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can be used to attach and remove the cutouts to the
applicator in an easy and fast manner. It is important to
note, that the mounted frame must not interfere with the
radiation field. Hence, the frame has an identical size as
the used applicator. VELCRO® strips were deployed as an
attachment modality between applicator, frame and
cutout. Those were glued to a PMMA frame with the same
size of a standard applicator which attach to VELCRO®
strips elongating around the applicator. The skin facing
side of the frame is also covered with VELCRO® strips. In
addition, cutouts were covered on the one side with
VELCRO® strips and on the other side with a plastic foil,
respectively. Straight-lined cutouts can be adjusted within
seconds to shape TVs.
Results
All used straight-lined shields from our database could be
exactly reproduced contouring 100% of the TV using the
developed attachment system. Nevertheless, the system
increases the distance of the applicator to the patient skin
by non-negligible 8.8mm. This has to be taken into
account by an output factor (OF). A comparison of the
measured and calculated (by the ISL) OFs shows a
maximum deviation of ±3.6% (12keV) and ±0.8% (>12keV)
for our set-up.
Conclusion
Shielding healthy tissue in kilovoltage x-ray beam therapy
using the reusable frame-based system promises a fast
exchange of cutouts. It also ensures a high reproducible
accuracy to irradiate multiple TVs in a row with the same
applicator but with different frames and cutouts.
Moreover, it provides a complete coverage of healthy
tissue of straight-lined TVs. Furthermore, a measured OF
should be considered to the prescribed dose using the
presented frame-based attachment system.
EP-1516 Prediction of secondary cancer risk from
lateral electrons transport from pediatric radiotherapy
A. Chaikh
1
, J. Balosso
2
1
CHU de Grenoble - A.Michallon, Radiothérapie et
physique médicale, Grenoble, France
2
University Hospital of Grenoble- University Grenoble-
Alpes, Department of Radiation Oncology and Medical
physics, Grenoble, France
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