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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
region of the image, it was possible to find the geometry
(i.e., ball center and field outline) in much more detail
than just subpixel accuracy. Using a set of at least 8
images with various gantry and collimator angles we could
accurately obtain the isocentric accuracy per gantry
angle. A consecutive set of 16 images allowed for an
analysis giving the distance of the table rotation axis to
the collimator rotation axis. We were able to adjust the
table position slightly to obtain accuracies necessary for
stereotactic application.
Results
The method was tested, and we found an accuracy (1 SD)
of 0.01 mm. Four new Elekta accelerators (Versa HD) were
analyzed according the procedure. The main contribution
to isocentric inaccuracy for Elekta linacs is the gantry sag.
By adjusting the table rotation axis to a position between
the collimator rotation axes at gantry 0° and 180°,
isocentric accuracy can be optimized. The table presents
the results that were obtained.
Table: r
isoc
: the size of the isocenter quantified by the
radius of the sphere containing rotation axes when
applying several gantry angles at zero table angle. d
table-
coll
: the distance between rotation axis of the table and
the rotation axis of the collimator.
Linac
r
isoc
(mm)
d
table-coll
(mm)
A
0.68
0.20
B
0.53
0.14
C
0.77
0.06
6
0.38
0.01
Figure: an EPID image of a 10x10 cm
2
field and the ball
bearing. The field outline and the detected ball are
overlayed.
Conclusion
With our method it is possible to quickly obtain a measure
for isocentric accuracy. In combination with table rotation
we achieved accuracies better than 0.9 mm, after
adjusting the table.
EP-1755 Multi-modality end-to-end audit by the ACDS
J. Lye
1
, F. Gibbons
1
, M. Shaw
1
, A. Alves
1
, S. Keehan
1
, I.
Williams
1
1
Australian Radiation Protection and Nuclear Safety
Agency, Australian Clinical Dosimetry Service,
Melbourne- Victoria, Australia
Purpose or Objective
The Australian Clinical Dosimetry Service (ACDS) has
commissioned a custom phantom and audit incorporating
conformal, IMRT, VMAT, and FFF modalities. The design
covers future inclusion of small field and SABR modalities.
The vision of the ACDS is to provide a comprehensive suite
of audit modalities covering all common clinical practice,
ultimately to ensure patient safety and to improve
national dosimetry. The ACDS also aims to provide
dosimetric information that can be used domestically and
globally in the clinical trial setting
Material and Methods
To ensure efficient delivery of the audit service, all
modalities relevant to a facility’s clinical practice are
measured in a single audit visit. The incorporation of new
audit modalities requires a consideration of phantom
design suitable for multiple modalities and limitations on
facility and ACDS workload. Classification of new