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contour OAR located within 3cm from PTV
opt
, based on the
daily MR imaging (SMART
3CM
). 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 (SMART
FULLOAR
).
Baseline plans for OAR were created that had identical
plan quality as were achieved for SMART
3CM
. 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 SMART
3CM
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%). SMART
3CM
uniformly
resulted in plans which complied with the V33Gy dose
constraint for OARs, whereas SMART
FULLOAR
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, SMART
3CM
resulted in
a
lower
V25Gy
than
SMART
FULLOAR
(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. Utz
1
, A. Ableitinger
1
, A. Zechner
1
, M. Mumot
1
, M.
Teichmeister
1
, P. Steininger
2
, H. Deutschmann
2
, M.
Stock
1
1
EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
2
medPhoton 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
is indexed on the patient couch. The cube can be moved
to a predefined translational and rotational offset position
on the couch. Two planar images were acquired and
registered to the CT.The calculated correction vector was
applied by the patient positioning system. This workflow
was repeated at three index positions (equal distributed
along the treatment volume), payloads (0kg, 100kg,
150kg), cube offsets (long.: 10mm, lat.: 15mm, vert.: -