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ESTRO 36
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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
modalities and choice of associated cases need to take
into consideration the utility for clinical trials.
Results
The Level III audit is an end-to-end test using a humanoid
thorax phantom (CIRS, Norfolk, VA). The custom phantom
has a central insert for either conformal modality with two
farmer chambers, or for IMRT and VMAT with seven CC13
ion chambers as the primary detectors. The IMRT/VMAT
central insert includes a film holder for supplementary
measurements. The custom phantom includes removable
lungs that are replaced with solid water inserts to
investigate the effect of inhomogeneity on IMRT and VMAT
deliveries. Figure 1 shows the custom phantom. The CC13
chambers are connected to the TomoTherapy
®
TomoElectrometer, an 8 channel reference class
electrometer for simultaneous measurement on all
chambers for each audit case.
The IMRT and VMAT planning cases were designed for
addition to the current Level III audit. Clinical plans were
prepared based on the AAPM Publication TG119 [1] and
adapted for use in the ACDS audit program. Table 1 shows
an example of how the audit outcomes are reported. Each
modality is scored separately, and assigned a Pass
(Optimal Level), Pass (Action Level), or Out of Tolerance
outcome. Field trials on the IMRT, VMAT and FFF
modalities began in September 2016. In 2017 the new
modalities scoring criteria will be finalised and the new
modalities will go live, and field trials on a SABR modality
are scheduled to begin.
Table 1
Example of modality scoring in the ACDS Level III
audit.
Figure 1
Images of the custom CIRS phantom for the new
ACDS Level III audit, showing the removable lungs and
removable central insert.