ESTRO 36 Abstract Book

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S763 ESTRO 36 2017
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recombination factors were measured with the two
voltage technique for different depths and field sizes. The
effect of polarity was evaluated using both polarities for
measurements. Measurements with different detectors
were carried out for a set of field sizes, ranging from 5x5
to 40x40 cmxcm and SSD 100 cm SSD.
Results
It was found that parallel plate chambers show the closest
agreement between PDD curves acquired with different
polarities, being the differences below 0.1% for all depths
and 40x40 cmxcm.PDDs for one single polarity and
different ion chambers have been corrected for
recombination and compared. The largest difference in
PDD among different ion chambers, once corrected for
recombination, has been found for the Scanditronix Roos
chamber at 350 mm deep (excluding build up) for all field
sizes, which would amount to: 0.7% for 6 FFF and 0.6% for
10 FFF for a 40x40 cmxcm field. Differences between
recombination corrected and uncorrected PDDs, and
among PDDs measured with different detectors, increase
with field size. Differences between recombination-
corrected and uncorrected PDDs were found ranging from
1.2% for PTW Semiflex ion chamber to 2.5% for PTW Roos
ion chamber, both measured for a 40x40 cmxcm at 350
mm deep.
Conclusion
Results show that plane parallel ion chambers can be used
for photon PDD measurements, with minimal polarity
effects, if recombination effects are corrected for as
needed. Medical physicists should use their own clinical
judgement to decide about whether or not PDDs must be
corrected for saturation effects.
EP-1447 Dose Determination in a CT Control Room
Using TLD and Monte-Carlo-Method-Based FLUKA Code
A.H. Yeşil (Turkey), M.G. Aksu, Y. Ceçen
1 Akdeniz University- School of Medicine, Department of
Radiation Oncology, Antalya, Turkey
Purpose or Objective
Computer Tomography (CT) scan is a diagnostic process
where patients are exposed to X-rays on the order of
hundred keVs. X-rays interact with different structures of
the body such as bone, soft tissue, lung etc. They also
interact with other materials present in the room. At the
end they are either absorbed or scattered out of the room.
The CT rooms are designed with sufficient shielding and
licenced by the local authorities however, it is always a
good idea to check for weak spots and ensure that the
radiologists are working in a safe environment. This study
aims to map the radiation dose in the CT control room and
determine the weak spots, if any.
Material and Methods
The work was carried out with both thermoluminescent
dosimeters (TLDs) and Monte Carlo method based FLUKA
code. The radiation dose recieved by the radiologists has
been measured by the TLDs and the results were compared
with the Monte Carlo simulations.
In this study, a third generation 4-slice helical GE Light
SpeedRT CT scanner was used. Scanner has a 80 cm wide
gantry opening and its standart operation is at 120 kV.
TLD-600s were used as passive dosimeters. 15 of them
were located in different positions within the control
room. 30 patients were scanned in a week by 120 kV X-
rays for a total of 90 minutes. Calibrations and readouts
were performed by PTW-TLDO TLD oven and RADOS
RE2000 TLD reader.
FLUKA Code was used to model the CT and the room
around. The doses at the TLD locations were obtained by
the simulation.
Results
The mean value of the TLD measurements was 2.54
µSv/week. FLUKA simulation results had a mean dose of
2.2±0.2 µSv/week. Maximum X-ray dose in the control
room was measured just behind the door 3.73 µSv/week.
The FLUKA simulations also agreed with the
measurements, 3.4±0.3 µSv/week.
Conclusion
Results of this study show that radiologists receive weekly
doses under the limits (0.1 mSv/week) which is compatible
with the literature. Study also shows that the CT model
of the FLUKA code is accurate and can be used in various
X-ray dose studies.
Electronic Poster: Physics track: Dose measurement and
dose calculation
EP-1448 Epid-based in vivo dosimetry for SBRT-VMAT
treatment dose verification
S. Cilla1, A. Ianiro1, M. Craus 1, P. Viola 1, A. Fidanzio 2, L.
Azario2, F. Greco2, M. Grusio2, F. Deodato 3, G. Macchia3,
V. Valentini4, A. Morganti5, A. Piermattei2
1Fondazione di Ricerca e Cura "Giovanni Paolo II"-
Università Cattolica del Sacro Cuore, Medical Physics
Unit, Campobasso, Italy
2Policlinico Universitario "A. Gemelli"- Università
Cattolica del Sacro Cuore, Medical Physics Department,
Roma, Italy
3Fondazione di Ricerca e Cura "Giovanni Paolo II"-
Università Cattolica del Sacro Cuore, Radiation Oncology
Unit, Campobasso, Italy
4Policlinico Universitario "A. Gemelli"- Università
Cattolica del Sacro Cuore, Radiation Oncology
Department, Roma, Italy
5Università di Bologna, Radiation Oncology Center-
Department of Experimental- Diagnostic and Specialty
Medicine - DIMES, Bologna, Italy
Purpose or Objective
In vivo dosimetry (IVD), a direct method of measuring
radiation doses to cancer patients during treatment, has
shown unique features to trace deviations between
planned and actually delivered dose distributions.
Extracranial stereotactic radiotherapy (SBRT) involves the
delivery of high doses in a few fractions (1-5) for ablative
purposes. Then SBRT treatments strongly benefit from IVD
procedures, as any uncertainties in dose delivery is more
detrimental for treatment goals or patient safety. We
assessed the feasibility of EPID-based IVD for complex
clinical VMAT treatments for SBRT.
Material and Methods
15 patients with lung, liver, bone and lymphnodal
metastases treated with Elekta VMAT were enrolled. All
plans were generated with Masterplan Oncentra and
Ergo++ treatment planning systems (Elekta, Crawley, UK)
with a single 360° arc VMAT. All targets were irradiated in
5 consecutive fractions, with total doses ranging from 40
to 50 Gy depending on anatomical sites. All patients
passed pre-treatment 3%/3mm g-analysis verification. IVD
was performed using SOFTDISO (Best Medical Italy), a
dedicated software implemented in our clinic for
conformal, IMRT and VMAT techniques. IVD tests were
evaluate by means of (i) R ratio between isocenter daily
in-vivo dose and planned dose and (ii) γ-analysis between
EPID integral portal images in terms of percentage of
points with γ-value smaller than one (γ%) and mean g-
values (γmean), using a global 3%-3 mm criteria. Alert
criteria of ±5% for R ratio, γ% <90% and γmean > 0.67 were
chosen, the last two in order to accept only 10% of the
values to exceed 3%/3mm and an average discrepancy of
the order of 2%/2mm, respectively.
Results
A total of 75 transit EPID images were acquired. Five
images (6.6%) were removed from analysis for image
deterioration and/or electronic acquisition failures. The
overall mean R ratio was equal to 0.999 ± 0.021 (1 SD) for
all patients, with more than 98% of tests within 5% alert
criteria. The 2D portal images g-analysis show an overall