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used for alignment and positioning. The xyz-stage also
allows for scanning and accurate field-size measurements
at the reference position. The system has been
characterized using an ensemble of 11 thimble ionization
chambers of the types PTW 30013, IBA FC65G, NE 2571,
and NPL2611.
Results
Automated procedures were implemented for
measurement of absorbed dose to water calibration
coefficients. Source irradiations and positioning was found
to be highly reproducible. The relative standard deviation
of dose-rate measurements with the 11 ionization
chambers was less than 0.03% within each specific
measurement session carried out over a period of 120
days. The collimator and the shutter systems were
characterized using randomized tests run continuously
over 24 h periods. Setting the field size to different values
in a random order resulted in a relative standard deviation
for dose rates within each filed size of less than 0.05%.
Conclusion
The ability to computer control irradiations has enabled
development of automatic calibration and measurement
procedures. This in turn has resulted in an improved
quality of measurements and implementation of more
comprehensive measurement sequences relative to what
would have been feasible using an irradiator system with
only manual source control. The special rig and the optical
alignment system allowed for precise (better than 0.1
mm) positioning of ionization chambers. The system was
therefore found to be highly suitable for research and
calibrations involving ionization chambers and other
dosimeters used in radiotherapy
.
EP-1438 Experimental determination of correction
factors for reference dosimetry in Gamma Knife
Perfexion
E. Zoros
1
, E.P. Pappas
1
, K. Zourari
2
, E. Pantelis
1
, A.
Moutsatsos
1
, G. Kollias
3
, C.I. Hourdakis
2
, P. Karaiskos
1
1
National and Kapodistrian University of Athens, Medical
School - Medical Physics Laboratory, Athens, Greece
2
Greek Atomic Energy Commission, Division of Licensing
and Inspections, Athens, Greece
3
Hygeia Hospital, Gamma Knife Center, Athens, Greece
Purpose or Objective
To experimentally determine machine-specific reference
(msr) field
correction factors (CFs) for
a set of commercially available ion chambers and two
dosimetry phantoms which are commonly used for the
calibration of the Gamma Knife Perfexion (GK PFX)
radiosurgery unit.
Material and Methods
Measurements were performed for both plastic spherical
phantoms, referred to as acrylonitrile butadiene styrene
(ABS) and Solid Water (SW), which are used in GK PFX
reference dosimetry. CFs were obtained for IBA CC01, IBA
CC13, PTW 31010 and Exradin A1SL ion chambers using the
formalism proposed by Alfonso
et al.
(2008) for the
dosimetry of small and non-standard photon fields. The
determination of absorbed dose to water in phantom
material for the msr field (16mm collimator size) was
performed using EBT3 radiochromic films and alanine
pellets as reference passive dosimeters whose calibration
is traceable to a primary standard and do not exhibit
substantial beam quality dependence. However, in order
to determine absorbed dose to water in water, film and
alanine measurements were corrected using phantom-
dose conversion factors obtained by Monte Carlo
simulations using a recently introduced EGSnrc simulation
model. Special custom made inserts to accommodate ion
chambers and alanine pellets were fitted into the inserts
of the ABS and SW phantoms. Detectors’ central axis was
aligned with the z axis of GK PFX stereotactic space for
SW measurements, while placed on x-y plane for the ABS
phantom. A scanning technique was implemented for the
accurate alignment of detectors’ reference point of
measurement with GK PFX radiation focus. In order to
estimate statistical uncertainties of the CFs five
measurements were performed for each detector.
Regarding ion chambers, measurements were averaged for
positive and negative polarity and the obtained readings
were corrected for ion recombination, temperature and
pressure effects.
Results
For the majority of ion chambers in SW phantom CFs were
up to 1.01, except of the IBA CC01 were a correction of 4%
is needed mainly due to perturbation of the high density
central electrode. Regarding ABS phantom larger
corrections are needed up to 1.05 for IBA CC01 and CC13
and up to 1.02 for PTW 31010 and Exradin A1SL, attributed
to the different orientation of the detectors in GK PFX
stereotactic space.
Conclusion
An experimental procedure is proposed for the
determination of CFs for the GK PFX radiosurgery unit and
CFs were determined for a set of ion chambers allowing
for accurate dosimetric measurements.
Acknowledgement: This work was financially supported by
the State Scholarships Foundation of Greece through the
program “Research Projects for Excellence IKY/SIEMENS”.
EP-1439 Small field dosimetry: preliminary
characterization of a nano-chamber with a focus on
stem effect
A. Stravato
1
, G. Reggiori
1
, P. Mancosu
1
, F. Lobefalo
1
, L.
Paganini
1
, F. Zucconi
1
, V. Palumbo
1
, S. Tomatis
1
, M.
Scorsetti
1
1
Istituto Clinico Humanitas, Department of radiotherapy
and radiosurgery, Rozzano Milan, Italy
Purpose or Objective
Micro and nano-chambers cannot be as small as solid state
detectors but present some advantages in terms of energy
independence and absolute dose measurement that make
them fundamental for small field dosimetry in the SBRT
scenario. A preliminary characterization of a nano-
chamber prototype (Razor Nanochamber, IBA) was carried
out with a particular focus on stem effect. Response under
10 MV FFF beams was observed too.
Material and Methods
The study included characterization of leakage, dose rate
and dose per pulse dependence, measurement of small
beam profiles, and depth dose curves. Profiles were
acquired both in orthogonal (i.e chamber axis orthogonal
to beam axis) and parallel (i.e chamber axis parallel to
beam axis) configuration. Ten repeated inline profile
measurements were performed in head-foot and foot-
head direction to better quantify the stem effect. Ion
collection efficiency and polarity effects were measured.
The values of P
ion
were verified with
1/V
versus
1/Q
curves
(Jaffé plots). The 6 MV and 10 MV FFF photon beams of a
Varian EDGE were used. Output factors for small fields
were compared with Razor Diode (IBA) and FOD scintillator
values.
Results
The 2mm diameter guarantees a very high spatial
resolution comparable to some commercially available
diodes, with penumbra values 0.5-0.8mm larger than
those measured with a Razor Diode for the same fields
(Figure 1). When used with the chamber axis
perpendicular to the beam axis a strong stem (and cable)
effect was observed leading to asymmetric inline profiles