ESTRO 36 Abstract Book

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S816 ESTRO 36
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Conclusion
The results showed medium to large differences between
the PB and MC doses which could be addressed totally or
partially by adding a correction term during the
optimization. Since MC beamlets calculation remains
time-consuming, this hybrid PB-MC optimization seems a
good compromise between accuracy and speed.
EP-1520 Stereotactic body radiation therapy
treatment planning using target volume partitioning
J. Robar1
1 Dalhousie University, Radiation Oncology, Halifax,
Canada
Purpose or Objective
The aim of this study was to evaluate a novel approach to
Volumetric Modulated Arc Therapy (VMAT) plan
optimization for stereotactic body radiation therapy of the
spine involving partitioning of the Planning Target Volume
(PTV) into simpler sub-volumes. Treatment plan quality
was compared to that provided by a standard VMAT
approach.
Material and Methods
The new technique investigated in this work relies on a
partitioning of the PTV that is dedicated to spinal
anatomy. The spine PTV is segmented into multiple sub-
volumes using a k-means algorithm, such that each sub-
volume minimizes concavity. Each sub-volume is then
associated with a separate arc segment for VMAT
delivery. The rationale of this approach is that the
delivery of dose to multiple, mainly convex target volumes
provides flexibility to the VMAT optimizer in prioritizing
spinal cord sparing. Treatment plans were established
with the novel algorithm using the Spine SRS Element
(Brainlab, AG, ver 1.0 beta) and compared to clinical
treatment plans generated using standard VMAT planning
approach in our centre (Rapidarc, Varian Medical
Systems). Test cases included a range of spinal target
volumes, including the vertebral body only, vertebral body
and pedicles, or spinous process only. Plan quality was
compared with regard to PTV coverage, PTV dose
homogeneity, dose conformity, dose gradient, sparing of
spinal cord PRV and MU efficiency.
Results
PTV coverage and dose homogeneity were equivalent,
however improved high-dose (90%) conformity was
observed for the new approach (p=0.002). Sharper dose
gradient was produced in 75% of cases but did not reach
statistical significance. The percent volume of the PRV
spinal cord receiving 10 Gy was reduced (p=0.05). Despite
the fact that the new method involves delivery of dose to
PTV sub-volumes with separate arc segments, MU
efficiency was approximately equivalent to the status-quo
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
1David 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 this 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. Fusella1, S. Bacco1, N. Pivato1, A.
Roggio1, M. Rossato1, R. Zandonà1, M. Paiusco1
1Istituto 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