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– 63.1%) for VMAT and 21.5 % (0.7%- 60.1%) for B-VMAT.
For V10, the VMAT medium value was 23.2% (0%-
37.2%) and the B-VMAT medium value was 8.9% (0%-
24.4%).
Conclusion
B-VMAT for mediastinal tumors is clearly superior to usual
VMAT for breast doses, mainly the low doses, and
equivalent in the rest of dosimetric parameters. Although
the inclusion of more patients is needed, our preliminary
results show B-VMAT like a great technical advance in
mediastinal radiotherapy.
Electronic Poster: Physics track: Basic dosimetry and
phantom and detector development
EP-1433 Photoneutron Flux Measurement via NAA in a
Radiotherapy Bunker with an 18 MV Linear Accelerator
T. Gulumser
1
, Y. Ceçen
1
, A.H. Yeşil
1
1
Akdeniz University- School of Medicine, Department of
Radiation Oncology, Antalya, Turkey
Purpose or Objective
In cancer treatment, high energy X-rays are used which
are produced by linear accelerators (LINACs). If the energy
of these beams is over 8 MeV, photonuclear reactions
occur between the bremsstrahlung photons and the
metallic parts of the LINAC. As a result of these
interactions, neutrons are also produced as secondary
radiation products (γ,n) which are called photoneutrons.
The study aims to map the photoneutron flux distribution
within the LINAC bunker via neutron activation analysis
(NAA) using indium-cadmium foils.
Material and Methods
The radiotherapy bunker hosts a Philips SLI-25 LINAC which
is used for experimental studies. The measurements are
taken at the highest energy of the LINAC which
corresponds to 18 MeV bremsstrahlung photons. Indium
and cadmium foils were used at 91 different points within
the bunker. Neutron activation was performed by
irradiating the room with 10000 monitor units (MU) at
different gantry angles. The field was 40x40 cm
2
open. The
activated indium foils are then counted in a High Purity
Germanium (HPGe) detector system.
Since indium has a high absorption cross section for
thermal and epithermal neutrons, bare indium foil
irradiation results in flux information of that region.
However cadmium has high absorption cross section in the
epithermal and fast region. If one filters the indium foils
by cadmium coatings, the difference in the count yields
thermal fluxes which are of interest for the doses to the
patients in radiotherapy.
Results
Result of the analysis shows that the maximum neutron
flux in the room occurs at just above of the LINAC head
towards to gun direciton. This is expected since most of
the neutrons are produced when the electron beam hits
the tungsten target and then the primary collimation
occurs. Both the target and the primary collimator are
located at the top of the gantry head. The maximum
thermal
neutron
flux
obtained
is
3x10
5
neutrons/cm
2
.second which is higher than a standard
americium-beryllium
(Am-Be)
neutron
source.
At the isocenter plane (SSD=100 cm), the fluxes were
5.4x10
4
at the center, 1.5x10
4
at 2.5 m away and 9.9x10
3
n/cm
2
.s at the room wall which is 3.8 m away from
isocenter. The flux at the maze entrance was measured
nearly six in a ten thousand less (81 n/cm
2
.s).
Conclusion
The neutron flux distribution within the bunker was
measured with detail using 91 points. Neutron flux
distribution within the bunker found and the graph was
plotted. Thus neutron flux can found any desired point in
the room by iterations. The flux decreases as we move
away the isocenter which is compatible with the
literature. The magnitude of the neutron fluxes shows that
there is a significant amount of neutron dose within the
room. The corresponding neutron dose to the patient
however is only 0.1-0.3 % of the total dose. However,
neutrons have a high RBE and this unwanted dose is not
calculated with the TPS. The future work would be to
compare the results with the Monte Carlo simulations.
EP-1434 Comparison of small-field output factor
measurements
C. Oliver
1
, V. Takau
1
, D. Butler
1
, I. Williams
1
1
ARPANSA, Radiotherapy, Yallambie, Australia
Purpose or Objective
The Australian Radiation Protection and Nuclear Safety
Agency (ARPANSA) held a comparison in April 2016
whereby participants came to ARPANSA and measured the
output factor of a 5 mm cone . The goal of the comparison
was to compare the consistency of the small-field output
factor measured by independent medical physicists with
their own apparatus.
Material and Methods
The participants measured the output factor of the 5 mm
cone using a 6 MV photon beam at a source to surface
distance of 95 cm and depth in water of 5 cm. ARPANSA
provided a 3D scanning water tank for detector positioning
but all detectors were brought by participants. The
participant was asked to measure the output factor as
accurately as possible. All post measurement analysis,
correction factor determination and uncertainty
calculations were supplied by the participant.
Results
Fifteen groups travelled to ARPANSA and a total of thirty
independent measurements of the output factor were
made. The most popular method of measurement was with
film but measurements were also made with ionisation
chambers, semiconductor detectors, diamond detectors
and a scintillation detector. A large volume ionisation
chamber measuring dose area product was also used in the
comparison. The standard deviation of all the
measurements was 5.6 % with the maximum variation
between two results being 42 %.
Conclusion
This exercise gave an indication of the consistency of the
small-field dosimetry being performed in Australia at the
present time. There is no currently accepted protocol for
these measurements and a wide range of detectors are
being used with correction factors being applied from a
variety of sources. The dissemination of the small-field
methods and techniques currently being used will aid the
consistency of these measurements.
EP-1435 Evaluation of single material and multi-
material patient-specific, 3D-printed radiotherapy
phantoms
D. Craft
1
, E. Burgett
2
, R. Howell
1
1
The University of Texas MD Anderson Cancer Center,
Radiation Physics, Houston, USA
2
Idaho State Univeresity, Department of Nuclear
Engineering, Pocatello Idaho, USA