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ESTRO 36 2017
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MCsquare
(Souris et al. Med Phys 2016) able to compute a
final dose in less than 1 minute.
The hybrid optimization strategy calculates the optimal
spot weights (
w
) using the analytical beamlets matrix (
P
PB
)
and a correction term
C
. After a first optimization where
C
= 0, the method alternates optimization of
w
using
P
PB
with updates of
C = D
MC
–
D
PB
, where
D
MC
results from a
regular MC computation (using 10
8
protons to ensure good
statistical accuracy) and
D
PB
= P
PB
* w
. Updates of C can be
triggered as often as necessary by running the MC engine
with the last corrected values of
w
as input.
The performance of the method is illustrated on two
extreme cases: prostate (relatively easy case) and lung
(considered to be complex due to the high heterogeneity).
For simplicity, we created PTV-based plans but the
findings can be equally applied to robust optimized plans.
Results
For the prostate case, the recomputed MC dose after
initial optimization (
C
=0, before correction) revealed a
decreased target coverage (D95=90% of the prescribed
dose, D
p
) that improved significantly after just one
correction (D95
corrected
=97%D
p
).
For the lung case, the difference between MC and PB doses
before correction was very large: D95=63%D
p
and
D5=137%D
p
. But still the hybrid strategy was able to
partially improve target coverage (D95
corrected
= 84%D
p
) as
well as reducing overdose (D5
corrected
= 111%D
p
), after two
updates of C.
In both cases, further corrections did not lead to better
results.
The results proved that the hybrid method allows us to
improve dose accuracy even for very complicated cases as
lung tumors. However, the success of the correction is
limited by the order of magnitude of the term C, i.e, very
large difference between MC and PB doses are only
partially corrected.
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. Robar
1
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