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Minimal Skin Dose Increase in Longitudinal Rotating Biplanar Linac-MR Systems: Examination of Radiation Energy and Flattening Filter Design

B Fallone

B Fallone*, A Keyvanloo, B Burke , J St. Aubin , D Baillie , K Wachowicz , B Warkentin , S Steciw , Cross Cancer Institute, Edmonton, AB


SU-G-JeP2-9 (Sunday, July 31, 2016) 4:30 PM - 5:00 PM Room: ePoster Theater

To quantify increase in entrance skin-dose due to magnetic fields of the Alberta longitudinal linac-MR by examining the effect of radiation energy and flattening filter, using Monte Carlo calculations and accurate 3-D models of the magnetic field.

The 3-D magnetic fields generated by the bi-planar Linac-MR are calculated with FEM using Opera-3D. BEAMnrc simulates the particle phase-space in the presence of the rapidly decaying fringe field of 0.5T MRI assembled with a Varian 600C linac with an isocentre distance of 130 cm for 6 MV and 10 MV beams. Skin doses are calculated at an average depth of 70 μm using DOSXYZnrc with varying SSDs and field sizes. Furthermore, flattening filters are reshaped to compensate for the significant drop in dose rate due to increased SAD of 130 cm and skin-doses are evaluated.

The confinement effect of the MRI fringe field on the contaminant electrons is minimal. For SSDs of 100 – 120 cm the increase in skin dose is ~6% – 19% and ~1% – 9% for the 6 and 10 MV beams, respectively. For 6MV, skin dose increases from ~10.5% to 1.5%. for field-size increases of 5x5 cm2 to 20x20 cm2. For 10 MV, skin dose increases by ~6% for a 5x5 cm2 field, and decreases by ~1.5% for a 20x20 cm2 field. The reshaped flattening filter increases the dose rate from 355 MU/min to 529 MU/min (6 MV) or 604 MU/min (10 MV), while the skin-dose increases by only an additional ~2.6% (all percent increases in skin dose are relative to Dmax).

There is minimal increase in the entrance skin dose and minimal/no decrease in the dose rate of the Alberta longitudinal linac-MR system. There is even lower skin-dose increase at 10 MV.

Funding Support, Disclosures, and Conflict of Interest: Funding: Alberta Innovates - Health Solutions (AIHS) Conflict of Interest: Fallone is a co-founder and CEO of MagnetTx Oncology Solutions (under discussions to license Alberta bi-planar linac MR for commercialization)

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