ESTRO 36 Abstract Book

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S421 ESTRO 36 2017
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• QUASAR phantom (Modus Medical)
• MATLAB R2015b software (MathWorks)
Methods
4 treatment plans with various prescription doses were
selected: liver SBRT [3x20Gy], lung SBRT [3x18 Gy], breast
IMRT [27x1.85Gy@breast; 27x2.44Gy@boost] and head-
and-neck IMRT [33x2.12Gy@high-risk; 33x1.8Gy@medim-
risk; 33x1.6Gy@low-risk]. Plans were copied onto the
QUASAR phantom. All treatments were planned with 6 MV
RX but the liver SBRT that was planned with 15 MV. All
plans had 7-11 fields.
Two 6 cmx25.4 EBT3 film strips were attached to the
QUASAR phantom along the axial plane. Plans were
delivered on two twin linacs with different table tops. All
experiments were repeated thrice.
Films were read with FilmQA software. Dose maps were
exported and then imported by MATLAB to apply the
equivalent depth correction factors (EDCF) to EBT3 films
(on areas not having any direct contact with the table top)
to get dose at the ICRU skin depth (70µm). EDCFs have
been determined from measurements made in a previous
work [1] using a PTW23392 Extrapolation ion Chamber and
measurements made now with EBT3 films. The application
of other correction factors was not required as a result of
other study presented by the authors at this meeting
where EDCFs for EBT3 are also reported.
We compared dose profiles along the middle of the strips
for the two table tops.
Results
Figure 1 shows one example of the dose maps
corresponding to the two strips for one session for each
plan and table top. The effect of the Exact C ouch grid is
visible, and it can increase the dose in contact to the grid
up to 180%, but doses in contact to the IGRT Couch are
much larger (see for example the two bottom subplots
scale).
Table I shows the maximum total doses measured on dose
profiles along the centre of the strips for the two table
tops and the relative increase in dose due to the IGRT
couch. For the IGRT couch top there are areas larger than
2 cm with a dose larger than twice the dose measured for
the Exact Couch top.
Conclusion
· IMRT techniques can deliver skin doses above the
threshold for deterministic effects.
· The main factor affecting skin dose is the table top.
· The skin dose for the IGRT Couch Top can be the triple
than that for the IGRT Couch top.
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.
PO-0799 Fast protocol for radiochromic film dosimetry
using a cloud computing web application
J.F. Calvo Ortega1, M. Pozo-Massó1, S. Moragues-
Femenía1, J. Casals-Farran1
1Hospital Quiron Barcelona, Radiation Oncology,
Barcelona, Spain
Purpose or Objective
To propose a fast protocol to evaluate plans computed by
a treatment planning system (TPS) by using radiochromic
film dosimetry.
Material and Methods
Gafchromic EBT3 films and an Epson V750 Pro scanner
were used in this study as dosimetry system. Film
dosimetry was conducted using the triple-channel method
implemented in a cloud computing application
). Batch calibration curve (up to
5 Gy) was obtained using several film pieces that were
scanned 24 hours after exposure (24 h-calibration).
So far, radiochromic film dosimetry has been performed in
our department for patient specific quality assurance (QA)
by scanning the films 24 hours after their irradiation.
However, in this study we have investigated the feasibility
of a "fast protocol" that enables to obtain measurement
results within 1 hour for dose verification. This protocol
combines the 24-h calibration and measurements acquired
using three film pieces: 1) one is exposed to the clinical
plan (verification film); 2) a film piece is homogenously
irradiated to the expected maximum dose of the clinical
plan, and 3) an unexposed film piece. The three films are
simultaneously digitized in the fast protocol in order to
obtain the absolute dose distribution in the verification
film.
To evaluate this fast protocol, ten IMRT plans (sites:
prostate, breast, brain, lung and head and neck) were
delivered onto EBT3 films on a Varian linac. Absolute dose
distribution of verification film was derived for each plan