984 / 1096
S968
ESTRO 36
_______________________________________________________________________________________________
Results
The differences in leaf positions compared with film and
light field are beyond 0.1 mm and 1 mm (light field edge
detection has a much bigger uncertainty). The acquisition
and analysis for one strip-test take less than 4 min.
Conclusion
The methodology employed analyzes a MLC strip-test in an
Elekta LINAC in a fast and accurate way.
EP-1758 Towards Clinical Implementation of an Online
Beam Monitoring System
M. Islam
1
, M. Farrokhkish
2
, Y. Wang
2
, B. Norrlinger
2
, R.
Heaton
1
, D. Jaffray
1
1
Princess Margaret Cancer Centre and University of
Toronto, Medical Physics, Toronto, Canada
2
Princess Margaret Cancer Centre, Medical Physics,
Toronto, Canada
Purpose or Objective
Continual advancement of Radiation Therapy techniques
and consequent complexity in planning and delivery
require constant vigilance. To address this, the idea of
independent real-time beam monitoring has been
proposed. In this presentation, we describe initial steps
towards introducing the Integral Quality Monitoring (IQM)
system into clinical practice.
Material and Methods
The IQM system (manufactured by iRT, Germany) consists
of a large-area ion chamber mounted at Linear
Accelerator’s (Linac) accessory slot, which provides a
spatially dependent “dose -area- product” per field
segment. The system monitors beam delivery in real-time
by comparing the expected and measured signals.
Initial evaluation of the system included: reproducibility
and stability, agreements between calculated vs.
measured signals, sensitivity and specificity for errors. A
multiphase approach was considered for clinical
implementation. First, IQM data are collected during
conventional dosimetric QA tests. Results of the QA tests
with and without the IQM chamber in the beam are
compared. During this phase a reference dataset of
measured IQM signals vs. calculated is generated to help
determine the tolerance in the predicated
signals. Second, IQM system is introduced as the primary
pre-treatment QA tool. Gain in work-flow efficiency and
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.