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S801
ESTRO 36
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use of similar components as in the other beam lines as
well as a static beam monitoring system. Simulations were
performed for 5 representative energies (62, 96.5, 157.4,
205, and 252.3 MeV) also considering the use of range
shifter and ripple filter. Nozzle designs were based on the
MC model of the MedAustron fixed beam line nozzle, which
was verified with measurements at isocenter for 20
representative energies.
Results
The influence of the gantry nozzle filling with vacuum or
helium (as used in some commercial systems) was found
to have only a minor impact on spot size (< 2%).
Compacting all nozzle elements towards the nozzle exit
and reducing the nozzle dimension in beam direction by
25% lead to a reduction of the spot size of up to 20%,
depending on the initial energy as depicted in Fig. 1. Using
higher energies in combination with range shifter also
decreased the delivered spot size for shallow seated
tumors, as already demonstrated in other studies (Parodi,
et al. PMB 57(12), 2012).
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 Fez
1
, M.A. Duch
2
, L. Muñoz
2
, N. Jornet
1
,
M. Lizondo
1
, C. Cases
1
, A. Latorre-Musoll
1
, T. Eudaldo
1
,
A. Ruiz
1
, M. Ribas
1
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 cm
3
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 cm
2
square
pieces marked to keep track of their orientation for
scanning. Readout of each film corresponded to the mean
value within a 1×1cm
2
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.