TOPAS: An innovative proton Monte Carlo platform for research and clinical applications
TOPAS application uses and extends the standard Geant4 simulation toolkit. The only element that the user needs to write is the user parameter file, a simple text file that controls the simulation. The user parameter file may in turn include additional parameter files that the user may write or may obtain from other users at their own institution, from colleagues at other institutions, or from hardware vendors.
By the include mechanism, the UserFile pulls in additional parameters defined in the OtherFile which in turn pulls in parameters defined in the DefaultFile. If the same parameter is in more than one file, the value from UserFile overrides the value from the OtherFile, the value from the OtherFile overrides the DefaultFile.
Multiple chains of parameter files. The UserFile pulls in parameters from patient, gantry and imager files. Values from the UserFile override values from the other files.
The treatment head for scanning beam delivery at MGH shown at four different times during a scan. The proton trajectories through the treatment head are shown along with the much shorter delta ray tracks.
UCSF proton beam line used for eye treatment as built in TOPAS. Shown are the exit window (X), wire chamber (WC), ion chambers (IC), rotating propeller (Prop), collimators (Coll), the position of the water column (H2O). The proton trajectories through the treatment head are shown along with the much shorter delta ray tracks.
The TOPAS parameter chain for UCSF eye treatment simulation. Default_BeamLine parameter file includes initial beam characteristics and all component description except rotating propellers, which are implemented in separate parameter files, i.e., Propeller_10, Propeller_15, Propeller_20, and Propeller_24. A user parameter file for SOBP simulation needs to include Default_BeamLine and one of those propeller implementations while a user parameter file Bragg peak simulation needs only Default_BeamLine.
Water tank expanding over time to facilitate measurement of Bragg peak. The thicknesses are 0.01, 1.0, 1.7, and 5.0 cm, respectively, and the water phantom expands at 5 mm/s in one direction.
Time versus average kinetic energy of primary protons at four positions along the beam path; PS1 at downstream exit window, PS2 in between the wire chamber and the first collimator, PS3 downstream of the propeller, PS4 at the isocenter. The propeller rotates once every 150 ms.
Energy spectra of primary protons at isocenter grouped according to time as described in the text, averaged over a 1 × 1 cm2 area (left). The partial SOBP for the full phase space from each time group is shown along with the summed SOBP for the 24 mm propeller (right). The dose was averaged over a 1 cm × 1 cm × 0.05 cm volume. The published measurement for this propeller is also shown (Ref. 48).
(Left) STAR Radiosurgery Beamline at MGH (proton beam enters from the left). (Right) SOBP as measured (circles) and simulated (histogram) for the STAR beamline in a water tank.
One of the IBA gantry treatment heads at MGH in double-scattering mode.
Spread out Bragg peak in water for three different range and modulation width options. The TOPAS simulation (squares) is compared with ion chamber measurement (triangles) from the MGH gantry treatment delivery systems in double-scattering mode. A total of 5 × 106 histories were simulated and the energy deposited in a 3 cm radius around the center of the beam was scored. The mean of the SOBP dose was normalized to unity. The SOBP region is shown in a zoomed view in the lower plots. From left to right, one of the best matches (a) and (b), an average match (c) and (d), and the worst case (e) and (f).
A compensator consisting of just a uniform half block of Lexan was placed in the beam path upstream of a water phantom.
Profile simulated with TOPAS and measured with an ion chamber. (Left) The simulated dose distribution in the water tank, the white arrow indicates the path of the measurements. (Right) Normalized profiles at Z = 9 cm.
Time dependence of X and Y dipole magnet fields, and triangular modulation of X field to compensate for patient motion (left). Number of particles simulated each time interval (right).
The fluence distribution at treatment system isocenter. (Left) Applying only the Field X and Field Y time features from Fig. 15. (Right) Applying also the Motion Field X time feature.
Comparison between XiO planned dose (left), TOPAS dose calculation (middle), and dose difference distribution (right) for one CT slice in the CTV for two patients, a head and neck (top) and a prostate patient (bottom). Shown are complete plans including all fields. Doses are given in Gy.
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