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Chamber Volume Effect On Absolute Dosimetry in A Magnetic Field

O Green

O Green1*, S Goddu1 , H Li1 , S Mutic1 , I Kawrakow2 , (1) Washington University School of Medicine, St. Louis, MO, (2) ViewRay, Inc., Oakwood, OH


TH-CD-304-7 (Thursday, July 16, 2015) 10:00 AM - 12:00 PM Room: 304

Purpose: To evaluate the effect of ionization chamber volume size and orientation on dosimetric accuracy in the presence of a magnetic field.
Methods: The magnetic resonance image-guided radiotherapy (MR-IGRT) system (ViewRay, Inc., Oakwood, OH) was placed into clinical use after extensive calibration measurements with ionization chambers and verification tests using thermo- and opti-luminescent dosimeters. To further investigate the effect of the magnetic field on the ionization chamber results, measurements were made using four orientations and two different chambers with significantly different volumes: the Standard Imaging (Middleton, WI) A18 and the PTW N30013 Farmer chamber, with collecting volumes of 0.123 and 0.6 cc, respectively. High-resolution computed tomography scans were obtained of the chambers to ensure accurate modeling in the Monte Carlo (MC) calculations with EGSnrc. The four different orientations of the chambers relative to the magnetic field direction were achieved by having the chambers parallel to the magnetic field (pointing into the bore) and perpendicular to the magnetic field (pointing down), using a 10.5x10.5 cm² open field incident on the chambers from either 90 or 270 degrees (IEC).
Results: MC calculations and measured data showed dependence on both orientation and volume in the presence of magnetic field. The greatest differences for both chambers were at 270 degrees pointing down, with A18 at 3.2% and Farmer at 4.4% as compared to the same setup with no magnetic field. The difference to zero magnetic field is minimized when the chamber axis is oriented parallel to the field. The measured and simulated chamber doses showed reasonable agreement. The MC simulations demonstrated a strong dependence of results when small air gaps between the chamber and build-up cup are present in the perpendicular orientation.
Conclusion: Accurate absolute dosimetry is possible in presence of low-strength magnetic fields, but care must be taken in chamber selection and orientation.

Funding Support, Disclosures, and Conflict of Interest: Drs. Green & Mutic served as non-compensated consultants for ViewRay, Inc., and have received honoraria and travel reimbursement.

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