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Polymer Gels as 3D QA Devices in Magnetic Resonance-Guided Radiation Therapy

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Y Roed

Y Roed1,2*, M Kadbi3 , J Wang2 , L Pinsky1 , G Ibbott2 , (1) University of Houston, Houston, Texas, (2) UT MD Anderson Cancer Center, Houston, TX, (3) MR Therapy, Philips healthTech, Cleveland, OH


SU-G-JeP2-11 (Sunday, July 31, 2016) 4:30 PM - 5:00 PM Room: ePoster Theater

To demonstrate the potential of polymer gels to serve as 3D QA devices in MR-guided radiotherapy.

Custom-designed polymer gel dosimeters (MGS Research, Madison,CT) were centered on the penumbras of a radiation field, parallel to the magnetic field lines inside a 1.5 T MR scanner integrated with a 7 MV linac (MR-Linac). 15 Gy were delivered perpendicular to the magnetic field. Coronal MR images were acquired with a 2D balanced-Fast Field Echo (b-FFE) sequence in real-time during irradiation and with a 2D spin echo sequence at different times after irradiation.
Signal intensities (SI) were measured inside and outside the radiation field on b-FFE images obtained during irradiation. Spin-spin relaxation rate (R2) maps were calculated from images after irradiation and penumbra widths were calculated from these maps.

A difference in SI between areas of the dosimeter inside and outside of the radiation field could be measured immediately after the beam was turned on. The difference in SI increased throughout the duration of the exposure. R2 values from immediately after irradiation were 83% of those measured 24h post-irradiation. At 1h after irradiation, the R2 values were 87% of those at 24h indicating that polymerization of the gel was still ongoing. At 24h post-irradiation the width of the 80/20 penumbra on the left field edge measured 6.5 mm and on the right 5.6 mm.

The MR-Linac allows for MR imaging during treatment and measurements of 3D dose distributions with steep dose gradients without having to transfer dosimeters for read-out. The results showed promise for polymer gels as relative 3D QA devices to verify radiation treatment plans delivered by an MR-Linac. To our knowledge, this was the first time that polymer gels were used for real-time image acquisition. Further investigations into the application of polymer gel dosimetry on MR-Linac are ongoing.

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