ESTRO 36 Abstract Book

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S412 ESTRO 36 2017
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Magnetic Resonance Image (MRI) has the potential to
increase the accuracy and effectiveness of proton
therapy. Previous studies on that topic demonstrated that
corrections in dose calculation algorithms are strictly
required to account for the dosimetric effects induced by
external magnetic fields. So far, a real dose calculation
possibility including a trajectory corrected approach was
missing. In this study, we developed a pencil beam
algorithm (PBA) for dose calculation of a proton beam in
magnetic fields.
Material and Methods
MC simulations using the GATE 7.1 toolkit were performed
to generate first benchmarking data and subsequent
validation data for the PBA. The PBA was based on the
theory of fluence weighted elemental kernels. A novel and
non-symmetric exponential tailed Gauss fitting function
was used to describe the lateral energy deposition profiles
in water. Nuclear corrections, multiple scattering and
charged particle drifting were accounted by means of a
look-up table (LUT) approach. Longitudinal dose
depositions were estimated from the LUT and corrected
using a water-equivalent depth scaling. In a first step
proton beams in the clinical required energy range 60 –
250 MeV with transverse external magnetic fields ranging
from 0 – 3T were analyzed in a 40x40x40 cm3 water
phantom. Next validation simulations were performed for
different phantom configurations, e.g. using a simple
water box or slab-like geometries with inhomogeneities of
different materials and volumes. Percentage depth dose
curves (PDD) and two-dimensional dose distributions were
calculated to assess the performance of the PBA.
Results
For PDD in water discrepancies between the PBA and MC
of less than 1.5% were observed for all the depth values
before the Bragg-Peak (see Figure 1). An increasing value
of up to 6% was found in the distal energy falloff region,
where dose values represents around 1% of the maximum
dose deposition. In all cases, maximum range deviations
of the results were less than 0.2 mm. Deviations between
two dimensional dose maps obtained with PBA and GATE
remained below 1% for almost all the proton beam
trajectory, reaching a maximum value up to 4% in the
Bragg-Peak region, see Fig. 2. As expected, agreement
became worse for high energy protons and high intensity
magnetic fields.
Fig. 1. PDD curves comparing the PB algorithm with MC
simulations for proton beams in water. Relative
discrepancies are shown in the top region of the graph.
Fig. 2 Relative dose difference map for a 240 MeV proton
beam in water exposed to a 3T transverse field.
Conclusion
The proposed pencil beam algorithm for protons can
accurately account for dose distortion effects induced by
external magnetic fields. Corrections of dose distributions
using an analytical model allows to reduce dose
calculation times considerably, making the presented PBA
a suitable candidate for integration in a treatment
planning system. The current work demonstrates that
proton MRI is feasible from a dosimetric point of view.
PO-0786 Energy dependence investigation for
detectors used in out-of-filed dosimetry
L. Shields1, L. Leon-Vintro2, B. Mc Clean3
1St Luke's Hospital, Medical Physics, Dublin, Ireland
2University College Dublin, Schoool of Physics, Dublin,
Ireland
3St. Luke's Radiation Oncology Network, Medical
Physsics, Dublin, Ireland
Purpose or Objective
Traditionally, energy dependence of a range of detectors
used in radiotherapy has been investigated mainly in the
Cobalt-60 and 6-15MV photon range. However, when
considering detectors for use in out-of-field dosimetry, it
is more important that the energy dependence is
investigated over a much lower range. This study
examined (i) the mean incident energy of radiation out-
of-field for a 6MV photon beam and (ii) the energy
dependence of a range of clinically available detectors to
the typical energies experienced out-of-field and (iii)
Monte Carlo (MC) calculated and detector measured out-
of-field dose profiles.
Material and Methods
An Elekta Synergy Linac operating at 6MV and a water
phantom at 90cm SSD was defined in BEAMnrc. Phase
spaces were scored at 6 different planes in the water
phantom - 0.2, 1.4 (dmax), 5, 10, 15 and 20cm. Each phase
space file was analysed using the EGSnrc program package
BEAMDP to extract energy spectra from each of the phase
space files to examine the change in energy spectra with
increasing distance from the field edge and depth in the
phantom.
The energy dependence of each of the detectors was
examined using 70, 100, 125 and 200 kV beams on a
Gulmay D3225 Orthovoltage Unit and a 6MV Elekta Synergy
beam. The kV energies lied within the range of energies
which were found to be dominant out-of-field in a 6MV
beam. A dose of 1 Gy was delivered to each detector as
determined by their respective calibration protocols, and
the signal was recorded for all energies.
In-plane and cross-plane profiles were measured by each
detector and compared to MC calculated.