High resolution entry and exit Monte Carlo dose calculations from a linear accelerator 6 MV beam under the influence of transverse magnetic fields

• Published in: Medical physics (Lancaster) Vol. 36; no. 8; pp. 3549 - 3559
• Main Authors: Oborn, B. M., Metcalfe, P. E., Butson, M. J., Rosenfeld, A. B.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-08-2009
• Subjects: biological effects of fields; biological effects of gamma‐rays; biomedical MRI; dosimetry; Dosimetry/exposure assessment; Effects of ionizing radiation on biological systems; Field size; GEANT4; high resolution; Ionization chambers; Leptons; linear accelerators; Linear Models; Magnetic fields; Magnetic Resonance Imaging; Magnetics; Models, Biological; Monte Carlo; Monte Carlo Method; Monte Carlo methods; MRIgRT; MRI‐linac; phantoms; Phantoms, Imaging; Photons; Positron beams; Radiation Dosage; skin; Skin - radiation effects; skin and surface dose; Surface Properties; Water; MRI-linac; GEANT4; Monte Carlo; skin and surface dose; MRIgRT; high resolution
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3157203

A current concern with 6 MV transverse field MRI-linac hybrid systems is the predicted increases in skin dose (both the entry and exit sides) caused by the effects of the magnetic field on secondary electrons. In this work high resolution GEANT4 Monte Carlo simulations have been performed at the beam central axis in the entry and exit regions of a water phantom to predict surface (
0

μ
m
depth) and skin (
70

μ
m
depth) doses when placed in such a hybrid system. A
30
×
30
×
20

cm
3
water phantom with
10

μ
m
thick voxels has been simulated by being irradiated perpendicularly with a 6 MV photon beam (Varian 2100C) of sizes of
5
×
5
,
10
×
10
,
15
×
15
, and
20
×
20

cm
2
. Uniform transverse magnetic fields of 0.2, 0.75, 1.5, and 3 T with varying thickness above the phantom have been investigated. Simulations with and without lepton contamination have been performed. In the entry region the high resolution scoring has yielded unexpected surface and skin doses. There is a small amount of nonpurged air-generated lepton contamination that originates immediately above the phantom surface and delivers its dose over very short longitudinal distances in the entry region. At 0.2 T the surface and skin doses are not accurately predicted using lepton-contamination-free simulations and extrapolated lower resolution scoring. Lepton-free simulations are up to 7% of
D
max
lower than simulations with leptons. However, compared to 0 T, entry skin dose is reduced at 0.2 and 0.75 T but increases to 28%–31% of
D
max
at 3 T. For skin doses at the central axis in the exit region, high resolution scoring shows relative increases of 38%–106%, depending on the magnetic field strength and field size. These values are also up to 20% higher than lower resolution results. The shape of the exit dose profiles varies unpredictably and so extrapolation of low resolution data is insufficient. In order to achieve accurate Monte Carlo skin dosimetry in a transverse field MRI-linac system, the authors recommend using high resolution scoring. In systems of 0.2 T the inclusion of air-generated lepton contamination is also recommended.