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Surface Dose Measurements in An MR Image-Guided Radiation Therapy System Using OSLDs: Comparison with and Without a Static 0.35 T Magnetic Field

J Cammin*, A Price , B Cai , S Mutic , O Green , Washington University School of Medicine, St. Louis, MO


(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 7

Purpose: To compare surface dose measurements without and with the magnetic field in a 0.35T MR image-guided LINAC treatment unit using OSLD dosimeters.

Methods: OSLD dosimeters were placed on the surface of a 9cm solid water phantom. The phantom was placed in an MR image-guided LINAC (ViewRay, Inc.) at an SAD of 90cm to the OSLDs. Dose measurements were performed in a 9.96cm×9.96cm 6 MV flattening-filter-free photon beam without and with the 0.35T field. The angular dose dependence was measured on the surface with gantry angles of 0 degree (perpendicular to the surface), ±40, ±60, and ±80degrees. In addition, depth dose was measured at 10cm.

Results: The surface dose with magnetic field for a photon beam perpendicular to the surface (gantry at 0 degree) was reduced by 16% relative to the dose without magnetic field. The surface dose was enhanced by 26% for very oblique incidence (+80 deg) with magnetic field, but no enhancement was observed for oblique incidence at –80deg. Relative dose measurement, after normalizing to the dose at 0deg, show enhancement for all oblique incident angles compared to the situation without magnetic field. Relative dose measurements were also compared to previous results obtained on a MR image-guided Co60 treatment system. Dose enhancement and asymmetry for oblique incidence was less pronounced in the Co60 system. No difference was found for depth dose.

Conclusion: There is a significant difference for the surface dose with and without a magnetic field in certain radiation beam orientations due to the influence of secondary electrons from head-scatter and in the build-up region. The degree of changes depend also on beam quality. The effect was observed to be stronger in an MR-LINAC system compared to an MR-Co60 system. Surface-dose dosimetry in a magnetic field should take this asymmetry into account.


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