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Measurement of Dose at Interfaces Using a Heterogeneous Phantom in a MR-Linac

S Ahmad*, M Paudel , A Sahgal , A Sarfehnia, B Keller , Sunnybrook Odette Cancer Centre, Toronto, Canada and University of Toronto, Department of Radiation Oncology, Toronto, Canada


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

Room: Exhibit Hall | Forum 7

Purpose: To measure the interface dose effects for various materials in an MR-linac (MRL) and to compare these measurements with calculated results from the treatment planning system.

Methods: A polystyrene slab phantom with dimensions of 29x30x19cm³ was used for the measurements on an Elekta MRL (Elekta AB, Stockholm, Sweden). Several EBT3 Gafchromic films (ISP, New Jersey, USA) were placed horizontally on the entrance and exit sides of a heterogeneity with one film placed horizontally at a depth of 2 cm for dose normalization. The inhomogeneities used were aluminum, stainless steel and bone placed at a depth of 6 cm from the surface. In a separate homogeneous polystyrene phantom, a vertically placed film was used to measure the central axis depth dose and the entrance and exit doses. The heterogeneous phantom was CT scanned and these images were imported into the Monaco treatment planning system (Elekta, research version 5.19.3) to model the dose using the Monte Carlo dose calculation algorithm GPUMCD. The energy, field size, and SSD were: 7.2 MV, 3x3cm², and 142.5 cm.

Results: In the absence of the magnetic field, the measurements performed at the polystyrene-bone, polystyrene-aluminum, and polystyrene-stainless steel entrance interfaces showed a 6.3%, 7.5 %, and 22 % increase respectively in dose compared to the homogeneous case at an upstream distance of 0.5 mm from the interface. The corresponding calculations, using the TPS, showed a 4.4%, 9.2%, and 24% increase respectively in dose at a 0.5 mm upstream distance away from the interface. The uncertainty associated with the film measurements was about 3 %.

Conclusion: Measurement of dose at the interfaces of various materials, in the absence of the magnetic field, showed agreement with the MRL treatment planning system within experimental uncertainty.


Radiation Dosimetry


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined dose measurement

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