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in the process, to assess the impact of each sub-step and
to rank the most relevant errors by setting a numerical
value- Risk Priority Number (RPN) obtained with the scores
attributed to the occurrence, severity and detectability by
questionnaires submitted to staff (doctors, physicists and
therapists).
Results
For breast the unanimous responses between professional
classes were initial placement, lateral displacement in the
location of the isocenter and image acquisition. The
causes of positioning errors were during treatment for
physicians losses marks on the skin is the most important
factor, to the physicists, error in the use of accessories
results in major failures and for therapists, changes in the
weight of the patient may cause major errors. For H&N
cases there was not unanimous response. In simulation-CT
scan, doctors point out patients lack of cooperation as the
leading cause of errors, physicists an improperly made
mask generates the greater number of failures and
therapists did not have unanimous answers. In the initial
position sub-step, the most important point proved to be
the inclusion or exclusion of tracheostomy/nasal probe for
therapists and physicists. Physicists also considered non-
coincidence of location marks a factor of great
importance. In location of the treatment isocenter sub-
step, physicists and therapists pointed to the poor
positioning of the mask as a cause of failure, but with
different impact in the treatment. For physicians, the
wrong initial displacement is the main cause of errors. In
acquisition of portal sub-step, the most frequent cause of
errors was inaccurate comparison of images and mistaken
correction, for all. For therapists and physicists, the use
of DRR associated with other phases was the root cause of
failures in this step. Positioning errors causes during
treatment received different answers: for doctors, the
main causes of failure are problems with the mask
accessories and change in patient weight. For physicists
the patient's weight change was the most important
failure.
Conclusion
The FMEA introduces a subjective analysis, since it is
dependent on personal judgment criteria relevant points
were highlighted in the analysis of positioning routine. To
the answers with relevant frequency or high RPN, solutions
could be suggested in order to prevent failures and
minimizing human erros.
Further studies are in progress to
other anatomical sites.
EP-1518 Various activation foils for photo neutron
measurements in medical linac
A.H. Kummali
1
, S. Cyriac
2
, S. Deepa
3
, A. BAKSHI
3
1
Nanavati Hospital, Medical Physics, Mumbai, India
2
Apollo Hospitals Navi Mumbai, Medical Physics, Navi
Mumbai, India
3
BARC, RPAD, Mumbai, India
Purpose or Objective
Photo neutrons produced from medical linear accelerators
while operating above 10 MV is a concern for radiation
protection and safety for patients and radiation workers
[1]
. Different methods are used to quantify the neutron
production in clinical situation. In our study we used
various activation foils for the photo neutron
measurements in medical LINAC. This study discusses the
measurement techniques of neutron absorbed dose for
various treatment parameters of clinical importance.
Material and Methods
Absolute measurements of photo-neutrons using the
Indium activation foil
[2]
having both thermal and fast
neutron cross-sections through the nuclear reactions
115
In
(n, γ)
116m
In and
115
In (n, n’)
115m
In, the thermal neutrons
using
197
Au(n,γ)
198
Au,
63
Cu (n,γ)
64
Cu were evaluated in the
present study. Photo-neutron measurements for various
field size opening using MLC, and for various wedge angles
for 15 MV photon beam from a Medical LINAC model Elekta
Precise have been carried out in the present study.
Results
Photo neutrons were measured using 3 foils mentioned
above for various field sizes
[3]
such as 10 x 10 cm
2
to 20 x
20 cm
2
and for 40 x 40 cm
2
. Irradiation time for each field
size took approximately 10 min and the total MU delivered
is 5000 at a dose rate of 590 MU/min. Dose calculated at
Dmax is 50Gy and 10 cm back up of PMMA phantom is
ensured for the scattering and to mimic the TPS treatment
planning. The result shows that, the total neutron dose
increases as the field size increases from 10 x 10 cm
2
to 20
x 20 cm
2
and for 40 x 40 cm
2
. The photo neutron
measurements using activation foils for Omni wedged
fields in Elekta LINAC is uniquely studied. The irradiation
time of about 20 min were taken to deliver 50 Gy at Dmax
with the dose rate of 640 Mu/min. Wedged fields were
defined for a field size of 30 x 30 cm
2
and the wedge used
for each set of measurements are 15°, 30° and 60°. The
fast neutron dose decreases and thermal neutron dose
increases with wedge angles from 15°, 30° and 60°. Open
beam gives the highest fast neutron dose and the lowest
thermal neutron dose.
Conclusion
Insensitivity nature of activation foils for gamma/photons
and the possibility of absolute measurements using the
primary quantity of nuclear reaction cross-section makes
activation foil best suited for photon induced neutron
measurement. The present results indicate that the total
neutron dose represents a small contribution to the
therapeutic photon dose, meaning that it is much smaller
than 1% of the photon dose delivered to the patient.
However, the amount of this extra dose in the vicinity of
the patient position cannot be neglected in view of
radiological protection assessment related to the patients.
Electronic Poster: Physics track: Treatment plan
optimatisation: algorithms
EP-1519 Implementation of a hybrid superfast Monte
Carlo-Pencil beam dose optimizer for proton therapy
A.M. Barragán Montero
1
, K. Souris
1
, D. Sánchez-
Parcerisa
2
, A. Carabe-Fernández
3
, J.A. Lee
1
, E. Sterpin
1,4
1
Université Catholique de Louvain- Institute of
Experimental & Clinical Research, Molecular Imaging-
Radiotherapy and Oncology MIRO, Brussels, Belgium
2
Universidad Complutense de Madrid, Departamento de
Física Atómica- Molecular y Nuclear, Madrid, Spain
3
Hospital of the University of Pennsylvania, Department
of Radiation Oncology, Philadelphia, USA
4
KU Leuven - University of Leuven, Department of
Oncology, Leuven, Belgium
Purpose or Objective
Monte Carlo (MC) dose calculation plays an important role
in treatment planning for proton therapy due to the
limited accuracy of analytical algorithms, especially in
very heterogeneous tumor sites. The new dedicated MC
engines for pencil beam scanning (PBS) achieve reduced
computation times for a single dose calculation. However,
computing spot-per-spot doses is still very time-
consuming, since typically 10000 to 20000 spots are
needed. The presented strategy combines the speed of
analytical algorithms and the accuracy of MC to get the
best outcome for PBS treatment planning in a reasonable
amount of time for clinical practice.
Material and Methods
An in-house treatment planning system was used to create
the plans. The optimizer combines the analytical pencil
beam (PB) algorithm in
FoCa
(Sánchez-Parcerisa et al.
Phys Med Biol 2014) and the super-fast Monte Carlo engine