Program Information
Evaluation of Detectors for Relative Dosimetry Measurements in the Presence of a 1.5 T Magnetic Field From An MR-Linac
D. J. O'Brien1*, J. Dolan2, S. Pencea2, N. Schupp2, G. O. Sawakuchi1, (1) The University of Texas MD Anderson Cancer Center, Houston, TX, (2) Elekta A. B., Maryland Heights, MO
Presentations
TU-FG-205-7 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: 205
Purpose: To assess the behavior of commercial dosimeters for relative dosimetry in a magnetic (B-) field from an MR-linac.
Methods: An MR-compatible 3D-scanning water phantom was used with a 7 MV 1.5 T Elekta MR-Linac. Percentage Depth Dose (PDD) curves, lateral profiles and output factors (OFs) for 2x2 to 22x22 cm² fields at 133.5 cm SSD were measured with PTW detectors including microDiamond, Diode P, Diode SRS, 30013 Farmer, Semiflex 3D, and PinPoint 3D. Measurements were performed with and without the B-field. The detectors were positioned using the onboard MV imaging system. Due to limited space, measurement depths were restricted to between 0 and 10 cm. Lateral profiles and OFs were measured at a depth of 10 cm.
Results: In the B-field, the upstream offset of the effective point of measurement (EPOM) of each ionization chamber was found to be approximately half its normal value. We also found a lateral shift to the EPOM which manifested as shifts in the measured position of the lateral profile. The largest was reported by the Farmer chamber (2.4 mm) and the smallest by the shielded diode (0.6 mm). Lateral profiles measured with the shielded diode underestimated the asymmetry of the lateral profiles. Measured PDDs for each detector agreed within 0.5% without a B-field. With a B-field, similar agreement was found for the 10x10 cm² field but the differences increased up to 2% as the field size decreased. OFs for each detector in the B-field were within 1% of the mean value for all field sizes studied.
Conclusion: The B-field alters the effective point of measurement of ionization chambers in the beam direction. It also introduces a lateral shift from the point of measurement to the EPOM. Shielded diodes underestimated the asymmetry and lateral shift of the lateral profiles in the B-field.
Funding Support, Disclosures, and Conflict of Interest: This work was partly supported by Elekta. J. Dolan, S. Pencea and N. Schupp are Elekta employees.
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