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ESTRO 36
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Objective’ optimization tool in Varian Eclipse (v 13.6)
incorporating the Photon Optimizer algorithm (v 13.6.23).
The RA approach currently used in this study implements
two complete arcs to deliver at least 95% of the prescribed
dose to the Planning Target Volume (PTV) while
minimizing dose to the surrounding Organs at Risk (OAR).
In general, RA tends to use fewer MUs per treatment
fraction than Intensity Modulated Radiation Therapy
(IMRT) with an associated reduction in the risk of
secondary induced cancers. The MU Objective tool offers
the possibility to further decrease total Monitor Units
while maintaining clinically acceptable plan quality.
Material and Methods
Thirty clinically approved RA plans (prostate only n=22,
prostate and nodes n=8) were selected for re-optimization
using the MU Objective tool. This tool allows variation of
the Minimum MU, Maximum MU and Strength (S). The ‘S’
parameter weights the optimizer to reach the MU goal
within the defined Min MU and Max MU limits. Based on a
previous study [1], the Min MU was set to 0%, the Max MU
to 50% of the total clinical plan MUs for the non-optimized
RA plan and ‘S’ was set to the maximum value of 100. The
prescribed doses were either 74Gy in 37 Fractions (or 60
in 20), collimator angles were 30
⁰
and 330
⁰
to minimize
the tongue-and-groove effect, jaw tracking was enabled
and all plans were treated at 6MV and 600 MU/minute
maximum dose rate. The dose/volume objectives for the
PTV and OAR were unchanged. Dose calculations were
performed using the Anisotropic Analytic Algorithm (v
13.6.23) with a calculation grid size of 2.5 mm, taking into
account inhomogeneity correction and disregarding air
cavity correction. To determine the quality of the
absorbed dose distributions resulting from smoothing, the
Paddick Conformity Index (CI
PAD
) and the International
Commission on Radiation Units (ICRU) Homogeneity Index
(HI) were calculated for all plans [2].
CI
PAD
= (TV
PI
)
2
/(PI x TV)
Where PI is the volume of the prescription isodose line
(95%), TV
PI
is the target volume within the PI, and TV is
the target volume.
HI = (D
2%
-D
98%
)/D
50%
Where D
50%
is the dose received by 50% of the target
volume and so on.
Results
The MU Objective tool resulted in a reduction of total
prostate RA plan MUs by approximately 29%. The average
ICRU HI for the prostate patients varied from 0.055 to
0.111 (σ = 0.015, CI: 0.07-0.08). The CI
PAD
varied overall
from 0.617 to 0.860 (σ = 0.067, CI: 0.72-0.78).
Conclusion
The MU Objective tool facilitates the reduction of total
prostate RA plan MUs with PTV coverage and OAR sparing
maintained. A lower total MU number should translate to
lower leakage from the linear accelerator and less scatter
within the
patient.
EP-1530 Validation of RayStation Fallback Planning
dose-mimicking algorithm
L. Bartolucci
1
, M. Robilliard
1
, S. Caneva- Losa
1
, A. Mazal
1
1
Institut Curie, Radiotherapy, Paris, France
Purpose or Objective
To demonstrate with end-to-end tests the ability of
RayStation v5.02 (RaySearch Laboratories AB, Stockholm,
Sweden) fallback planning module (RFP) to perform an
accurate Helical Tomotherapy (HT) to volumetric
modulated arc therapy (VMAT) plan conversion by
validating the dose-mimicking algorithm used during the
automatic optimization of the fallback plans.
Material and Methods
Thirty patient plans of various treatment sites previously
treated with HT were switched to 6 MV dual-arc VMAT
plans using RFP and default dose-mimicking algorithm
parameters. For the purpose of this study no further
optimizations were performed and delivery quality
assurance (DQA) were designed for each fallback plan.
DQA were delivered on a TrueBeam linear accelerator
(Varian Medical Systems, Palo Alto, CA) and
planar/absolute dose measurements were acquired using
the ArcCHECK diode array (Sun Nuclear Corporation,
Melbourne, FL) with an insert containing an Exradin A1SL
ionization chamber (Standard Imaging, Middleton, WI). 3D
dose distributions in the patient geometry were
reconstructed within 3DVH software (Sun Nuclear
Corporation, Melbourne, FL) by using ArcCHECK Planned
Dose Perturbation (ACPDP). Agreement between planned
and delivered dose was eventually evaluated with global
and local 2D/3D gamma-index analysis (3%/3mm and
2%/2mm criteria) and DHV-based comparisons were
performed using the following dosimetric parameters:
quality of coverage (Q=D98%/Dref), mean dose to target
(MDT=Dmean/Dref) and integral dose to organs at risks
(ID_OAR=∑·Di·Vi).