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ESTRO 36
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Purpose or Objective
Modern dose calculation algorithms in radiotherapy
treatment take into account the scattered dose and
lateral electrons transport, such as point kernel model.
The impact of scattered radiation dose from radiotherapy
treatment is more significant for children. In this study,
secondary cancer risk (SCR) resulting from scattered dose
and the contribution of electrons transport were
compared.
Material and Methods
Clinical examples of treatment plans for pediatric
medulloblastoma were used to estimate the SCR for lungs.
For each case, two treatment plans with conformal
radiotherapy were generated. The same dose
prescriptions for posterior fossa and craniospinal
irradiation were used for both plans. The dose in first plan
was calculated with algorithm taking account only
scattered dose. The dose in second plan was calculated
taking account scattered dose and lateral electron
transport, as point kernel algorithms. The organ
equivalent dose (OED) concept with a linear, linear-
exponential and plateau dose response curves was applied
to dose distributions, dose volume histograms, for lungs to
estimate SCR. The excess absolute risk ratio (EAR) was also
evaluated as EAR = OED from scattered dose divided to
OED from scattered with lateral electrons transport doses.
Results
The calculated DVH with algorithm modeling lateral
electron transport were significantly increased predicting
more average dose for lungs by a factor of 1 to 1.1. The
SCR was also increased (8%-16%) depending on model
prediction. The EAR ratio were 1.08, 1.2 and 1.13,
respectively, using linear, linear-exponential and plateau
models.
Conclusion
The considerable impact of dose calculation methods in
radiotherapy, integrated in TPS, can significantly
influence the secondary cancer risk prediction and plan
optimization, since OED is calculated from DVH for a
specific treatment. The modern algorithms such as AAA,
Acuros XB or Monte Carlo showed a better prediction of
dose distribution. On the other hand, they provided more
“trust” DVH metrics, as input in the SCR models, avoiding
the uncertainties of dose distribution as well as
significantly contribute to better estimations.
EP-1517 Analysis of radiotherapy risk profile applied
to the patient positioning
G. Menegussi
1
, M.M. Vasques
1
, G.R.D. Santos
1
, L.
Furnari
1
, L.N. Rodrigues
1
1
Hospital das Clinicas -FMUSP, Radiotherapy, Sao Paulo,
Brazil
Purpose or Objective
The purpose of this work is to recognize and understand
the risks of the processes of Radiotherapy positioning.
Material and Methods
Risk analysis methods were applied Failure Mode Effect
Analysis (FMEA) to key steps in each sub-step of the
positioning process (simulation, initial positioning,
displacement, images acquisition and treatment) of
patients in the treatment of breast and head&neck (H&N)
tumors. This tool enabled us to identify the risks involved
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