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Investigation of 1.5T Magnetic Field Dose Effects On Organs of Different Density

H Lee

H Lee*, A Rubinstein , G Ibbott , UT MD Anderson Cancer Center, Houston, TX


SU-E-J-203 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: For the combined 1.5T/6MV MRI-linac system, the perpendicular magnetic field to the radiation beam results in altered radiation dose distributions. This Monte Carlo study investigates the change in dose at interfaces for common organs neighboring soft tissue.

Methods: MCNP6 was used to simulate the effects of a 1.5T magnetic field when irradiating tissues with a 6 MV beam. The geometries used in this study were not necessarily anatomically representative in size in order to directly compare quantitative dose effects for each tissue at the same depths. For this purpose, a 512 cm³ cubic material was positioned at the center of a 2744 cm³ cubic soft tissue material phantom. The following tissue materials and their densities were used in this study: lung (0.296 g/cm³), fat (0.95), spinal cord (1.038), soft tissue (1.04), muscle (1.05), eye (1.076), trabecular bone (1.40), and cortical bone (1.85).

Results: The addition of a 1.5T magnetic field caused dose changes of +46.5%, +2.4%, -0.9%, -0.8%, -1.5%, -6.5%, and -8.8% at the entrance interface between soft tissue and lung, fat, spinal cord, muscle, eye, trabecular bone, and cortical bone tissues respectively. Dose changes of -39.4%, -4.1%, -0.8%, -0.8%, +0.5%, +6.7%, and +10.9% were observed at the second interface between the same tissues respectively and soft tissue. On average, the build-up distance was reduced by 0.6 cm, and a dose increase of 62.7% was observed at the exit interface between soft tissue and air of the entire phantom.

Conclusion: The greatest changes in dose were observed at interfaces containing lung and bone tissues. Due to the prevalence and proximity of bony anatomy to soft tissues throughout the human body, these results encourage further examination of these tissues with anatomically representative geometries using multiple beam configurations for safe treatment using the MRI-linac system.

Funding Support, Disclosures, and Conflict of Interest: NSF GRFP Grant Award #LH-102SPS

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