ESTRO 36 Abstract Book

S416

ESTRO 36

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detectors. A rotational symmetric Gaussian horizontal

beam profile and exponential decaying depth dose profile

in the vicinity of the pellet was fitted to the measured

profiles. Both 6 MV and 6 MV FFF beams were considered.

Results

The fit of the beam profile in three dimensions was based

on two parameters: the variance for the Gaussian profile

and the gradient of the depth profile. The parameters in

turn were both changing as function of FS. Using the fitted

beam profiles, an analytical model was developed for the

calculation of volume correction factors k

for given FS

(see Table 1).

Table 1: Calculated volume correction factors k

temperature and volume corrected output factors (OF)

with SD being one standard deviation (SD) are displayed

for the 6 MV and 6 MV FFF beams as function of the field

size FS.

Conclusion

Volume averaging was found to influence the alanine

measurements by up to 6 % for the smallest field size. For

a cylindrical detector irradiated along the symmetry axis

of the detector, simple analytical expressions of the

volume correction factors were obtained. The analytical

expression gives valuable insight in the volume correction

factor k

as function of field size and the radius of the

sensitive volume of the detector. The method presented

here would be applicable for other detectors. With a

defined geometry of the sensitive volume of the detector

relative to the central axis of the beam the volume

correction factor can either be calculated analytically or

numerically as function of FS.

Poster: Physics track: Dose measurement and dose

calculation

PO-0785 A pencil beam algorithm for protons including

magnetic fields effects

F. Padilla

, H. Fuchs

, D. Georg

Medizinische Universität Wien Medical University of

Vienna, Department of Radiation Oncology, Vienna,

Austria

Purpose or Objective

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 cm

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