ESTRO 36 Abstract Book

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S791 ESTRO 36 2017
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Figure 1: Spot size over initial proton energy with (full
symbols) and without (empty symbols) passive elements,
for the non-optimized (Basic) and the most compacted
nozzle.
Conclusion
The optimum in terms of spot size can be reached if all
nozzle elements are as close as possible to the nozzle exit
as a reduction in distance to the isocenter proved very
effective. Therefore, MedAustron will focus on a compact
nozzle design with retractable snout.
EP-1495 Should we use correction factors for skin
dose measurements with radiochromic films?
P. Carrasco de Fez1 , M.A. Duch2, L. Muñoz2, N. Jornet1,
M. Lizondo1, C. Cases1, A. Latorre-Musoll1, T. Eudaldo1,
A. Ruiz1, M. Ribas1
1 Hospital de la Santa Creu i Sant Pau, Servei de
Radiofísica i Radioprotecció, Barcelona, Spain
2 Universitat Politècnica de Catalunya, Institut de
Tècniques Energètiques, Barcelona, Spain
Purpose or Objective
The election of detector for skin dose measurements is
critical (see fig. 1a). This work is aimed to study the
surface dose in high-energy x-ray beams and to derive
potential correction factors (CFs) to be applied for in-vivo
skin dose measurements when using EBT3.
Material and Methods
• 6 and 15 MV x-ray beams from a Clinac 2100 CD
(Varian)
• EBT3 radiochromic films + Film QA Pro 2014
software (Ashland) + EPSON EXPRESSION
10000XL scanner
• 30X30X30 cm3 Plastic Water (PW) phantom
(CIRS)
• Low-density polyethylene plastic sleeve
• TLD-2000F (Conqueror)
Methods
TLDs and EBT3 films were attached to the centre of the
PW phantom surface side facing the radiation beam.
The main parameters affecting surface dose as reported
in literature were studied (field size and angle of
incidence) with both EBT3 and TLDs. The field size was
changed between 3.5 and 25 cm, the angle of incidence
between 0 and 90º, and the SSD between 75 and 100cm.
The effect of a plastic sleeve to be used for in vivo
measurements was assessed. Incidence angle and field
size CFs for EBT3 films could be derived from comparison
against measurements made with TLDs because TLDs are
known as not having any dependence on the incidence
angle or the field size.
The equivalent depth correction factors (EDCF) for EBT3
films have been determined using measurements made
with a PTW 23392 Extrapolation ion chamber in a previous
work [1] and measurements made with EBT3 films in this
work. EDCF allows determining dose @ the ICRU skin depth
(70 µm) from EBT3 measurements (active depth@120µm).
The effect of SSD was studied with EBT3.
For film dosimetry, EBT3 films were cut into 3 cm2 square
pieces marked to keep track of their orientation for
scanning. Readout of each film corresponded to the mean
value within a 1×1cm2 ROI centred in the film piece.
Several pieces for each measurement were read 3 times
with random position in the central part of the scanner to
account for scanner and film non-uniformity in the
uncertainty.
Results
Fig 1b shows surface dose increases linearly as a function
of the field size measured with every detector.
Fig 1c shows that surface dose increases slowly up to an
angle of incidence of 30º and very fast from angles
between 60 and 80º.
There is no significant difference between measurements
made with EBT3 and with TLD. The effect of the plastic
sleeve is negligible considering the uncertainty.
SDD: Fig 1d shows deviation to the inverse square law of
0.004% for 6 MV and 0.13% for 15 MV, much lower than
EBT3 overall uncertainty (≈3%).
EDCF were 0.709±0.044 for 6MV and 0.872±0.127 for 15
MV.
Fig 2 shows the consistency of applying EDCF on data of
Fig 1b: EBT3 agree with TLD within the uncertainty. All
error bars are k=2.
Conclusion
No other CFs than the EDCF have to be applied for skin
dose measurements with EBT3 films.
This work has been partially financed by the grant
Singulars Projects 2015 of the Spanish Association Against
Cancer (AECC).
[1]Detector comparison for dose measurements in the
build-up zone. M.A Duch et al. 3rd ESTRO FORUM. 2015.
EP-1496 A portal dosimetry dose prediction method
based on CT images of Electronical Portal Imaging
Device
J. Martinez Ortega 1, N. Gomez Gonzalez1, P. Castro
Tejero2, M. Pinto Monedero1, N. Tolani3, L. Nuñez
Martin 1, R. Sanchez Montero4
1Hospital Universitario Puerta de Hierro, Radiofisica y
PR, Majadahonda - Madrid, Spain
2Hospital Universitario La Princesa, Radioterapia,
Madrid, Spain
3ME De Bakey VA Medical Center, Radiotherapy
Department, Houston, USA
4Universidad de Alcala, Signal Theory and