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Skin dose in longitudinal and transverse linac-MRIs using Monte Carlo and realistic 3D MRI field models
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10.1118/1.4754657
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    Affiliations:
    1 Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    2 Department of Physics, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    3 Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    4 Department of Physics, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    5 Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    6 Jack Ady Cancer Centre, 960 19th St. S., Lethbridge, Alberta T1J 1W5, Canada and Departments of Oncology and Physics and Astronomy, University of Calgary, Tom Baker Cancer Centre, Calgary, Alberta T2N 4N2, Canada
    7 Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    8 Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Departments of Oncology and Physics, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    a) Electronic mail: keyvanlo@ualberta.ca
    Med. Phys. 39, 6509 (2012); http://dx.doi.org/10.1118/1.4754657
/content/aapm/journal/medphys/39/10/10.1118/1.4754657
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/10/10.1118/1.4754657
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Partial sections of the three-dimensional linac-MRI geometries simulated with the finite element method. Both (a) parallel (longitudinal) and (b) perpendicular (transverse) configurations are illustrated, showing the biplanar magnet assembly, the superconducting coil, the treatment assembly, and the gantry support link.

Image of FIG. 2.
FIG. 2.

Partial section of the magnetic treatment assembly, including the electron gun casing, passive magnetic linac shielding, linac base, secondary collimator base, and MLC base. With reference to the Monte Carlo particle simulations, the dotted regions depict the approximate locations of (a) the electron gun and linac waveguide, (b) the primary collimator, flattening filter, and monitor chambers, (c) the movable secondary collimator jaws, and (d) the movable MLC leaves.

Image of FIG. 3.
FIG. 3.

(a) Schematic diagram of the longitudinal linac-MR system with the isocenter at 126 cm. (b) The CAX magnetic field maps of our realistic 3D models versus a 1D model (Ref. 15). The 1.0 T MRI of the 1D model extends only to the end of the magnet in Fig. 2 of Ref. 15.

Image of FIG. 4.
FIG. 4.

(a) The electron trajectories from the EGSnrc, GEANT4, and OPERA-3D are shown in a 3D plot, where the electron originates at (0,0,0). (b) The Euclidean distance between the tracks in (a).

Image of FIG. 5.
FIG. 5.

Energy fluence spectra of contaminant electrons for a 10 × 10 cm2 field scored below the linac head (MLC), below the air column, and at the phantom surface (21.5 cm air gap) with (a) no magnetic field, (b) 0.56 T superconducting (CCI) magnet, and (c) 1D (1/r 2) model (Ref. 15). These data were extracted from the central 5 × 5 cm2 region of the phase space. The energy fluences are normalized to the initial particle history in the Monte Carlo simulations.

Image of FIG. 6.
FIG. 6.

(a) First 2 cm of CAX PDD's for realistic 3D B-field models versus a 1D model (Ref. 15). (b)The magnification of the first 1 mm of (a). The data shown are for a 10 × 10 cm2 photon beam and with the phantom surface at 21.5 cm air gap.

Image of FIG. 7.
FIG. 7.

(a) Entry skin dose line profiles through the CAX for our longitudinal linac-MR system in the presence/absence of the realistic 3D B-field model. (b) Comparison of the entry skin dose of the longitudinal linac-MR system with the 0.56 T superconducting magnet to that without a magnetic field as a function of the field size (air gap of 21.5 cm). Lateral voxel sizes of 0.2 and 2 cm were used in (a) and (b) respectively.

Image of FIG. 8.
FIG. 8.

Comparison of the entry skin dose of the longitudinal linac-MR system with the 0.56 T superconducting magnet to that without a magnetic field as a function of the air gap (field size of 10 × 10 cm2). A lateral voxel size of 2 cm was used for all air gaps.

Image of FIG. 9.
FIG. 9.

(a) The CAX PDD of our transverse linac-MR system in the presence of the realistic 3D B-field model. (b) The magnification of the first 5 mm of (a) is displayed. The data shown are for a 10 × 10 cm2 photon beam and with the phantom surface at 136 cm from the linac source.

Image of FIG. 10.
FIG. 10.

(a) Entry skin dose line profiles through the CAX and (b) entry skin dose as a function of the field size for our transverse linac-MR system in the presence of the realistic 3D B-field model. The phantom surface is at 136 cm from the linac source.

Image of FIG. 11.
FIG. 11.

The CAX entry skin dose of the transverse linac-MR system as a function of the surface angle (nonperpendicular beams), for a 10 × 10 cm2 photon beam. The surface of a 20 cm thick phantom was placed at a 136 cm distance from the linac electron gun (i.e., isocenter at 10 cm depth).

Image of FIG. 12.
FIG. 12.

(a) Exit skin dose line profiles through the CAX and (b) exit skin dose as a function of the field size for our transverse linac-MR system in the presence of the realistic 3D B-field model. The phantom surface is at 136 cm from the linac source.

Image of FIG. 13.
FIG. 13.

The CAX exit skin dose of the transverse linac-MR system as a function of the surface angle (nonperpendicular beams), for a 10 × 10 cm2 photon beam. The surface of a 20 cm thick phantom was placed at a 136 cm distance from the linac target (i.e., isocenter at 10 cm depth).

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/content/aapm/journal/medphys/39/10/10.1118/1.4754657
2012-10-03
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Skin dose in longitudinal and transverse linac-MRIs using Monte Carlo and realistic 3D MRI field models
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/10/10.1118/1.4754657
10.1118/1.4754657
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