ESTRO 36 Abstract Book

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S415 ESTRO 36
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Results
The detector presents dependency on energy that is
reflected in the response variation with depth and field
size (2.2% under-response for 6 MV, 20x20 cm2 at 20 cm
depth).
The anisotropy study shows important deviations: 28% for
lateral incidences and 7% for posterior incidence.
The detector sensitivity for leaf positioning measurement
is 1.8 % per tenth of millimeter in the penumbra.
The output factor corresponding to 6 MV and 1x1 cm 2
shows +2% deviation compared with the measurements
obtained using a SFD diode and a CC13 gas ionization
chamber. The results are normalized to a 5x5 cm2. For a
10x10 cm2 this deviation is -1%. If the energy increases the
deviations decrease (+1% for 1x1 cm2 and -0.5% for 10x10
cm 2 in 10 MV and 15 MV).
In the measurement of small field profiles the gamma
comparison between measurements with the liquid
ionization array and radiographic film shows 100% passing
rates with tolerances 1% - 1mm.
Several patient treatments have been verified. In table 1
the comparison between the treatment planning system
and the array measurement for a particular case is shown.
We show differences in gamma passing rates when
anisotropy corrections are applied or not. Figure 1 shows
one of such comparisons.
Conclusion
A new detector array is presented for the verification of
patient treatments of high complexity.
The detector presents a small dependence on
energy, which causes a small over-response for the output
factors of small fields and an under-response for output
factors of large fields. The anisotropy of the device is
significant (28% and 7% for lateral and posterior
incidences), but can be compensated during treatment
verification by using angle-dependent correction factors.
The usefulness for the patient treatment verification has
been demonstrated by measuring different patient
treatments. The results obtained confirm the validity of
this array for dose distribution measurements of complex
treatments with small fields and high gradients.
PO-0783 Planverification in Robotic Stereotactic
Radiotherapy with the Delta4-Dosimetry-System
W. Baus 1, G. Altenstein1
1 Universität zu Köln, Department of Medical Physics,
Köln, Germany
Purpose or Objective
Stereotactic robotic radiotherapy with the CyberKnife
(Accuray, Sunnyvale) might not be fluency modulated
radiotherapy (IMRT) in the strict sense. However, the
technique is comparable in complexity because of a large
number of small (5 to 60 mm), highly non-coplanar fields.
Therefore, the manufacturer recommends individual plan
verification (DQA, Delivery Quality Assurance), though
only on a point dose measurement basis. The report of the
AAPM task group on robotic radiotherapy (TG 135) [1]
advises the use of film. However, because film dosimetry
is rather cumbersome in most cases, it foregoes to demand
it for every plan. Film dosimetry provides 2D-
measurement, but also laborious calibration, fading and
non-linear sensitivity. Arrays of dosemeters have the
advantage of comparably easy and direct evaluation,
though at a distinctively lower resolution. The aim of this
work is to investigate the usefulness of an existing
dosimetry system based on diode arrays – the Delta4+
Phantom (ScandiDos, Uppsala) – for DQA of the CyberKnife
robotic treatment system.
Material and Methods
Several patient plans with PTVs ranging from 5.6 to 112
cm³ were investigated. The irradiation was performed
with a CyberKnife (G4, rel. 9.5, 6 MV photons, no
flattening filter), treatment planning system was
Multiplan (rel. 4.5). The Delta4+ dosimetry system consists
of a PMMA-cylinder of 22 cm diameter in which two
orthogonal silicon diode arrays are housed, adjecent to
electrometers. There are 1069 detectors, an inner region
with detector spacing of 5 mm (6x6 cm²) and 10 mm
spacing in the outer region. The measurements were
carried out with software Vers. 2015/10. The
measurements were compared to film (Gafchromic EBT3
Film, ISP, Wayne) and a high-resolution ion chamber array
(Octavius 1000 SRS, PTW, Freiburg). A speciality of the
CyberKnife treatment is the fact that correct image
guided positioning – using X-ray opaque markers, e.g. – is
mandatory. Therefore, special considerations have to be
taken for marker placement.
Results
Positioning and localisation of the Delta4 was possible and
the plan verification could be carried out. The evaluation
with the scandidos software produced results with good
agreement between plan and measurement (see fig. 1).
The evaluation was somewhat compromised by system
breakdowns (maybe caused by treatment times of
hour) and the non-complanarity of the plans which
prevented the correction for gantry angle usually
exploited by the software.
The scandidos software allows for a 3-dimensional
evaluation of the dose distribution. The lower spacial
resolution compared to film or the 1000 SRS seems to be
less important, on the other hand.
Conclusion
In principle, the Delta4 dosimetry system seems to be
highly suited for DQA of CyberKnife treatment. However,
the manufacturer should improve the system in terms of
radiaton resistance and a proper implementation of
fiducial markers to make it wholly suitable for Cyberknife
DQA.
[1] S. Dieterich et al., Report of AAPM TG 135, Med. Phys.
2011
PO-0784 Volume correction factors for alanine
dosimetry in small MV photon fields
H.L. Riis1, S.J. Zimmermann1, J. Helt-Hansen2, C.E.
Andersen 2
1Odense University Hospital, Department of Oncology,
Odense, Denmark
2Technical University of Denmark, Center for Nuclear
Technologies, Roskilde, Denmark
Purpose or Objective
Alanine is a passive solid-state dosimeter material with
potential applications for remote auditing and dosimetry
in complex fields or non-reference conditions. Alanine has
a highly linear dose response which is essentially
independent of dose rate and energy for clinical MV
photon beams. Alanine is available as pellets with a 5 mm
diameter, and irradiations in flattening filter-free
(FFF) beams or other non-uniform beams are therefore
subject to volume averaging. In this work, we report on a
simple model that can provide volume correction factor
for improved output factor measurements in small MV
photon beams.
Material and Methods
The x-ray beam was delivered by an Elekta Versa HD linac
with an Agility MLC160 radiation head. Square field sizes
(FS) 0.8, 1.0, 1.4, 2.0, 3.0, 4.0, 5.0, 7.0, 10.0, 20.0, 30.0,
40.0 cm were investigated. The data were acquired at
SSD=90 cm, depth 10 cm. The alanine pellets were the
standard Harwell/NPL type (Ø4.83×2.80 mm). The pellets
were placed in water with a latex sleeve to protect against
water. A Bruker EMX-micro EPR spectrometer equipped
with an EMX X-band high sensitivity resonator was used to
read out the dose deposited in the alanine pellets. The
horizontal beam profiles were measured using the IBA
Dosimetry photon field detector (PFD) for all FSs while
depth dose profiles were measured using the PTW
microLion (FS < 8 cm) and PTW semiflex (FS > 8 cm)