915 / 1096
S899
ESTRO 36
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Conclusion
IVD with EPID, is a powerful tool that can be inserted in
an overload radiotherapy department. It can be helpful
daily to monitor the accuracy of the treatment and enable
a quickly correction of misalignment or discrepancies
occurred during the treatment course.
EP-1655 Improved patient setup for breast cancer
patients using the predicted (absolute) couch position.
M. Essers
1
, S. Hol
2
, I. Maurits
2
, W. Kruijf
1
1
Dr. Bernard Verbeeten Instituut, Department of Medical
Physics, Tilburg, The Netherlands
2
Dr. Bernard Verbeeten Instituut, Radiotherapy, Tilburg,
The Netherlands
Purpose or Objective
Usually, patient setup is performed by obtaining a
reference position at the first treatment fraction
(“relative couch position”) and then applying on-line or
off-line setup protocols . In our institute, a method is used
in which the couch position is predicted before the
treatment (“absolute couch position”)
1
. The purpose of
this work was to investigate whether the patient setup for
breast cancer patient is improved using the ‘absolute
couch position”method.
Material and Methods
At the time of this study, accurate patient setup was
ensured by applying an on-line setup protocol using the
patient anatomy (mainly vertebrae, lung tops and
sternum) visible on orthogonal (AP and lateral) MV images,
and checking the residual deviation of the lung wall on an
MV image in the direction of the mediolateral tangential
field (ML image).
1. For 83 patients positioned using relative couch
positioning as well as 83 patients positioned using absolute
couch positioning, the difference in image registration (MV
images compared to DRRs) using sternum only or vertebrae
only
was
determined.
2. For the same patients, the residual deviation of the lung
wall on the ML image was determined.
Results
1.
Using relative couch positioning, the difference
between sternum and vertebrae match was
smaller than 2 mm for 80% and larger than 5 mm
for 12% of the fractions, and for absolute couch
positioning, 90% of the fractions showed a match
difference smaller than 2 mm and 7% larger than
5 mm. These figures indicate that the patient
posture at the linac is slightly in better
agreement with the posture at the CT for
absolute couch positioning.
2.
On the ML images, a residual deviation in lung
wall position of 5 mm or more was present for
5.1% of the fractions for relative couch
positioning and for 2.7% for absolute couch
positioning. This also implies a slightly improved
patient setup using absolute couch positioning.
Conclusion
The patient posture as well as patient setup for breast
cancer patients is slightly improved using the predicted,
or absolute, couch position.
1
W.J. de Kruijf, R.J. Martens, Reducing patient posture
variability using the predicted couch position. Med.
Dosim, 40:218-21; 2015.
EP-1656 The inter-fraction variation of the
supraclavicular- and the axilla-area in breast cancer
patients
S. Gerrets
1
, L. Kroon- van der
1
, M. Buijs
1
, P. Remeijer
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiotherapy, Amsterdam, The Netherlands
Purpose or Objective
Currently a volumetric modulated arc technique (VMAT)
for whole breast irradiation, including the supraclavicular-
and axilla-area is being implemented at our institute. In
contrast to our currently used tangential fields, VMAT
requires a CTV and PTV with corresponding margins. From
our clinical experience we know that the setup of the
shoulder can be very challenging..The purpose of this
study was to quantify the inter-fractional variation of the
supraclavicular- and the axilla-area in order to quantify
CTV to PTV margins.
Material and Methods
So far 6 right sided and 6 left sided breast cancer patients,
were randomly selected in this ongoing study. Patients
were positioned on a Macromedics MBLXI breastboard with
upper- and lower- arm trays. During the acquisition of the
planning CT skin marks were drawn extended to the
humerus to improve reproducibility of the arm positioning.
Setup verification and correction was performed based on
bony anatomy registration (ribs and sternum) using Cone
beam CT and an offline shrinking action level (SAL)
protocol. Retrospectively, the residual inter-fraction
errors of the supraclavicular area and the axilla were
measured by performing bony anatomy registrations using
a rectangular region of interest representative for these
areas/regions (see Figure 1 'Region of interest”), and
determining their difference from the registration on ribs
and sternum. From these residual errors, the random and
systematic errors were computed and corrected for the
use of a SAL protocol (N=3 and α=9mm). Using previously
determined setup data from Topolnjak et al [1],
Subsequently, the CTV to PTV margins were determined
according to the standard margin recipe: 2.5∑+0.7σ.
Results
In total 88 Cone beam CT were analyzed; 5-10 scans per
patient. Computed residual errors for the supraclavicular
region and axilla region are shown in Table 1.The random
and systematic residual errors for the axilla regions are
larger than the supraclavicular region, as expected.
Notable is the small residual error for supraclavicular in
LR-direction. The total margins are 0.59cm LR, 0.76cm CC
and 0.81cm AP for the supraclavicular region and 0.84cm
LR, 0.89cm CC and 0.98cm AP for axilla region.
Conclusion
For the introduction of a VMAT planning technique for
breast and axilla irradiation specific PTV margins adapted
for supraclavicular and axilla inter-fraction motion need
to be introduced.
1
Topolnjak, et al, IJROBP, Volume 78, Issue 4, 15
November 2010, Pages 1235–1243
EP-1657 Clinical use of transit dosimetry to analyze
inter-fraction motion errors
F. Ebrahimi Tazehmahalleh
1
, C. Moustakis
1
, U.
Haverkamp
1
, H.T. Eich
1
1
Universitaetsklinikum Muenster, Klinik fuer
Strahlentherapie, Münster, Germany
Purpose or Objective
The aim of this work was to inquire the correlation
between the target and organ at risks motions and volume
discrepancy with the dosimetric variations at hospital
levels. The high resolution, large active area, and
effectiveness of the Electronic portal imaging devices
offers it to be used for in vivo dosimetry during radiation