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ESTRO 36 2017
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may lead to better palliation and possibly increase survival
with the advantage of a short overall treatment time. The
superior soft tissue contrast of MRI might improve tumour
contouring and MRI is capable of tumour motion
quantification. We want to investigate the technical
feasibility and safety of MRI-guided SBRT for LAPC.
Material and Methods
From July 2013 to January 2016, 20 patients with LAPC or
medically unresectable pancreatic cancer without distant
metastasis were included in this study (Table). A custom
made abdominal corset was manufactured to reduce
breathing induced tumour motion. Contouring of the gross
tumour volume (GTV) and organs at risk (OARs) was
performed on 4D treatment planning CT and
multiparametric MRI. A GTV-to-PTV margin of 3 mm was
applied. We quantified tumour motion with cine MRI. After
treatment planning, the static dose distribution was
convolved with the cine MRI based motion trajectory to
simulate and evaluate the delivered dose to the GTV, PTV,
and OARs. SBRT was carried out up to a dose of 24 Gray in
3 fractions in one week. Online position verification was
performed with 4D CBCTs, with 4D matching based on gold
fiducial markers at the midventilation position.
Results
All patients underwent uncomplicated endoscopic fiducial
marker placement. Tumours and OARS were clearly visible
with contrast enhanced CT and multiparametric MRI
(Figure). On the 4D planning CT and on the 4D CBCT scans,
the fiducial markers were clearly visible. The corset
decreased peak-to-peak tumour motion in craniocaudal
direction on average from 11.3 to 7.2 mm. With
incorporation of the tumour motion trajectory, the dose
distribution was blurred and, in this way, the actual
delivered dose was simulated. In all patients, an adequate
dose distribution was achieved with acceptable dose in the
OARs (Table). Position verification based on 4D marker
matching was feasible. No grade 3 or higher treatment
related toxicity was observed in these patients to date.
Figure: Contouring with the aid of MRI and CT. Red: GTV.
Green: pancreatic head. Blue: duodenum.