ESTRO 36 Abstract Book

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S992 ESTRO 36 2017
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comparison of treatment plans whether Hexapod is
applied.
Results
The average errors of the patients’ position were Lateral
(X-axis) direction of 0.1±1.4 mm, Longitudinal (Y-axis)
direction of 0.0±1.4 mm, Vertical (Z-axis) direction of -
0.4±1.2 mm Pitch of -0.29±0.61°, Roll of -0.42±0.98° and
Yaw of -0.53±0.98°. If the position error takes absolute
value, average error on three directions of translation was
1.06±0.14 mm. Rotation error was 0.82±0.14° which is
larger than the translation error.
Through DQA evaluation, the average error rate of point
dose in the case of rotational error existed is 0.89±0.012%.
and in the case without rotational error is 0.24±0.015%.
Gamma pass rate in the case without rotational error is
99.71±0.328% in average and in the case of rotational error
existed is 89.33±3.874% which is 10% lower so it is
statistically significant.(p<0.05)
The mean values of dose difference on each ROI before
and after rotational error correction in treatment plan are
2.17 Gy of Brain_max, 0.28 Gy of brain_mean, and –3.58
Gy, -4.43 Gy of Brain_stem max and mean respectively.
Also, the value of Lt_Eye_max is 1.34 Gy and the value of
Rt_eye_max is –0.71 Gy individually. There is dose
difference whether correction of rotational error is
existed or not.
Conclusion
When VMAT with Hexapod Couch is applied for patients
with brain tumors, it is considered to increases
reproducibility on patients positioning and treatment
efficiency and at the same time, decreases side effects.
EP-1838 First IGRT results for SBRT bone and lymph
node oligometastases within the pelvic region.
L. Wiersema1, G. Borst1, S. Nakhaee 1, H. Peulen1, T.
Wiersma1, M. Kwint1, A. Smit1, M. Romp1, P. Remeijer1,
A. Van Mourik1
1 Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, radiotherapy, Amsterdam, The Netherlands
Purpose or Objective
Purpose: There is a growing demand for application of
stereotactic body radiation therapy (SBRT) to
oligometastatic disease, like bone and lymph node
metastases. Based on our clinical experience with
common SBRT sites (such as lung, spine and liver), a
comprehensive set of treatment execution guidelines was
developed for bone and lymph node locations eligible for
SBRT. To our knowledge, we present the first combined
IGRT positioning data of bone- and lymph nodes SBRT
treatments in the pelvic region.
Material and Methods
Materials and Methods: The IGRT data for 32 patients
treated with SBRT in the pelvic region for oligometastases
were reviewed; 16 on gland and 16 on bone. Radiotherapy
schedules ranged from 24 -45 Gy in 3 fractions to 25-50 Gy
in 5 fractions. These patients were immobilized with a
personal vacuum bag, knee-fix, head rest and arm
support. The Gross Tumor Volume (GTV) was expanded
with a 5mm Planning Target Volume (PTV) margin for bone
and 7mm for lymph node treatments. All patients were
treated on an Elekta linear accelerator, with 10MV and a
coplanar, dual arc, volumetric Modulated Arc Therapy
(VMAT) technique. A Cone Beam CT (CBCT) based online
imaging protocol was used for set-up, couch correction
verification and intra-fraction motion (IFM) assessment.
Rigid registrations were performed on the bony anatomy
adjacent to the GTV. If the residual translation setup error
(i.e. after couch correction) was larger than 2 mm, the
correction-verification procedure was repeated and if
residual rotation setup errors were larger than 3° the
patient was repositioned. The coverage of GTV within PTV
was checked visually. To calculate the IFM, the difference
between translation and rotation errors of the inline (i.e.
during treatment) or post treatment CBCT and the residual
setup errors was calculated.
Results
Mean, systematic and random components of residual
setup and intra-fraction errors (translations and rotations)
are summarized in the table 1 for bone and lymph node
cases. The correction-verification procedure was repeated
in 3.8% and 10% of the fractions for bone and lymph node
cases respectively.
Conclusion:
The setup and IFM errors of patients treated with SBRT for
oligometastatic disease in the pelvic region (for bone or
lymph nodes locations) are very small, demonstrating the
reproducibility and robustness of the positioning protocol.
Consequently, the contribution of these errors to the GTV-
PTV margin is limited and margins may be reduced. For
the lymph node locations, research is ongoing to improve
image registration methods (e.g. shaped region of interest
registration).
EP-1839 Towards planning organ at risk volumes for
rectum and bladder using cone beam CvT in prostate
cancer.
R. Seuntjens 1, T. Convents1, G. De Kerf2, A . Sprangers2,
K. Van Belle1, D. Verellen2, P. Dirix2
1Odisee University College- campus T erranova, School of
Medical Imaging, Brussels, Belgium
2Iridium Cancer Network GZA Sint-Vincentius,
Department of Radiation Oncology, Antwerp, Belgium
Purpose or Objective
To analyze planning organ at risk volumes (PRV) for the
rectum and bladder using daily cone beam computed
tomography (CB-CT) images acquired during prostate
radiotherapy.
Material and Methods
From February 2015 to October 2015, 18 consecutive
prostate cancer patients received daily CB-CT imaging
after routine set-up imaging. All patients had
intermediate- to high risk disease and received 37
fractions of 2.0 Gy to the prostate (CTV_high) and 1.5 Gy
to the seminal vesicles (CTV_low) through volumetric arc
radiotherapy (VMAT). Treatment simulation (with both CT
and MRI on the same morning) was performed according
to a strict protocol: patients were advised to place a
Microlax® Fleet enema 1 hour before the appointment. At
the same time, they were asked to empty their bladder
and then drink 400 cc of water. Bladder voidance should
then be avoided until the CT was taken. First, the CT was
taken. If the rectum was too filled and/or the bladder was
too empty, the patient was removed from the table and
advised to empty the rectum and/or drink some more
water. Afterwards, patients were advised to empty the
bladder and again drink 400 cc of water. About an hour
later, the MRI was performed at the radiology department.
Before each treatment, the same protocol was followed
but a Glycerin® suppository was subscribed instead of the
enema.
Results
A total of 666 CB-CT’s were evaluated. All CB-CT images
were deemed of sufficient quality to identify the CTV’s as
well as the rectum & bladder. Both CTV’s as well as the
bladder and rectum(from the lowest level of the ischial
tuberosities to the connection anteriorly with the sigmoid)
were delineated on all CB-CT’s. The manual delineation of
each CB-CT took around 18 minutes in total. There was
considerable individual variation in rectal and bladder
volume on CB-CT’s during treatment for each patient (see
Figure). The mean rectal volume on daily CB-CT (84.1 cc)
was not significantly different from the mean rectal
volume during simulation (86.0 cc) on a paired student t-
test. The mean bladder volume on daily CB-CT (160 cc)
was significantly larger than the mean bladder volume
during simulation (140 cc, p < 0.01). There were no
significant differences in CTV volumes. PRV margins were