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S115
ESTRO 36 2017
_______________________________________________________________________________________________
Purpose or Objective
Recently the 0.35T CO
60
MRIdian system (Viewray Inc.,
Cleveland) is clinically implemented at our institution.
IMRT online adaptive patient treatment is daily routine.
Quality assurance (QA) on dose delivery (both absolute and
position), and end to end test (per fraction and overall)
are prerequisites for safety and quality. Hereto, a reliable
dosimeter is required. EBT GafChromic film might
appropriate. However, a magnetic field can cause
perturbations to absolute and relative dosimetry
measurements. In addition, the amount of radiation
induced dipole polymers might be influenced by the
orientation of the film in the B
0
-field, i.e. an angular
dependency in absolute dose response might be present.
The purpose of this study is to investigate suitability of
EBT3 GafChromic film for MRIdian QA purposes.
Material and Methods
GafChromic film sheets were irradiated in a water
phantom using the MRIdian and a Linac (Varian 2300C/D,
6MV) as comparison system. The film sheets are placed in
a rotatable device positioned in the water phantom such
that the B
0
field is frontal or sagittal to the film surface in
initial position (figure 1). The film was rotated over 7
angles from 0° to 90° and irradiated with 3Gy. On each
machine 4 measurements were performed for each
orientation. The irradiation setup is 5cm depth, 100cm SSD
and 10.5x10.5cm
2
/10x10cm
2
(MRIdian/Linac) field size.
Optical density (OD) to dose (D) calibration measurements
(4x) were performed up to 8Gy both on MRIdian and Linac,
with the film perpendicular to the beam axis.
The film sheets were scanned in portrait mode with the
A4 Epson 1680 Expression Pro flatbed scanner and at the
film rotation point OD and D values were analyzed for the
red, green and blue channel.
The following comparisons were made:
a. Reproducibility of MRIdian and Linac film measurements
b. Angle dependent film response with (MRIdian) and
without B
0
(Linac) influence
c. Angle dependent film response in frontal or sagittal
setting
Results
Table 1 represents (a) the reproducibility of film OD values
for both machines, (b) the influence of B
0
on film dose
response over all angles and (c) the mean dose values
averaged over all angles for frontal and sagittal planes.
The blue channel shows the widest range, which is i.a. due
to the fact that the blue channel is barely dependent on
dose and is sensitive to the dye homogeneity in the film.
Using triple color correction on the results in (c) gives
better mutual correspondence.
Conclusion
All data indicates that there is no dose deviation between
MRIdian and Linac measurements neither in frontal or
sagittal setting. In addition, within experimental
uncertainty there is no B
0
influence on absolute film dose
response. This means that EBT3 GafChromic film can be
used for absolute dosimetry regardless its orientation in
the field. This makes it a suitable dosimeter for QA
measurements and end to end testing in a 0.35T CO
60
machine.
OC-0232 Development of a novel ‘end to end’
dosimetry audit of motion management in
radiotherapy
A.L. Palmer
1
, D. Nash
1
, S. Jafari
1
, S. Muscat
1
1
Portsmouth Hospital NHS trust, Medical Physics
Department, Portsmouth Hampshire, United Kingdom
Purpose or Objective
External dosimetry audit is valuable to assure quality,
safety, and enable improvements. However, motion
management in radiotherapy has not previously been
subject to rigorous audit. 4DCT allows assessment of
motion at treatment planning, but verifying the accuracy
of mitigation strategies at treatment planning and
delivery is poorly reported. This includes any detrimental
effect of MLC, gantry and target motion interplay. We
have developed an end-to-end dosimetry audit system to
objectively assess the success of motion management
strategies. The audit system may be used for
interdepartmental dosimetry audit as well as to provide
improved understanding and accuracy of motion managed
radiotherapy.
Material and Methods
The system uses a respiratory motion lun g-phantom
(Quasar), custom designed insert with targe t (~40 mm
diameter irregular structure) and lung OAR. Dose plane
measurement is made using triple-channel film dosimetry
(EBT3 and FilmQAPro) with uncertainty reduction
strategies. Measurements are performed with the
phantom static to provide a conventional end-to-end
evaluation of accuracy and then moving (+/- 7 mm
sinusoid) to add separately the effect of motion. A novel
dose-area-histogram, via Matlab, was used to assess dose
coverage of the moving target (GTV and ITV) in the film
plane. Agreement between treatment planning system
(TPS) calculation and measured dose was performed using