980 / 1096
S964
ESTRO 36
_______________________________________________________________________________________________
in CT and MRI as contrast agents, but also can be feasible
radiosensitizers in radiotherapy. Hence they are attractive
candidates for multimodal dose enhancement studies. In
this study, the ability of dose enhancement of these
nanoparticles using MAGIC-f polymer gel under the
internal Iridium-192 and the external Cobalt-60
radiotherapy practices were investigated.
Material and Methods
The Bi2O3-NPs less than 40 nm in diameter and 0.1 mM
concentration were synthesized. To increase the precision
of the gel dosimetry a Plexiglas phantom was designed and
made, all of the gel filled vials (with and without the
nanoparticles) were irradiated to an Ir-192 radioactive
source simultaneously. Also, Irradiation was carried out
with a Co-60 teletherapy unit.
Results
The maximum dose enhancement factors under the
internal Iridium-192 radiotherapy were 31% and 22% in the
presence of Bi2O3-NPs and Gd2O3-NPs, respectively,
whereas these amounts were reduced to 1% in external
radiotherapy by Co-60 photons.
Conclusion
The results of our research approves the use of Bismuth
and Gadolinium based nanoparticles in brachytherapy.
Additionally, the polymer gel dosimetry can be a feasible
material for verification and estimation of dose
enhancements in the presence of nanoparticles.
EP-1751 A comparison of tools for Delivery Quality
Assurance in TomoTherapy
T. Santos
1
, T. Ventura
2
, J. Mateus
2
, M. Capela
2
, M.D.C.
Lopes
2
1
Faculty of Sciences and Technology, Physics
Department, Coimbra, Portugal
2
IPOCFG- E.P.E., Medical Physics Department, Coimbra,
Portugal
Purpose or Objective
A TomoTherapy HD unit has recently been installed in our
hospital. The purpose of the present work is to establish
an accurate and efficient method of patient specific
delivery quality assurance (DQA). Four available tools
(EBT3 Grafchromic film, Dosimetry Check –DC –,
ArcCHECK
TM
and RadCalc®) have been tested and
compared.
Material and Methods
Standard patient plan verification in TomoTherapy is done
through film dosimetry in the Cheese Virtual Water
TM
phantom. Also point dose measurements can be performed
inserting ionization chambers in this phantom. A well-
established film absolute dosimetry methodology using
EBT3 Gafchromic film and applying a multichannel
correction method was developed in-house, adapting the
standard approach in the DQA Tomo station. The
treatment plans of the first 21 patients were
retrospectively verified using also Dosimetry Check
software (Math Resolutions, LLC) and ArcCHECK
TM
(Sun
Nuclear). A beta version of RadCalc®6.3 (LifeLine
Software Inc.) for TomoTherapy has been used for
independent treatment time calculation.
DC uses the MVCT detector sinogram to reconstruct the 3D
dose distribution. In this work it was used in pre-treatment
mode with the couch out of the bore. The transit dose
mode where the patient delivered dose reconstruction is
obtained was not assessed in this work. ArcCHECK
TM
records the signal of 1386 diodes embedded as a helical
grid on a cylindrical phantom, enabling 4D volumetric
measurements.
The Gamma passing rate acceptance limit was 95% using a
3%/3 mm criterion in all cases.
Results
All the used QA tools showed a good agreement between
measured and planned doses. Film and DC achieved similar
results with mean Gamma passing rates of 98.8±1.6% (1SD)
for EBT3 film and 97.9±1.6% (1SD) for DC. Moreover, a
correlation was found between those results: when
passing rates using film were poorer (<97%), the same
happened with DC, while passing rates over 97% for DC
corresponded to the same range using film. This
correspondence was not verified with ArcCHECK
TM
where
Gamma passing rates were always close to 100%
(99.6±0.7% (1SD)).
Concerning the independent treatment time verification
with Radcalc®, the percentage difference to the Tomo
TPS calculation was 0.2±2.5% (1SD), on average.
Conclusion
DC and ArcCHECK
TM
allow volumetric dose comparisons
between calculated and measured doses. Moreover DC
displays DVHs and isodose lines for the considered
structures in the plan while 3D-DVH in ArcCHECK
TM
is not
available for TomoTherapy.
DC seems to be a valuable tool for performing patient-
specific DQA however, considering the present Pencil
Beam algorithm and its known limitations, a verification
using film dosimetry and ionization chamber
measurements should be done in case of any significant
discrepancy.
Concerning the beta version for TomoTherapy in RadCalc®
software, it seems to be a valid tool for independent
treatment time verification, easily incorporable in routine
treatment
workflow.
EP-1752 A simple technique for an accurate shielding
of the lungs during total body irradiation
H. Mekdash
1
, B. Shahine
1
, W. Jalbout
1
, B. Youssef
1
1
American University of Beirut Medical Center, Radiation
Oncology, Beirut, Lebanon
Purpose or Objective
During total body irradiation (TBI), customized shielding
blocks are fabricated and positioned in front of the lungs
at a close distance from the patient’s surface to protect
the lungs from excessive radiation dose. The difficulty in
such treatments is to accurately position the blocks to
cover the entire lungs. Any error in the positioning of lung
blocks can give higher doses in the lungs than intended
and can lead to underdosage in the body/target volume.
The conventional technique for the positioning of lung
blocks is based on a time-consuming trial and error
procedure verified at each trial with radiographic films. A
new technique based on CT simulation was developed to
determine the exact position of lung blocks prior to
treatment for each specific patient. This technique of
accurate shield positioning serves the purpose of reducing
lung toxicities and most importantly reduces patient’s
pain and discomfort by minimizing the length of the
overall treatment session.
Material and Methods
Patients were CT simulated in their lateral recumbent
treatment position and lungs were contoured with the aid
of a treatment planning system. Horizontal AP/PA fields
were designed with MLC aperture conforming to lung
contours. The fields were used to project a light field on
the patient’s skin representing the extent of the lungs,
which was subsequently marked on the patient’s anterior
and posterior skin as seen in Figure 1. Prior to each
fraction, the lung blocks were positioned with their
shadow matching the lungs’ marks. The position of the
shielding blocks was radiographically verified prior to the
delivery of each beam as per the usual procedure (Figure
2). Three patients underwent TBI with this new technique.
Each patient received in total six fractions of AP/PA beams
including two fractions with shielded lungs. The lungs
received in total 8 Gy and the rest of the body was
irradiated with the prescribed dose of 12 Gy. To evaluate
the efficiency of this technique, the number of repeated