the TPS’s, and then compared with the OSLD measurement.
Results: For 5 out of 6 patients, the MR-Linac OSLD dose was higher than for the conventional linac (11.5%-24.8% higher). The one patient where the MR-Linac dose was lower (-7.5%) had four non-coplanar beams for the conventional linac plan, which might explain the outlier (MR-Linac plans use only coplanar beams). There was better agreement between the MR-Linac measurements and Monte Carlo TPS (RMSE=7.3%) than between the convolution TPS and the conventional linac OSLD doses (RMSE=17.4%).
Conclusion: MR-Linac patient skin dose measurements are higher than in an equivalent plan on a conventional linac. Monte Carlo dose calculation agrees closer to the OSLD measurements than that of the convolution TPS.
Keywords
Surface Dose, MRI, Optical Dosimetry
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