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delivered and accepted based on certificates provided by
the manufacturer and only minimum testing is performed.
At MedAustron, advanced acceptance testing procedures
of the QA equipment were additionally developed and
implemented by medical physicists. QA equipment passing
the advanced acceptance testing procedure was further
commissioned. This works consolidates preliminary
understanding for range and spot measurement equipment
(Grevillot et al, PTCOG2016), using additional data
obtained during beam delivery commissioning (e.g. long
term reproducibility of QA equipment). Additional
detectors were also commissioned, such as a 2D array of
ionization chambers and a diamond detector. The
commissioning methods developed allowed determining
the accuracy of the QA devices in clinical conditions and
better define the QA tolerances for periodic quality
assurance of the beam delivery system. The purpose of
this work is to guide medical physicists in the
implementation of dosimetry equipment and associated
phantoms as a pre-requisite to acceptance testing,
commissioning and further QA checks of the facility itself.
Material and Methods
A x-ray check was carried out for each ionization chamber
to verify the integrity of its construction. Positioning
accuracy of the water phantom scanning mechanism was
evaluated in 3D against laser tracker measurement. Range
measurement equipment was carefully calibrated by
measuring the entrance WET of the device. Long term
reproducibility of a multi-layer ionization chamber
detector used for daily QA of beam ranges was determined
and used to define morning QA tolerances of the beam
delivery system. Spot measurement equipment was
evaluated in terms of spot size, position and 2D
homogeneity against radiochromic films. Transverse
profiles acquired in water with diamond detector were
evaluated against pin-point detectors (Figure 1). An
ionization chamber-based 2D array was evaluated in terms
of effective depth of measurement, recombination, 2D
homogeneity and absolute dose against ROOS chamber.
Extensive commissioning procedures were developed
specifically for each piece of QA equipment, based on its
intended use.
Results
The advanced acceptance testing procedures developed
resulted in the identification of defects in several devices
from different manufacturers before clinical use. The
commissioning procedures allowed maximizing the
performance of the QA equipment and consequently the
quality of medical commissioning activities. Main overall
uncertainties
are
presented
in
Table
1.
Conclusion
The procedures implemented at MedAustron significantly
improved the knowledge and the performances of the
dosimetric equipment and therefore the quality of
medical commissioning activities at the facility. Our
experience shows that commissioning of QA equipment is
a necessary step towards high precision radiation therapy.
EP-1451 Validation of local tolerances for VMAT
patient specific QA using the IBA Compass system
E. Crees
1
, R. Hulley
1
, G. Kidane
1
, Y. Miao
1
1
Queen's Hospital, Department of Medical Physics,
Romford, United Kingdom
Purpose or Objective
The aim of this study was to review the locally set
tolerances for VMAT Head and Neck, Brain and Prostate
and Pelvic Nodes patient specific QA using the IBA
COMPASS MatriXX
evolution
measurements and computations
with the IBA COMPASS® system. Radiotherapy treatment
planning requires independent verification of both the
treatment planning system (TPS) dose calculation and the
patient dose delivery system. The verification of the dose
delivery system can be carried out independently to the
TPS dose calculation (in which case individual treatment
plans may be verified using a 2
nd
calculation alone) or can
be incorporated into individual patient specific
measurements. The pass/fail tolerances of patient
specific QA using the Compass system have been set
locally. The calculation method verification tolerance was
set for the comparison between the Compass dose
calculation and the treatment planning system
calculation. The measurement method verification
tolerance was set by comparing the MatriXX
evolution
measured dose to the treatment planning system
calculated dose.
Material and Methods
A retrospective study was performed on Head and Neck,
Brain and Prostate and Pelvic Nodes VMAT plans that were
produced on the Eclipse TPS using the AAA algorithm. Each
treatment plan was re-calculated using the Compass
software and the dose distribution of each plan was
measured using the Compass MatriXX
evolution
. Subsequently,
the Compass computed and measured dose distributions
were compared to the TPS calculated plan using gamma
index analysis. Bland Altman statistical analysis was
employed to compare gamma index results of the Compass
calculated and Compass measured dose distributions. The
analyses were performed based on the agreement
between the treatment planning system compared to the
measurement and Compass calculations. The tolerances
were set on absolute dose difference (2% for computation
and 3% for measurement on all points) and global gamma
index assessment (2%/2 mm criterion for 98% of points –
computation and 3%/3mm criterion for 95% of point -
measured).
Results
Across all treatment sites, the mean gamma index was
99.6% for the calculated dose and 98.3% for measured