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ESTRO 36 2017
_______________________________________________________________________________________________
P.H. Mackeprang
1
, D. Vuong
1
, W. Volken
1
, D. Henzen
1
, D.
Schmidhalter
1
, M. Malthaner
1
, S. Mueller
1
, D. Frei
1
, D.M.
Aebersold
2
, M.K. Fix
1
, P. Manser
1
1
Division of Medical Radiation Physics and Department of
Radiation Oncology, Inselspital- Bern University Hospital-
and University of Bern, Bern, Switzerland
2
Department of Radiation Oncology, Inselspital- Bern
University Hospital- and University of Bern, Bern,
Switzerland
Purpose or Objective
To implement and validate a Monte Carlo (MC) based dose
calculation framework to perform patient-specific quality
assurance (QA) for multi-leaf collimator (MLC) based
CyberKnife treatment plans.
Material and Methods
In order to calculate dose distributions independently
from the treatment planning system (TPS), an
independent dose calculation (IDC) framework was
developed based on the EGSnrc MC transport code. The
framework uses XML-format treatment plan input and
DICOM format patient CT data, an MC beam model using
phase space files, CyberKnife MLC beam modifier
transport using the EGS++ class library, a beam sampling
and coordinate transformation engine and dose scoring
using DOSXYZnrc.
Validation of the framework was performed against dose
profiles of single beams with varying field sizes measured
with a diode detector in a water tank in units of cGy /
monitor unit (MU) and against a two-dimensional dose
distribution of a full treatment plan measured with
Gafchromic EBT3 (Ashland Advanced Materials,
Bridgewater NJ) film in a homogeneous solid water slab
phantom. The film measurement was compared to IDC by
gamma analysis using 1% (global) / 1 mm criteria and a 10%
global low dose threshold.
Finally, the dose distribution of a clinical prostate
treatment plan was calculated and compared to dose
calculated by the TPS finite size pencil beam algorithm by
gamma analysis using either 2% (global) / 2 mm or 1%
(global) / 1 mm criteria and a 10% global low dose
threshold.
Results
Dose profiles calculated with the developed framework in
a homogeneous water phantom agree within 3% or 1 mm
to measurements for all field sizes. 87.1% of all voxels pass
gamma analysis comparing film measurement to
calculated dose. Gamma analysis comparing dose
calculated by the framework to TPS calculated dose for
the clinical prostate plan showed 99.9% passing rate for 2%
/ 2 mm criteria and 85.4% passing rate for 1% / 1 mm,
respectively. Dose differences of up to ±10% were
observed in this case near bony structures or metal
fiducial markers.
Conclusion
An MC based modular IDC framework was successfully
implemented and validated against measurements and is
now available to perform patient-specific QA by
independent dose calculation.
EP-1481 Testing algorithms in water and
heterogeneous medium using experimental designs
S. Dufreneix
1
, A. Barateau
1
, M. Bremaud
1
, C. Di Bartolo
1
,
C. Legrand
1
, J. Mesgouez
1
, D. Autret
1
1
Institut de Cancérologie de l'Ouest, Medical Physics,
Angers, France
Purpose or Objective
The IAEA Tecdoc 1580 and 1583 suggest several beam
configurations for testing, commissioning and ongoing
quality assurance of TPS. However, the large number of
tests makes it difficult to implement and results out of
tolerance are often left unexplained. Experimental
designs are a robust statistical method which minimizes
the number of tests to be performed and provides a
statistical analysis of the results. They were used to
compare computed and measured doses for several
algorithms.
Material and Methods
Tests were chosen using a Taguchi table L36 (2
11
x3
12
) to
enable the quantification of the influence of each
parameter. Five algorithms were studied: the AAA (version
11, Varian) is used in clinical routine and the collapsed-
cone convolution-superposition (CCCS) algorithm (version
1.5, Mobius Medical Systems) is used as a secondary dose
calculation plan check. The AcurosXB (AXB) algorithm
(version 11, Varian) was also investigated as well the
pencil beam (PB) and Monte Carlo (MC) algorithms
available on Iplan (version 4.5, Brainlab). Absorbed dose
was first calculated in water for 72 beams with varying
parameters: energy, MLC, depth, wedge angle, wedge
jaw, X, Y
1
and Y
2
dimensions. Computations were then
conducted for 72 beams in a CIRS Thorax phantom with
varying parameters: energy, wedge angle, wedge jaw, X
and Y dimensions, medium and gantry angle. Calculated
doses were compared to measurements conducted on a
Novalis TrueBeam STx (Varian) with a CC04 ionisation
chamber (IBA).
Results
In water, all algorithms gave a mean difference between
computed and measured doses centred on zero (within the
uncertainty). No studied parameter led to statistically
significant deviation. In the thorax phantom, the mean
difference between computed and measured doses was -
0.7 ± 1.1 % for AAA, -1.4 ± 1.4 % for CCCS, -2.5 ± 1.0 % for
AXB, 2.3 ± 2.2 % for PB and 0.3 ± 1.9 for MC. For AAA and
CCCS, calculations in bone medium led to a statistically
significant underestimation of the computed dose while
the other parameters had no influence on the results. For
MC, calculated dose was overestimated for gantry angle of
225° which was attributed to the modelization of the
treatment table by the TPS.
Conclusion
Experimental designs were used as a statistical method to
validate the AAA, CCCS and MC algorithms. The PB
algorithm was rejected for clinical use because it
overestimates the dose in heterogeneous medium. Results
showed that the AXB algorithm systematically
underestimates the dose in heterogeneous medium which
could be linked to the dose to water - dose to medium
conversion as referred in the literature. Further
investigation is needed before its implementation in
clinical routine, especially for modulated beams. The tests
described by the experimental designs were also used to
define the tolerance levels of the secondary plan check
software and are now part of the ongoing quality
assurance of the TPS
.
EP-1482 Signal Prediction for an On-line Delivery
Verification System
R. Heaton
1
, M. Farrokhkish
1
, G. Wilson
1
, B. Norrlinger
1
,
D.A. Jaffray
1
, M.K. Islam
1
1
Princess Margaret Cancer Centre University Health
Network, Radiation Physics, Toronto, Canada
Purpose or Objective
Dynamic radiation delivery techniques like VMAT
introduce challenges in treatment verification. Complex
treatments, as well as hypofraction and adaptive radiation
therapy, require new verification approaches to ensure
safe delivered. One approach is the introduction of
entrance fluence monitoring device, like the Integral
Quality Monitoring (IQM) System (iRT Germany), which
provides a spatially encoded dose area product signal as a
unique delivery fingerprint. Complementary to this
measurement is the signal calculation based on the
treatment plan. This work describes the calculation for
the IQM system and examines the impact of selected
components on clinical fields.
Material and Methods