ESTRO 36 Abstract Book

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S770 ESTRO 36
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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
effects of applicable corrections, particularly for light
scattering and non-uniform responses between a film layer
and a panel, dose errors before and after correction were
visualized and quantified. Each effect by sensitometric
curves and correction methods on detection of dose errors
in gamma analysis was evaluated in a square field of 10
cm, a 45° wedged field, and an intensity-modulated field
for prostate cancer.
Results
Both sensitometry curves of the red and the green channel
could reach two times higher OD of 2.2 at 20 Gy and the
gap of OD was gradully more distinguishable over 4 Gy in
scanning with high contrast. The sensitometric curve of
green channel showed differentiated linearity in the dose
range under 2 Gy. The difference in the gradient of OD
brought out maximum 10% difference of gamma passing
rate in both wedged and intensity-modulated fields. The
primary positions of failed points in gamma analysis were
different according to sensitometric curves. When the
optimal sensitometric curve was applied, uniformity
correction caused maximum 8% difference in gamma
passing rate.