ESTRO 36 Abstract Book

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S425 ESTRO 36
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Results
The first round of in-vitro tests in clinical ambient allowed
to demonstrate that fiber optical based dosimeters are
suitable for dosimetry in regimes such the ones in HDR
prostate brachytherapy. The versatility of this kind of
device and easiness of use allows application in other
radiotherapy modalities. The dosimeter response was
evaluated under irradiation with a 10.07 Ci Ir-192 HDR-
brachytherapy The dosimeter shows a linear response with
dose and is capable of detecting µGy dose variations like
an ionization chamber.
Conclusion
The first round of in-vitro tests in clinical ambient allowed
to demonstrate that fiber optical based dosimeters are
suitable for dosimetry in regimes such the ones in HDR
prostate brachytherapy. The versatility of this kind of
device and easiness of use allows application in other
radiotherapy modalities.
The dosimeter shows a linear response with dose and is
capable of detecting µGy dose variations like an ionization
chamber.
PO-0798 Identification of areas of high-risk skin toxicity
in SBRT and IMRT treatments
P. Carrasco de Fez1 , M.A. Duch2, N. Jornet1, M. Lizondo1,
T. Eudaldo1, A. Latorre-Musoll 1, C. Cases1, 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
Ciències Energètiques, Barcelona, Spain
Purpose or Objective
The AAPM TG176 shows examples of dramatic unexpected
skin effects. This work is aimed:
1. To develop an in-phantom method to identify
what areas of skin could be exposed to high
doses and are of potential risk in order to decide
where to perform skin in vivo dosimetry on SBRT
(3DCRT) and IMRT treatments (sliding windows).
2. To assess the impact of 2 table tops for these
techniques.
Material and Methods
• Two Clinac 2100 CD (Varian) with the Exact
Couch, one with the Exact Couch top (carbon
fiber grid) and one with the IGRT Couch top
• EBT3 films+Film QA Pro 2014 Software
(Ashland)+EPSON EXPRESSION 10000XL scanner
• 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.