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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
recalculation of delivered dose to the planning CT.
PO-0796 Optimisation of plan robustness to sinus filling
in a magnetic field.
A. Pollitt
1
, R. ChuteR
1
, P. Whitehurst
1
, R. MacKay
1
, M.
Van Herk
2
, A. McWilliam
2
1
Christie Hospital NHS, Radiotherapy, Manchester,
United Kingdom
2
University of Manchester, Division of Molecular and
Clinical Cancer Science, Manchester, United Kingdom
Purpose or Objective
The MR Linac (Elekta AB, Stockholm, Sweden) will provide
on-treatment MR imaging allowing for excellent soft tissue
imaging. Such a machine will become an integral part of
the drive towards daily online adaptive radiotherapy.
However, the presence of the magnetic field results in the
Lorentz force and will cause an increase or decrease in
dose to superficial tissues (Raaijmakers et al. 2007). This
is particularly pertinent for sinus cancers, of which 60%
are squamous cell carcinoma’s and primarily on the
surface layer of the nasal cavity. Recent studies (Bol et al
2015, Uilkema et al. 2015) have been performed to
determine the effect of the Lorentz force on low density
cavities in the body. This abstract aims to investigate the
effect of the magnetic field on plan quality and
optimisation for varying sinus filling and emptying states.
Material and Methods
Ten patients with PTV’s overlapping the sinus cavity were
selected from the clinical archive. For each patient four
plans were optimised at 60Gy in 30 fractions, 2 with no B-
field and 2 with the 1.5T B-field present. For each, 1 plan
assumed full sinuses with the volume overridden to 1gcm
-
3
and the other assumed empty sinuses with the volume
overridden to 0gcm
-3
. All plans were created using Monaco
(v5.19.07, Elekta AB Stockholm, Sweden) and met the
departmental constraints for Target and OAR doses. To
investigate the effect of a change in sinus filling, plans
were recalculated on their opposite filling state, i.e plan
optimised on a full sinus was recalculated on an empty
sinus. The difference in dose between the two plans for
target coverage and OARs was calculated. This comparison
will determine the magnitude of the effects from sinus
filling in each scenario. Investigating the range of dose
differences will provide information on how to optimise
these plans to minimise the effect of the Lorentz force.
Results
The change in dose to the Target for the different filling
and magnetic field combinations can be seen in Figure 1.
Several of the dose differences for plans optimized on an
empty cavity, for both with and without B-field show a
shift of the mean of the distribution which is greater than
2% (considered potentially clinically significant). i.e. mean
Dose = 2.36%; V
50%
= 2.26%; V
5%
= 3.12%; V
2%
= 3.21%. An
OAR which also saw a difference greater than 2% was the
Brainstem PRV 1cc max = 2.16 %.