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dependent quenching can be corrected by adjusting the
measured 3D optical density (OD) distribution using a
calibration model generated from a Monte Carlo (MC)
simulation and a calibration dosimeter.
Material and Methods
A radiochromic (leucomalachite green) silicone-based 3D
dosimeter was irradiated with three spatially separated,
unmodulated 60 MeV proton beams (Ø 10 mm collimator),
to plateau doses of 5, 10 and 20 Gy. Dosimeter read-out
was performed prior to and after irradiation, in a Vista 10
optical CT scanner with 0.25 mm
3
voxel resolution, thus
obtaining the change in OD caused by the radiation. MC
simulation of 10
8
protons was performed in SHIELDHIT-
12A, giving the dose and dose-averaged LET (dLET)
distributions. A calibration model with respect to dose and
dLET was generated from the 5 and 20 Gy Bragg peaks:
Linear fits to OD as a function of dose was computed for
all voxels within limited ranges in dLET. Only voxels within
the central part of the beam (Ø 5 mm) with doses above
1% of the maximum dose were used for the model. The
slopes and intercepts from the linear fits were smoothed
as a function of dLET (Fig. 1). Voxels in the 10 Gy Bragg
peak were calibrated using these variables and compared
to the dose distribution from the MC simulation (Fig. 2).
Results
Quenching results in a lower dose response in the Bragg
peak of the proton beam (0.013 cm
-1
Gy
-1
) compared to the
low-LET regions, such as the plateau (0.027 cm
-1
Gy
-1
at
2.94 keV/µ m), see Fig. 1. The dose response increased to
a sharp peak for dLET-values within the plateau region.
The calibrated optical measurement was close to identical
to the MC simulated dose in the central part of the beam.
Less good results were obtained towards the edges of the
beam (Fig. 2), which corresponded to areas where the MC
beam model was suboptimal. Dose errors larger than 2 %
of maximum dose (1.1 Gy) was found in 35 % of all
calibrated voxels, while 3.5 % had dose errors larger than
5 % of maximum dose (2.7 Gy).
Conclusion
We present the first LET-corrected 3D dose m easurements
in a radiochromic dosimeter for proton thera py, showing
that verification for single fields has been made possible.
Acknowledgements: PL-Grid
OC-0063 Energy resolution and range reproducibility
of a dedicated phantom for proton PBS daily QA
L. Placidi
1
, J. Hrbacek
1
, M. Togno
2
, D.C. Weber
3
, A.J.
Lomax
1
1
Paul Scherrer Institute PSI, Medical physics, Villigen PSI,
Switzerland
2
IBA, Dosimetry GmbH, Schwarzenbruck, Germany
3
Paul Scherrer Institute PSI, Radiation Oncology, Villigen
PSI, Switzerland
Purpose or Objective
Wedge phantoms coupled with a CCD camera (Fig.1) were
suggested as a simple mean to verify range consistency or
reproducibility for a specific energy. The method is based
on analysing of integral image created by a spot pattern
that passed through a wedge. We have investigated the
reproducibility and dependence of this method on setup
errors (shift and tilt) for a commercially available
phantom (Sphinx, IBA Dosimetry) and CCD camera (Lynx,