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Beamline at The Australian Synchrotron. We are also in the
process of performing a plan comparison study using
anonymised patient datasets, comparing kilovoltage MRT
plans with clinical megavoltage treatment plans.
Results
The Eclipse TPS performed well in calculating ‘peak’ doses
in a water phantom. Considering the simplicity of the
algorithm, the ‘valley’ dose and field profiles were also
produced with reasonable accuracy, albeit with some
underestimation of the valley dose for larger field sizes.
Preliminary studies of megavoltage treatment plan
comparisons have been performed. Compared to the
clinical megavoltage treatment plans, MRT plans
demonstrated adequate target coverage whilst meeting
normal tissue dose constraints when target volumes were
small and relatively superficial. As expected, planning
goals for deep seated tumours and target regions distal to
bone could not be met using MRT.
A screen shot of the treatment planning system. The peak
dose has been calculated for three treatment fields on a
head CT scan.
Conclusion
There are real advantages to using the familiar
environment of Eclipse with a new radiotherapy paradigm
such as MRT. Although, there are limitations to our MRT
calculation engine in Eclipse and further work is required,
the data generated in this work are overall encouraging
and indicate that the potential for this calculation engine
to be implemented in the future as part of a Phase 1
clinical trial.
EP-1564 Dosimetric assessment of pseudo-CT based
proton planning
G. Pileggi
1
, C. Speier
2
, G. Sharp
3
, C. Catana
4
, D.
Izquierdo-Garcia
4
, J. Pursley
3
, J. Seco
5
, M.F. Spadea
1
1
Magna Graecia University, Department of Experimental
and Clincal Medicine, Catanzaro, Italy
2
Friedrich-Alexander University Erlangen-Nürnberg,
Radiation Oncology, Erlangen, Germany
3
Massachusetts General Hospital, Radiation Oncology,
Boston, USA
4
Athinoula A. Martinos Center for Biomedical Imaging,
Radiology- MGH, Charlestown, USA
5
Deutsches Krebsforschungszentrum - DKFZ, Radiation
Oncology, Heidelberg, Germany
Purpose or Objective
The aim of this work is to use pseudo-CT (pCT) data,
obtained from T1 and T2 weighted MRI, for proton therapy
planning.
Material and Methods
Data of 15 patients, including T1 and T2 weighted MRI and
CT scans, were used in this study. The pCT was generated
according to the methodology described in Speier
et al,
by
segmenting the T1
w
and T2
w
MRI volume into 6 tissue
classes (grey and white matter, cerebrospinal fluid, bone,
skin and air). For each patient, three 18 Gy beams (2 axial
and 1 coronal,) were designed on the pCT volume, for a
total of 45 analyzed beams. The plan was then copied and
transferred onto the CT that represented the ground
truth. Range shift (RS) between pCT and CT was computed
at R
80
over 10 slices. The acceptance threshold for RS was
set to 3.5% of R
80
, according to the clinical guidelines of
our Institution.
Results
The median value of RS was 0.6 mm with lowest and
highest absolute values being 0.08 mm and 3.8 mm
respectively. 40 out of 45 beams passed the acceptance
test. Largest discrepancies occurred in correspondence of
the surgical hole of the scalp containing a metal plate.
This happened because the segmentation process did not
include metal classification, thus mis-assigning the
Hounsfield Unit to skin or air. In this circumstance, the
planned range on the pCT was deeper than the actual one
detected on the CT.
Conclusion
This study showed the feasibility of using pCT, derived
from MRI, for proton therapy treatment. The major
benefit of MRI acquisition lies in better soft tissue contrast
for tumor and organs at risk delineation. Further
improvements of the methodology are required for the
correct conversion of metal voxels to electron density.
EP-1565 Best of both worlds: 3D-CRT-based VMAT for
locoregional irradiation in breast cancer.
P.G.M. Van Kollenburg
1
, H.J.M. Meijer
1
, M.C. Kunze-
Busch
1
, P. Poortmans
1
1
UMC St Radboud Nijmegen, Department of Radiation
Oncology, Nijmegen, The Netherlands
Purpose or Objective
Purpose:
Postoperative locoregional radiation therapy (RT) is
increasingly applied in breast cancer patients as it has
been demonstrated to decrease the risk of any recurrence
and breast cancer mortality in patients with node-positive
disease after mastectomy or breast conserving therapy.
However RT has also been associated with side effects
such as fibrosis, cardiac and pulmonary toxicity, impaired
shoulder function and the induction of secondary
malignancies.
It is therefore essential to use treatment techniques that
enable the delivery of conformal and homogeneous doses,
adequately covering the target volumes and limiting the
dose to the organs at risk. The technique should also be
robust taking into account changes in the position and the
shape of the target volumes during treatment. We hereby
present the results of the technique as being used in our
department.
Material and Methods
Materials/Methods:
10 breast cancer patients with and an indication for
locoregional RT were selected for dosimetric comparison
between 3D-CRT and VMAT. All patients underwent a CT-
scan with 3-mm slice thickness. Patients with left-sided
breast cancer were scanned and treated with voluntary
moderately deep inspiration breathhold. The treatment
plans were created in the Pinnacle
3
treatment planning
system V.9.10 with the Auto-Planning module, using 6
and/or 10MV beams.
For each patient a CTV was delineated based on the ESTRO
guidelines. A margin of 7 mm was used to generate a PTV.
The following organs at risk were contoured: thyroid
gland, heart, lungs, esophagus and contralateral breast.
Treatment Planning:
Prescription dose was 42,56 Gy in 16 fractions of 2,66 Gy.
The 3D-CRT technique consisted of tangential beams for
the breast/thoracic wall, one anterior beam (15° or 345°)
for the medial periclavicular region and an anterior (15°