ESTRO 36 Abstract Book

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S761 ESTRO 36 2017
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to optimize the dose sensitivity and the Signal-to-Noise
Ratio. In particular, geometrical distortion, image
homogeneity, artifacts, image texture, dose accuracy and
resolution, limit of detectability (LOD) and quantification
(LOQ), Fe3+ spatial diffusion (Fricke-gels) and dose rate
dependence were evaluated. Finally, a pre-treatment
dosimetry of a SBRT plan was acquired and a relative
planar profiles comparison with a standard dosimeter
(Gafchromic EBT2) was performed. Ad hoc Matlab codes
were developed for images analysis.
Results
The chemical composition, MRI acquisition and
reconstruction parameters were optimized for each gel
system. No image correction maps were needed, since
geometrical distortion, artifacts and inhomogeneity were
always negligible, and no dependence on photon beam
dose rate was observed. 3D spatial resolution (voxel
dimension) was 1x1x3mm3. Dose accuracy was under 4% in
the range 18-25Gy, but worst for lower doses. Dose
resolution was about 1Gy, while LOD was less than 0.5Gy.
Differences between gel systems and Gafchromic profiles’
FWHMs were in the range 0,5mm – 5,5mm, mean dose
deviations in flat region were always around 2%, while
penumbra differences were about 2mm. Negligible
diffusion and time effects were observed up to 3 hours
from irradiation for all gel systems.
Conclusion
This study showed that both Fricke/Polymer- gel
dosimeter could be a suitable tool to perform pre-
treatment QA, with particular focus on SBRT and SRS
treatments, thanks to their optimal spatial resolution,
their practicability and their capability to perform 3D
dosimetry. Further studies are ongoing to standardize a
protocol to perform 3D pre-treatment dosimetry.
EP-1443 Measurement of 3D dose distributions from
an MR Linac with gel dosimetry
Y. Roed1,2, L. Pinsky1, G. Ibbott2
1 University of Houston, Physics, Houston, USA
2 The University of Texas MD Anderson Cancer Center,
Radiation Physics, Houston, USA
Purpose or Objective
To demonstrate the potential value of polymer gels to
measure 3D dose distributions delivered with an MR-image
guided radiotherapy delivery machine.
Material and Methods
Polymer gels were obtained from MGS Research Inc
(Madison, CT) in custom-designed glass cylinders of 4 cm
height and 5 cm diameter. Irradiations were delivered
with a non-clinical MR-linac pilot system (MR-Linac, Elekta
AB, Stockholm) that combined a 1.5 T MR scanner with a
7 MV linac. Two dosimeters were positioned separately in
a phantom with their midplanes at isocenter distance. A
total of 750 MU (~5 Gy) was delivered with 3x3 cm² fields
at three gantry positions. The gantry was positioned at 0 ⁰,
90⁰, and 180 ⁰ for the first irradiation and at 0⁰, 270⁰, and
180⁰ for the second irradiation. All four cardinal angles
weren’t feasible due an asymmetric phantom design.
MR images across the entire volume of the dosimeter were
acquired with a 3T GE scanner using a 2D spin echo
sequence (TR = 1000 ms, TE = 10, 20, 60, 100 ms) 24h after
irradiation. Spin-spin relaxation rate (R2) maps were
generated. Both field size and penumbra widths were
calculated on the central slice. R2 maps were
concatenated into a 3D matrix.
The experiment was performed while the magnet of the
MR component (B-field) was turned off and will be
repeated once the B- field is turned back on.
Results
The small fields were captured and resolved within each
dosimeter. The field width measured along the central
cross-plane R2 profile from each dosimeter was 28 mm and
29 mm, respectively. The penumbra widths were 5 mm at
both field edges in each dosimeter. The 3D R2 matrix
visualized the irradiated volume of the dosimeter well.
In order to study the influence of the B-field on the dose
distribution in 3D, the results in the presence and absence
of the MR component (B-field) will be compared and
presented.
Conclusion
Polymer gels offer an excellent means to measure 3D
relative dose distributions delivered with an MR-Linac in a
clinically relevant fashion.
Previous experiments with polymer gels have already
shown that steep dose gradients could be measured when
irradiated with an MR-Linac. The current study encourages
further study of polymer gels for measuring 3D dose
distributions in the presence of B-fields.
EP-1444 Reliable error detection in radiochromic film
dosimetry with optimal density curves and corrections
H. Park1, Y. Bae2, J. Park3, M. Kim1, Y. Oh 1, M. Chun1, O.
Noh 1, O. Cho1, J. Lee 2
1Ajou University Medical Center, Dept. of Radiation
Oncology, Suwon, Korea Republic of
2Konkuk University Medical Center, Convergent
Medical Physics and Dept. of Radiation Oncology, Seoul,
Korea Republic of
3University of Florida, Dept. of Radiation Oncology,
Gainesville, USA
Purpose or Objective
To minimize variation of dosimetric errors caused by
correction methods and to suggest optimal conditions in
gafchromic film dosimetry using a flatbed scanner,
feasible scanning and post analysis procedures were
investigated with impacts on error detection in gamma
analysis.
Material and Methods
When a rectangular piece (5 × 4 cm) of EBT3 film was
placed at a 5 cm depth of the water-equivalent solid water
phantom, doses were delivered to film pieces from 0 cGy
to 20 Gy with every 50 cGy under 500 cGy and 100 cGy
over 500 cGy. To find an optimal sensitometric curve
having a large range of optical density (OD) and linearity
in doses of interest, a set of exposed films was scanned in
a flatbed scanner with different conditions by adjusting
brightness, contrast, and highlight from -50 to 50.
Sensitometric curves of a red and a green channel were
obtained with each scanning condition and used to
compare gamma distributions. In addition, to clarify the