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ESTRO 36
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PO-0772 Patient-specific realtime error detection for
VMAT based on transmission detector measurements
M. Pasler
1
, K. Michel
2
, L. Marrazzo
3
, M. Obenland
4
, S.
Pallotta
5
, H. Wirtz
4
, J. Lutterbach
6
1
Lake Constance Radiation Oncology Center, Department
for Medical Physics, Friedrichshafen, Germany
2
Lake Constance Radiation Oncology Center- Martin-
Luther-Universität Halle-Wittenberg, Department for
Medical Physics- Naturwissenschaftliche Fakultät II,
Friedrichshafen, Germany
3
AOU Careggi, Medical Physics Unit, Florence, Italy
4
Lake Constance Radiation Oncology Center, Department
for Medical Physics, Singen, Germany
5
University of Florence- AOU Careggi, Medical Physics
Unit- Department of Biomedical- Experimental and
Clinical Sciences, Florence, Italy
6
Lake Constance Radiation Oncology Center,
Radiooncology, Singen, Germany
Purpose or Objective
To investigate a new transmission detector for online dose
verification. Error detection ability was examined and the
correlation between the changes in detector output signal
with γ passing rate and DVH variations was evaluated.
Material and Methods
The integral quality monitor detector (IQM, iRT Systems
GmbH, Germany) consists of a single large area ionization
chamber which is positioned between the treatment head
and the patient. The ionization chamber has a gradient
along the direction of MLC motion and is thus spatially
sensitive. The detector provides an output for each single
control point (segment-by-segment) and a cumulative
output which is compared with a calculated value.
Signal stability and error detection sensitivity were
investigated. Ten types of errors were induced in clinical
VMAT plans for three treatment sites: head and neck (HN),
prostate (PC) and breast cancer (MC). Treatment plans
were generated with Pinnacle (V.14) for an Elekta synergy
linac (MLCi2). Geometric errors included shifts of one or
both leaf banks for all control points toward (i) and away
(ii) from the central axis of the beam and unidirectional
shifts of both leaf banks (iii) by 1 and 2mm, respectively.
Dosimetric errors were induced by increasing the number
of MUs by 2% and 5%.
Deviations in dose distributions between the original and
error-induced plans were compared in terms of IQM signal
deviation, 2D γ passing rate (2%/2mm and3%/3mm) and
DVH metrics (D
mean
, D
2%
and D
98%
for PTV and OARs).
Results
For segment-by-segment evaluation, calculated and
measured IQM signal differed by 4.7%±5.5%, -2.6%±4.6%
and 4.19%±6.56% for MC, PC und HN plans, respectively.
Concerning the cumulative evaluation, the deviation was
-1.4±0.25%, -6.0±0.3% und -1.47%±0.97%, respectively.
Signal stability for ten successive measurements was
within 0.5% and 2% for the cumulative and the segment-
by-segment analysis.
The IQM system is highly sensitive in detecting geometric
errors down to 1mm MLC bank displacement and
dosimetric errors of 2% if a measured signal is used as
reference. Table 1 reports IQM signal deviations for a
range of introduced errors.
Regarding MLC errors affecting the field size, large
deviations from reference were observed in the IQM
signal, while unidirectional shifts introduced deviations
below detection limit. A similar behavior was observed for
2D γ and DVH parameters. Figure 1 illustrates the
correlation of D
mean
(PTV) and IQM signal deviation,
indicating that clinically relevant errors can be identified.
Conclusion
The deviation between calculated and measured signal is
relatively high, therefore a measurement should be
defined as reference. With this limitation, the system is
not yet capable of treatment plan verification but is a
powerful tool for constancy testing.
The detector provides excellent signal stability and is very
sensitive regarding error detection. The signal deviation
correlates with 2D γ and DVH metric deviations; this
information can be used for identifying action limits for
the IQM.
PO-0773 Three-dimensional radiation dosimetry based
on optically-stimulated luminescence
M. Sadel
1
, E.M. Høye
2
, P. Skyt
2
, L.P. Muren
2
, J.B.B.
Petersen
2
, P. Balling
1
1
Aarhus University, Department of Physics and
Astronomy, Aarhus, Denmark
2
Aarhus University Hospital, Department of Medical
Physic, Aarhus, Denmark
Purpose or Objective
Modern radiotherapy employs complex 3D radiation fields
to deliver therapeutic doses during treatment, and
detailed quality assurance is a prerequisite. Methods
based on luminescent passive detectors, such as optically
stimulated luminescence (OSL), are widely applied,
especially for personal dosimetry and phantom
measurements. Reusability is one advantage of using OSL
for dosimetry; the OSL particles can be reset by
temperature or light-bleaching. Furthermore, the OSL
material used in this study has a wide dynamic range and
linear dose response, and the dosimeter matrix consists of
a flexible material that can be cast into anthropomorphic