ESTRO 36 Abstract Book

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S881 ESTRO 36 2017
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Conclusion
96% of the patients trained were suitable for this
technique. In 80% of the cases, DIBH treatment was chosen
over FB. DIBH 3D RT for left breast cancer showed
significant dose reduction for heart in every patient, and
the left lung usually also benefits of it, achieving
comparable coverage to PTV.
EP-1645 Optimization of on-line setup verification and
adaptive radiotherapy for breast cancer patients
M. Essers 1, S. Hol2, I. Maurits2, Y. Nijs2, T. Donkers2, L.
Pontzen 2, S. Toemen2 , L. Mesch2, K. De Winter2
1 Dr. Bernard Verbeeten Instituut, Department of Medical
Physics, Tilburg, The Netherlands
2 Dr. Bernard Verbeeten Instituut, Radiothearpy, Tilburg,
The Netherlands
Purpose or Objective
In recent years, new imaging modalities besides the long
existing megavolt (MV) imaging using an electronic portal
imaging device (EPID) have become available at the linac,
to ensure accurate patient setup during the course of the
radiotherapy treatment. The purpose of this work was to
study the usefulness of all possible imaging modalities and
to develop the optimal online imaging protocol for breast
cancer patients in our institute.
Material and Methods
1. kV or MV orthogonal imaging?: The interobserver
variation in image registration was compared for 30 MV
and 30 kV imaged patients.
2. Is cine acquisition useful, and how?: For 30 patients,
cine images (“movieloop” during treatment fields) were
acquired and analysed during all treatment fractions.
3. Surgical clips or bony anatomy for image registration?:
For 30 patients, the setup changes based on patient
anatomy and surgical clips registrations were compared.
4. How to use MV images in the mediolateral tangential
direction (ML images) for adaptive radiotherapy? For 30
patients, the ML images during all treatment fractions
were analysed .
5. Should we use CBCT images?
Results
1. The interobserver variation reduced from 0.2mm for MV
to 0.1mm (1 SD) for kV image registration.
2. Breath hold is very reproducible and stable within one
fraction and during the course of fractions for the majority
of patients . 1 of 30 patients was not able to have a stable
(< 5mm) breath hold, for every fraction.
3. For 33% of fractions, the difference between anatomy
and clip match was > 5mm, due to different patient
(breast) posture or seroma or edema, or uncertainties in
anatomy match.
4. On the ML images, the entire breast with a ring
structure of 5 mm is projected. For 2 out of 30 patients,
the actual patient contour was projected outside the ring
structure for several consecutive fractions, resulting in a
new CT scan and treatment plan (adaptive RT). For 2.7%
of the fractions, the residual deviation in lung wall was >
5mm.
5. CBCT images (if necessary in breath hold) are only
acquired and analysed if indicated by the physicist based
on deviations on one of the above images.
Based on these results, an imaging protocol for breast
cancer patients has been developed. Results of this
protocol for (about 450) patients treated in the last half
year will be presented.
Conclusion
Based on the above results, the imaging protocol for
breast cancer patients is now as follows: cine images are
acquired during the first treatment fraction, and if
breathing motion is > 5 mm, further instructions are given
to the patient and extra cine imaging is performed.
Everyday, on-line setup verification using clips on
orthogonal kV imaging is applied, as well as the use of ML
verification images to 1) check and if necessary correct for
residual lung wall deviations and 2) apply adaptive
radiotherapy using an additional external contour ring
structure. CBCT images are acquired only if required by
the physicist based on deviations in one of the above
images.
EP-1646 Impact of interobserver variability and setup
uncertainty on dose in organs-at-risk
V. Prokic1, F. Röhner2
1Koblenz University of Applied Sciences, Faculty of
Mathematics and Technology, Remagen, Germany
2University Hospital Bonn, Department of Radiology,
Bonn, Germany
Purpose or Objective
Accurate target and organs-at-risk (OARs) contouring and
accurate and reproducible patient setup are crucial for
success in radiotherapy, in particular when volumetric-
modulated radiotherapy (VMAT) with high conformality
and steep dose gradients is applied. Interobserver
variability in contouring of OARs can have strong impact
on dose-volume histograms, as well as possible setup-
errors when setup-correction with on-board imaging is not
performed daily. The goal of this study is to quantify the
impact on delivered dose in parotid gland in patients with
head-and-neck cancer irradiated in VMAT technic: (1) due
to the interobserver variation in contouring of parotid
glands and (2) due to the patient setup without daily
image guidance.
Material and Methods
We have retrospectively analyzed seven patients who
underwent primary definitive radiotherapy for head-and-
neck cancer. Patient set-up is verified weekly using kV
CBCT. The prescription dose was 50Gy/70Gy to PTV1/2.
VMAT plans were generated using Eclipse 13.6 (Varian
Medical Systems USA), for TrueBeam Linac with HD-120
MLC and 6MV. Plans were optimized to meet a set of dose
constraints to OARs and the prescribed doses to the target
volumes. Three radiation oncologist have independently
delineated the parotid glands for this analysis. In order to
estimate the impact of set-up errors on the dose in parotid
glands that would have occurred without correction of
patient positioning, for each patient the isocenter of the
plan was shifted according to the weekly set-up error and
the dose distribution was recalculated in treatment
planning system for VMAT plan. Dosimetric impact due to
the intraobserver and setup variations was quantified in
terms of mean dose in parotid glands.
Results
In initial plans, the mean calculated dose in parotid glands
ranged from 24.1 to 26.2 Gy. There is significant variation
in parotid contouring. The degree of variation varied from
patient to patient, with maximum differences up to 23%
in mean dose to parotid glands. Maximum differences in
mean dose to parotid glands due to the uncorrected setup-
shifts was up to 12%.
Conclusion
Intraobserver variability in contouring of OARs and daily
variations in patient setup are significant contributors to
uncertainty in radiotherapy treatment planning, and
consequently in delivered dose. Our analysis indicates that
the not-precise contouring can lead to larger difference
between delivered and calculated dose.
EP-1647 Validation of a set up procedure for
IMRT/VMAT breast treatment using in vivo dosimetry
with EPID
S. Kang1, J. Li1, P. WANG1, X. Liao1, M. Xiao1, B. Tang1, X.
Xin1, L.C. Orlandini1
1Sichuan Cancer Hospital, Radiation Oncology, Chengdu,
China
Purpose or Objective
In vivo dosimetry (IVD) is an important tool able to verify
the accuracy of the treatment delivered and its
reproducibility. The change of a consolidated existing