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e.g. in calculation, positioning and movement, spatial
precision and absolute dose application. We present a test
that was introduced into the clinical workflow and
evaluated its sensitivity to those errors.
Material and Methods
Prior to the irradiation, a custom-built phantom insert for
the ArcCHECK (Sun Nuclear, USA) allowed for automatic
registration of the cone beam CT to reference data. A 12-
field plan including gantry and table rotations targeting a
spherical volume of approx. 2 cm diameter was measured
weekly using a Synergy accelerator with an Agility MLC
(Elekta, Sweden). Signals were obtained from all diodes
along the cylinder surface of the ArcCHECK and additional
dose was measured with an ionization chamber in the
phantom center. For each measurement the plan was
compared to the calculation of the treatment planning
system via gamma evaluation and every diode reading was
compared to the averaged diode readings from previous
weeks. Additionally, errors were induced to test the
sensitivity for phantom malposition, machine geometry
problems and MLC positional inaccuracies.
Results
Due to the phantom set up according to the cone beam CT
registration, the measurements were very reproducible
without any observable user-to-user differences. The
typical dose map for the diode cylinder is shown in fig. 1.
For all diodes, mean values with small standard deviations
were obtained from many consecutive measurements. Any
diode deviation observed for the correct application of the
test plan never exceeded three standard deviations, while
much larger discrepancies could be detected for all
induced errors (example: fig. 2).
Conclusion
We developed a fast end-to-end test for stereotactic
radiation therapy with the ArcCHECK phantom which
minimizes user influences for high reproducibility and was
easily included into clinical routine. It compares the dose
distribution on a helical diode array and a cumulative
central dose with the doses from the treatment planning
system. By additionally comparing each of the over 1300
diode values to a corresponding average dose derived from
previous measurements, the method simultaneously
serves as a constancy test of all involved components and
is able to reliably detect a vast variety of even very small
errors.
PO-0795 Comparison of Service graph log and Dynamic
linac log of Elekta Linacs for patient QA.
M. Kowatsch
1
, M. Meinschad
1
, G. Leitold
1
, P.
Szeverinski
1
, T. Künzler
1
1
LKH Feldkirch, Institut of Medical Physics, Feldkirch,
Austria
Purpose or Objective
The complexity of intensity modulated radiation therapies
(IMAT, IMRT) requires patient specific pretreatment
verification of calculated dose distributions which is time
consuming. Elekta linacs provide 2 different log files. One
is the Service graph (SG) with a resolution of 4 Hz and is
directly accessible through the service mode on the linac.
The second one is the Dynamic linac log (DLL) with a
resolution of 25 Hz. The aim of this study is to compare
both types of log files for dose recalculation with Monte
Carlo and beam statistics for an Elekta Synergy linac with
Agility MLC (Elekta, Crawley).
Material and Methods
To compare the log files 2 head & neck, a mamma left
side, an abdomen with simultaneous integrated boost, a
thoracic spine with 3 dose levels and 1 brain case were
chosen. Different parameters like leaf travel (LT), the sum
of travel of all leaves between the open jaws, leaf speed
(LS), leaf position (LP) and modulation complexity score
(MCS) (Masi, Med. Phys. 40, 071718, 2013) were compared
between the SG and the DLL. The DICOM RT file was used
as reference for comparing LT and MCS. Furthermore log
files were converted with an in-house Matlab script to .tel
files to recalculate the irradiated plans with Monaco 5.0
TPS (Elekta, Crawley). For recalculation a grid size of 3mm
and an uncertainty of 1% per control point were used
resulting in a final uncertainty of roughly 0.1%. Isodose and
DVH comparison were performed to evaluate equality of
recalculated and originally calculated plans.
Results
The difference for leaf travel between SG and DLL to the
Dicom-RT file was between -9.5% to 2.7% and -0.4% to
6.2%, respectively and between SG and DLL from -2.8 to -
11.3%. The differences of the MCI between the two log
files was -0.4% to 0.3% and up to 20% compared to the
DICOM file (see Table 1). The difference of 20% for plan 6
originates from the definition of LT. In this case, 2 beams
with 2 arcs were evaluated. For SG and DLL all beams were
evaluated as a single beam, the Dicom RT files were
evaluated beam-by-beam. The maximum LT for a
particular leaf between 2 control points (CP) showed big
discrepancies and was in one case 20.1 mm for the SG and
32.6 mm for the DLL. The differences originate from
writing errors between CPs in the SG and these errors are
still inexplicable.Random dose errors in DVH up to +-0.5
Gy can be seen by recalculation of both log files for the
entire plan. For linac parameter statistics (LT, LS, LP) the
SG cannot be used because of random writing errors.
Conclusion
Both file types are accurate for dose recalculation. The 4
Hz resolution and writing errors of the Servicegraph log
are limiting a robust statistical analysis of linac
parameters. Dynamic linac logs allow for dose
recalculation and for a more detailed statistical analysis
of the linac. Both types of log files can be taken for patient
QA to decrease the workload of measurements and for