ESTRO 36 Abstract Book

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S258 ESTRO 36 2017
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OC-0490 A robust and fast planning approach for
adaptive MR-guided treatment of pancreatic cancer
O. Bohoudi1, A. Bruynzeel1, B. Slotman1, S. Senan1, F.
Lagerwaard1, M.A. Palacios1
1 VUMC, Radiotherapy, Amsterdam, The Netherlands
Purpose or Objective
In May 2016, we implemented stereotactic MR-guided
adaptive radiation therapy (SMART) using the MRIdian
system (Viewray) for locally advanced pancreatic cancer.
Interfractional changes in the anatomy of adjacent organs-
at-risk (OARs) make daily online plan adaptation
desirable. The main challenge of online plan adaptation is
the requirement that it must be performed fast while the
patient remains in treatment position. We evaluated an
in-house developed re-planning strategy, which is
currently in clinical use.
Material and Methods
Before use of SMART, robust baseline IMRT plans for online
re-optimization are first produced with the MRIdian
planning system (ViewRay). The same planning software is
available at the treatment console for plan adaption. The
target structure used for optimization is defined as PTV opt
(GTV+3mm minus OARs). OAR contours are then spatially
partitioned in separate OAR portions located within 1, 2
and 3cm from the PTVOPT surface, thereby allowing direct
control over the spatial dose distribution (Fig. 1). The
optimization process relies on a model which predicts OAR
dose as a function of distance from PTV OPT, and generates
optimization objectives to achieve a robust baseline plan
for daily adaption. For daily SMART, physicians only re-
contour OAR located within 3cm from PTVopt, based on the
daily MR imaging (SMART3CM). Optimization structures are
automatically adapted to the new anatomy, and re-
optimization is performed using exactly the same plan
parameters. This limited re-contouring strategy was
evaluated by comparing 45 previously delivered fractions
against a simulated standard (re-)planning method using
full-scale OAR (re-)contouring, where optimization
objectives were used for the whole organs (SMARTFULLOAR).
Baseline plans for OAR were created that had identical
plan quality as were achieved for SMART3CM. Efficiency of
both strategies was scored according to the number of
optimizations needed to generate a high quality plan. Plan
quality was assessed using PTV coverage (V95%) and
institutional OAR constraints (V33Gy and V25Gy).
Results
The SMART3CM baseline plans required a lower number of
optimizations than SMART FULLOAR (4 vs 17 on average).
PTV OPT coverage with both strategies was identical in all
fractions (median V95%=93±6.8%). SMART3CM uniformly
resulted in plans which complied with the V33Gy dose
constraint for OARs, whereas SMARTFULLOAR failed in 35% of
the cases to adhere to the V33Gy dose constraint
according to the clinical protocol. Both strategies
achieved V25Gy values lower than 20 cc for all OARs in
every fraction. However, on average, SMART3CM resulted in
a lower V25Gy than SMARTFULLOAR (Fig.2).
Conclusion
This fast and robust (re-)planning approach for SBRT to
pancreatic tumors requires clinicians to only re-contour
OARs located within 3cm of the PTV OPT. Spatially
partitioned optimization structures within this 3 cm region
allowed for optimal OAR sparing, and adequate target
coverage, using exactly the same plan parameters.
OC-0491 Quality assurance of a novel table mounted
imaging device integrated in a patient positioning
system
A. Utz1, A. Ableitinger1, A. Zechner1, M. Mumot1, M.
Teichmeister1, P. Steininger2, H. Deutschmann2, M.
Stock1
1EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
2medPhoton GmbH, Medical Physics, Salzburg, Austria
Purpose or Objective
Image guided radiation therapy (IGRT) aims to reduce
margins and subsequently increase dose sparing for OAR.
The majority of image guidance procedures are based on
ceiling/floor- or gantry mounted imaging devices. In our
particle therapy center a novel approach for patient
alignment was introduced. The imaging system (imaging
ring) is mounted on the treatment table and as such,
allows high imaging flexibility e.g. CBCT or planar imaging
at different table positions. The goal was to establish a
phantom and a concept for a quality assurance procedure
for the whole IGRT workflow.
Material and Methods
The IGRT Phantom consists of a PMMA cube with steel
fiducials, which can be placed in predefined offset
positions on a baseplate to simulate clinical patient shifts.
An additional support structure is used to lift the cube.
Holes on the upper corners of the cube allow to
independently determine the absolute position with a
lasertracker (see figure 1a). A CT imageset of the cube in
a reference position serves as planning CT. The baseplate