ESTRO 36 Abstract Book

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S951 ESTRO 36 2017
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DC and ArcCHECKTM 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 ArcCHECKTM 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. Mekdash1, B. Shahine1, W. Jalbout1, 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
attempts to correctly position the shielding blocks was
recorded for each beam.
Results
We succeeded in positioning the shielding blocks from the
first attempt in 10 out of 12 beams for the three patients.
The position of the shielding blocks was adjusted only one
time prior to treatment in 2 out of 12 beams. These results
are compared to an average of 3 attempts per beam for
each patient using the conventional technique of trial and
error. The average time of a treatment session was 29 min
with a maximum time of 41 min compared to an average
of approximately 60 min in past treatments and a
maximum of 120 min.
Conclusion
Most of TBI patients are pediatric patients and it is
difficult to keep them immobilized for a long period of
time. This new technique succeeded in reducing the
length of the overall treatment session of the conventional
TBI procedure and hence reduced patient discomfort while
ensuring accurate shielding of the lungs.
EP-1753 Determining the effect of using lead as
electron cutout material compared to low melting
point alloy
M. Wanklyn1, S. Rizkalla1, T. Greener1
1Guy's and St.Thomas' Hospital NHS Foundation Trust,
Radiotherapy Physics, LONDON, United Kingdom
Purpose or Objective
The aim of this investigation was to determine whether
lead cut-outs are suitable for delivering MeV electron
treatments on a Varian TrueBeam which have been
planned using the eMC algorithm in Eclipse.
Due to the eMC algorithm beam data being configured
using Cerrobend low melting point alloy as the cut-out
material it is important to assess the dosimetric
differences between the lead and Cerrobend cut-outs.
Material and Methods
Unlike the Cerrobend cut-outs which are 1.5cm thick, the
lead cut-outs were made to 1cm thickness. This was done
to minimise the cost of lead.
Lead versions of all the standard Varian cut-outs were
made in house (6x6, 10x10, 6x10, 15x15, 20x20 &
25x25cm2). Two regular cut-outs were also made, a 4x8
cm2 cut-out for the 10x10 cm2 applicator and a 10x14cm2
cut-out in a 15x15 cm 2 applicator to determine the out-of-
field transmission.
Transmission factors through a 10x10 cm2 closed end plate
were calculated for the lead and Cerrobend materials for
a range of energies (6, 9, 12, 16, and 18MeV)
PDDs in water at 100cm SSD and output factors in solid
water at dmax at 100cm SSD were measured for the
standard applicators with both the lead and Cerrobend
inserts for all energies.
Cross line and inline profiles at dmax were taken in water
at 100cm SSD for all energies using the two regular cut-
outs.
Results
As can be seen in Figure 1, the transmission through a
closed lead endplate is comparable to that for the
Cerrobend.