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water tank filled with water with 20-50-mm thickness. For
calculation of photon-equivalent dose (Gy-Eq), blood 10B
concentrations, 10B tumor/blood concentration ration,
and CBE factor for 10B(n,α)7Li reaction were assumed to
be 25 ppm, 3.5, 4.0. Tolerance dose of the skin was
regarded as 18 Gy-Eq.
Results
In condition with no bolus, irradiation time was 121.6 min,
and tumor Dmax and Dmean were 125 Gy-Eq, and 74.3 Gy-
Eq, respectively. In condition with water-equivalent bolus
technique, irradiation time was 72.1% decreased (33.9
min) compared with no bolus condition. Also tumor Dmax
and Dmean were 54.4 Gy-Eq and 45.0 Gy-Eq, and the dose
homogeneity was dramatically improved. Skin Dmax
became greatly less than tolerable dose (11.5 Gy-Eq,
59.6% decrease).The bolus-like effect of covered
collimator with a mass of polycarbonate or water tank was
not sufficient. Dose homogeneity and irradiation time was
largery worse than the condition with a water-equivalent
bolus.
Conclusion
Although this study was examined for a single case of
melanoma patient, our results revealed that water-
equivalent bolus technique could have a great
effectiveness on dose improvement of AB-BNCT for
superficial
cancers.
EP-1437 New Cobalt-60 system for reference
irradiations and calibrations
C.E. Andersen
1
1
DTU Nutech Technical University of Denmark, Center
for Nuclear Technologies, Roskilde, Denmark
Purpose or Objective
Cobalt-60 plays an important role as reference beam
quality in radiation dosimetry and radiobiology. Only few
systems are available on the commercial market for the
therapeutic dose range (~1 Gy/min), and it is therefore of
interest for research and calibration laboratories that a
new irradiator (Terabalt T100 Dosimetric Irradiator) has
been introduced by UJP Praha, Czech Rebublic. In 2013,
DTU Nutech in Denmark acquired the first unit of this new
model, and the purpose of this contribution is to report on
(i) the main characteristics of this gamma irradiator found
during the commissioning work, and on (ii) additional
developments carried out in order to apply the irradiator
for highly precise, automated (i.e. computer controlled)
irradiations.
Material and Methods
The irradiator has a fixed horizontal beam axis about 110
cm above the floor. A collimator system enables field sizes
from 5x5 cm
2
to 40x40cm
2
at the reference point at 100
cm from the source. The irradiator is equipped with a
GK60T03 cobalt-60 source having an activity of 250 TBq
corresponding to a dose rate of about 1.1 Gy/min at the
reference point (Sep. 2016). The source is fully computer
controlled. A special rig of 10x10 cm
2
aluminum profiles
has been designed in collaboration with UJP Praha. This
rig is equipped with a water-tank lift and an xyz-stage for
precise positioning of ionization chambers and other
dosimeters at the reference point. An optical system is
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