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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. Wanklyn
1
, S. Rizkalla
1
, T. Greener
1
1
Guy'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 &
25x25cm
2
). Two regular cut-outs were also made, a 4x8
cm
2
cut-out for the 10x10 cm
2
applicator and a 10x14cm
2
cut-out in a 15x15 cm
2
applicator to determine the out-of-
field transmission.
Transmission factors through a 10x10 cm
2
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 d
max
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 d
max
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.
Figure 1: Transmission through a closed endplate in a
10x10cm^2 applicator for both materials.
There is higher transmission through the lead endplate
compared to the Cerrobend endplate for all energies but
even at the highest energy the difference is only 0.74%.
The measured PDDs agree with each other to within
1.2mm for all energies. The discrepancies were observed
near the surface of the PDD curves.
The output factors measured in solid water using the lead
inserts agreed with the commissioning values obtained
with the Cerrobend inserts to within ±1% apart from the
6e 10x6cm
2
applicator in which a 1.7% difference in output
factor was observed between the lead and Cerrobend. This
difference could be due to inaccuracies in the solid water
setup as the effective point of measurement of the NACP
was estimated to be 1mm.
The profiles measured agreed very well, with the largest
discrepancies occurring out of field for the higher energies
both crossplane and inplane. This is due to the higher
transmission through the lead cut-out at higher energies.
Conclusion
As there was very good agreement between the lead and
Cerrobend inserts and cut-outs for all the tests performed,
it can be concluded that using the lead cut-outs is
dosimetrically similar to the Cerrobend inserts with which
the eMC algorithm was configured.
EP-1754 Isocentric accuracy of Elekta VersaHD linear
accelerators
E. Kouwenhoven
1
, J. Van Egmond
1
, J. Van Wingerden
1
, M.
De Goede
1
, M. Mast
1
, J. Van Santvoort
1
1
Haaglanden Medical Centre Location Antoniushove,
Radiation therapy, The Hague, The Netherlands
Purpose or Objective
The demands on isocentric accuracy are high when
accelerators are used for stereotactic treatments. The
determination of the optical or mechanical isocenter is
inadequate for this purpose, and instead we aim for a
procedure to find the megavoltage isocenter. The radius
of the smallest sphere through which all rotation axes pass
when various collimator, gantry and table angles are
applied, is what we use as a measure to quantify the
isocentric accuracy. The purpose of the present study is to
(1) give an accurate measure of the size of the region
containing the megavoltage isocenter, and (2) establish
the distance between the rotation axes of table and
collimator, respectively. We developed a highly accurate
method to determine the isocentric accuracy of Elekta
VersaHD linear accelerators.
Material and Methods
The tests were performed on all 4 Elekta VersaHD
accelerators in our institute. We applied a modified
Winston-Lutz test, using a phantom containing a radio-
opaque ball, and imaged this phantom onto the EPID using
a 10 x 10 cm
2
field. Using the information of a considerable