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quality have been assessed. Subsequent real-time
monitoring issues have been considered.
Results
The presence of IQM chamber in the beam path changed
the pass rate of MapCheck (IMRT), and ArcCheck (VMAT)
within ±1% when %Dose–DTA criteria were employed, as
shown in Table-1. The calculated and measured IQM
signals on a Varian TrueBeam Unit for 340 randomly chosen
IMRT field segments from clinical plans show good
agreements (Fig.1): 95% of segments are within ±3; total
cumulative signals for VMAT delivery were within± 2%.
By introducing the system as a pre-treatment QA tool 700
hours of staff time and 180 hours of machine time can be
saved annually, for a facility treating 240 IMRT and VMAT
patients per day. Additionally, the segment by segment
dosimetry and independent gantry angle monitoring
provides added quality values for pre-treatment QA.
Daily monitoring of beam delivery may save more machine
and staff time by eliminating pre-treatment QA, while
improving patient safety. However, a number of issues
need to be addressed, such as: (1) Modification of TPS
beam model to include the effect of the IQM chamber on
the beam (2) The TPS should allow an accessory code in
IMRT and VMAT (3) The Linac manufacturer should make
an accessory code available for the on-line monitor.
Conclusion
Clinical implementation of the system in multiple phases
helps understanding the performance characteristics of
the system, allows smooth transition of QA practices,
make overall clinical workflow safe and effective for real-
time beam monitoring.
EP-1759 MLC positioning study based on EPID images
analyzed with the Dosimetry Check software
C. Avigo
1
, M. Mignogna
2
, S. Linslata
3
1
National Research Council, Institute of Clinical
Physiology, Pisa, Italy
2
Azienda USL Toscana nord ovest- S. Luca Hospital,
Radioterapia, Lucca, Italy
3
Azienda USL Toscana nord ovest- S. Luca Hospital, Fisica
Sanitaria, Lucca, Italy
Purpose or Objective
The IMRT requires extensive knowledge of the MLC
position accuracy and repeatability since when accurate
leaf positioning is lost significant dose delivery errors can
occur. Therefore, the MLC QA is crucial for a complete
control of the patient treatment. The use of EPID for this
scope can be very helpful in saving time providing images
with high spatial resolution and directly digitalized.
Dosimetry Check is a commercial software which uses EPID
images for pre-treatment verification, in vivo-dosimetry
and also MLC QA. The aim of this work was to validate the
combined system EPID-Dosimetry Check, in order to
control the leaf positioning of an Elekta Agility MLC.
Material and Methods
The leaf position is defined as the position of the 50% of
the dose profile. This measurement depends on relative
position of the beam source and of the leaves. In order to
validate the EPID measurements of the absolute leaf
positions, 10 dose profiles, at the center of different leaf
pairs of the same MLC field, were acquired with an Elekta
iViewGT EPID and with a diode positioned in a water
phantom. The comparison between the two detectors was
performed by Matlab. Garden Fence (GF) was chosen as
test of the leaf position accuracy and a preliminary study
on the gap width was conducted. Leaf position accuracy
was checked automatically with DC by acquiring GF at the
4 cardinal gantry angles and with all the beam energies (6,
10 and 15MV), while the reproducibility was tested with 5
GF repeated in one day and 6 repeated in a time interval
of 70 days.
Results
The difference between EPID and diode absolute
measurement of the leaf positions was less than 0.8mm
for all the analyzed leaves, resulting from the summation
of an error due to the isocenter identification (0,5mm)
plus the leaf positioning error (0.2mm). The gap width
study revealed that, because of the penumbra widening
observed in small fields, the leaf position could be
accurately measured as the 50% of the edge profile, only
if the gap width is equal or larger than 16mm with 6MV
beam. Therefore, GF with 20mm gap was chosen as leaf
position accuracy test for all the energies in order to
distinguish the effect of beam source from that of leaf
positioning. For the GF at different gantry angles the
difference between the measured and the prescribed
position was well within 1.0mm for all the leaves.
Moreover, reproducibility of each leaf position resulted to
differ from its average value less than 0.4mm.
Conclusion
This work permitted to assess the accuracy and the
repeatability of the Elekta Agility MLC leaf positioning by
the combined use of the Elekta IviewGT EPID and the
Dosimetry Check software through the acquisition and the
analysis of Garden Fence test. This system was validated
comparing the EPID with a diode in a water phantom and
assessing the minimum gap width necessary for an
accurate leaf position measurement at all energies which
is useful to distinguish issues related to beam symmetry
from those related to leaf positioning.
EP-1760 A simple method for estimating the
longitudinal isocentre shift due to gantry motion
R. Hudej
1
, D. Brojan
1
, S. Pulko
2
, P. Peterlin
1
1
Institute of Oncology Ljubljana, Department of
Radiophysics, Ljubljana, Slovenia
2
University Clinical Centre Maribor, Department of
Oncology- Radiotherapy Unit, Maribor, Slovenia
Purpose or Objective
The isocentre as a point of intersection of the three
rotational axes (gantry, collimator and treatment couch)
ideally remains fixed in space during the rotation of
gantry, collimator, or the treatment couch. Due to the
mechanical limitations, gantry sags slightly, and
consequently the radiation isocentre shifts slightly
towards the treatment couch when the gantry rotates
from the uppermost to the lowermost position. The
purpose of this study is to assess this shift.
Material and Methods
A strip of radiochromic film embedded in a suitable water-
equivalent phantom is irradiated with a cross-line half-slit
field from the top (0°). Then the gantry is rotated to the
lowermost position (180°) without moving the jaws and
the phantom is irradiated again. The film is scanned and
analysed with an image analysis script. The central lines
of both half-slit images are determined, then the
intersection angle between them is calculated, and finally
the distance between the intersections of extrapolated
lines with the 'sagittal” plane is calculated.