ESTRO 36 Abstract Book

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S411 ESTRO 36 2017
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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
1 Odense University Hospital, Department of Oncology,
Odense, Denmark
2 Technical 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)
detectors. A rotational symmetric Gaussian horizontal
beam profile and exponential decaying depth dose profile
in the vicinity of the pellet was fitted to the measured
profiles. Both 6 MV and 6 MV FFF beams were considered.
Results
The fit of the beam profile in three dimensions was based
on two parameters: the variance for the Gaussian profile
and the gradient of the depth profile. The parameters in
turn were both changing as function of FS. Using the fitted
beam profiles, an analytical model was developed for the
calculation of volume correction factors kV for given FS
(see Table 1).
Table 1: Calculated volume correction factors kV,
temperature and volume corrected output factors (OF)
with SD being one standard deviation (SD) are displayed
for the 6 MV and 6 MV FFF beams as function of the field
size FS.
Conclusion
Volume averaging was found to influence the alanine
measurements by up to 6 % for the smallest field size. For
a cylindrical detector irradiated along the symmetry axis
of the detector, simple analytical expressions of the
volume correction factors were obtained. The analytical
expression gives valuable insight in the volume correction
factor kV as function of field size and the radius of the
sensitive volume of the detector. The method presented
here would be applicable for other detectors. With a
defined geometry of the sensitive volume of the detector
relative to the central axis of the beam the volume
correction factor can either be calculated analytically or
numerically as function of FS.
Poster: Physics track: Dose measurement and dose
calculation
PO-0785 A pencil beam algorithm for protons including
magnetic fields effects
F. Padilla1, H. Fuchs1, D. Georg 1
1Medizinische Universität Wien Medical University of
Vienna, Department of Radiation Oncology, Vienna,
Austria
Purpose or Objective