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ESTRO 36 2017
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technique.
Conclusion
The novel target volume splitting technique offers an
efficacious new approach to VMAT optimization,
producing high dose gradients in the vicinity of the spinal
cord and allowing prioritization of spinal cord sparing.
EP-1521 Non-coplanar beam orientation and fluence
map optimization based on group sparsity
K. Sheng
1
1
David Geffen School of Medicine at UCLA, Radiation
Oncology, Los Angeles- CA, USA
Purpose or Objective
With the increasing availability of non-coplanar
radiotherapy systems in clinical set-tings, it is essential to
develop effective and efficient algorithms for integrated
non-coplanar beam orientation and fluence map
optimization. To achieve this goal, we investigate the
novel group sparsity approach for non-coplanar beam
orientation optimization.
Material and Methods
The beam orientation and fluence map optimization
problem is formulated as a large scale convex fluence map
optimization problem with an additional group sparsity
term that encourages most candidate beams to be
inactive. The optimization problem is solved using an
accelerated proximal gradient method, the Fast Iterative
Shrinkage-Thresholding Algorithm (FISTA).We derive a
closed-form expression for a relevant proximal operator
which enables the application of FISTA. The beam
orientation and fluence map optimization algorithm is
used to create non-coplanar treatment plans for six cases
(including two head and neck, two lung, and two
prostatecases) involving 500 - 800 candidate beams. The
resulting treatment plans are compared with 4treatment
plans created using a column generation algorithm, whose
beam orientation and fluence map optimization steps are
interleaved rather than integrated.
Results
In our experiments the treatment plans created using the
group sparsity method meet or exceed the dosimetric
quality of plans created using the column generation
algorithm, which was shown superior to that of clinical
plans (Figure shows a head and neck case). Moreover, the
group sparsity approach converges in about 5 minutes in
these cases, as compared with runtimes of more than an
hour for the column generation method. Table shows the
PTV dose statistics and runtime comparison.
Conclusion
This work demonstrates that the group sparsity approach
to beam orientation optimization, when combined with an
accelerated proximal gradient method such as FISTA,
works effectively for non-coplanar cases with a large
number of candidate
beams.Inthis paper we obtain an
orders of magnitude improvement in runtime for the
\group sparsity"approach to beam orientation optimization
by using an accelerated proximal gradient method to solve
the ℓ2;1-norm penalized problem. Furthermore, the
dosimetric quality of our group sparsity plans meets or
exceeds the quality of treatment plans created using a
column generation approach to beam angle selection,
which has been demonstrated in recent literature to
create high quality treatment plans.
EP-1522 Quantifying the operator variability reduction
driven by knowledge-based planning in VMAT
treatments
A. Scaggion
1
, M. Fusella
1
, S. Bacco
1
, N. Pivato
1
, A.
Roggio
1
, M. Rossato
1
, R. Zandonà
1
, M. Paiusco
1
1
Istituto Oncologico Veneto IOV-IRCCS, Medical Physics,
Padova, Italy
Purpose or Objective
The purpose of this study is to evaluate the potential of a
commercial knowledge-based planning (KBP) algorithm to
standardize and improve the quality of the radiotherapy
treatment. This study evaluates if the predicted DVH
constraints generated by the KBP algorithm can reduce the
inter-operator variability thus providing a better standard
of quality.
Material and Methods
Using Varian RapidPlan two models were created for
oropharynx and prostate VMAT treatments with
respectively 73 and 90 previously treated patients. Five
oropharynx and six prostate test patients, not included in
the training database, were anonymized and randomized.
Four operators, with different planning expertise, were
asked to manually obtain a clinical VMAT plan (mVMAT) for
each test patient. Subsequently, each operator replied the
planning procedure assisted by RapidPlan DVH predictions
obtaining a second VMAT plan (rpVMAT). The potential of
RapidPlan to reduce the inter-operator variability was
evaluated comparing rpVMAT with mVMAT plans in terms
of OAR sparing, target coverage and conformity.
Results
In the case of prostate treatments mVMAT and rpVMAT
plans resulted in similar target coverage while a net
reduction in OAR sparing variability was seen for rpVMAT
plans (a visual example is given in Figure). For the case in
figure, rectum V40Gy resulted 34.4±18.1% for mVMAT and
32.1±7.6% for rpVMAT. In general, a 40% reduction in inter-
planner OAR sparing variability has been registered when
planning was assisted by RapidPlan predictions.
For oropharynx treatments RapidPlan-assisted planning
leads to more homogeneous target dose distributions,
especially for the low-dose target. The low-dose PTV
standard deviation obtained in rpVMAT plans was 2.6±0.6%
while it resulted 3.2±1.5% for mVMAT ones. A variability
reduction of the order of 10% was also seen in parotids,
oral cavity and larynx sparing. For the less experienced
planner RapidPlan assistance also induced an overall
decrease of OAR mean doses by approximately 15%. Using
RapidPlan assistance the overall inter-planner variability
is reduced in every single patient and a general
improvement of plans statistics is achieved.
Conclusion
The use of RapidPlan predictions in VMAT planning driven
a homogenization of the planning outcome both in
prostate and oropharynx treatment for a group of 4
planners. OAR sparing variability can be reduced as much
as 40% maintaining similar target coverage when
RapidPlan is employed. This study provide a quantitative
measure of the RapidPlan potential as an instrument to
improve plan
quality.
This findings states that the use of a knowledge based
planning system allow for safer treatments.