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ESTRO 36 2017
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evaluate the RP quality relative to the clinical plans (CP).
Secondary, through normal tissue complication probability
(NTCP) estimations, the possible effective clinical benefit
in planning with RP is evaluated.
Material and Methods
83 patients presenting AHNC were selected from the
department database. The patients were chosen as their
plans were considered as dosimetrically optimal. All plans
were optimized for VMAT technique (RapidArc), with 2-4
arcs, 6 MV beam quality, treated on a department linac
equipped with Millennium 120-MLC or HD-MLC. Inverse
planning used the PRO optimizer, and final calculations
were with AAA. Dose prescription was to 69.96 Gy and
54.45 Gy to PTV2 and PTV1, respectively, in 33 fractions.
A RP model was generated for the OARs: spinal cord, brain
stem, oral cavity, parotids, submanidbular glands, larynx,
constrictor muscles, thyroid, eyes. To constrain the
uninvolved healthy tissue, the ‘body’ with all the targets
subtracted was included in the model. The optimization
objectives in the model included the line objective for all
OARs with generated priority. For serial organs, an upper
objective was added with generated dose at 0% volume
with a fixed priority of 90. For parotids and oral cavity, a
mean objective was added with generated dose and fixed
priority of 60. Targets upper and lower objectives were
placed in a very narrow interval, with priority 110 and 120.
The automatic Normal Tissue Objective NTO was added
with priority 280. The model was validated on a set of 20
similar patients selected from the clinical database. The
possible clinical benefit was evaluated through NTCP
estimation for some of the OARs, using the biological
evaluation availabile in Eclipse, based on LQ-Poisson
model.
Results
Regarding target dose homogeneity, the standard
deviation was reduced by 0.3 Gy with RP (p<0.05). The
mean doses to parotids, oral cavity, and larynx were
reduced with RP of 2.1, 5.2, and 7.0 Gy, respectively.
Maximum doses to spinal cord and brain stem were
reduced of 7.0, and 6.9 Gy, respectively (p<0.02). NTCP
reductions of 11%, 16%, and 13% were estimated for
parotids, oral cavity, and larynx, respectively, with RP
planning.
Conclusion
Model validation confirmed the better plan quality with RP
plans. NTCP estimation suggests that this dosimetric
effect could positively affect also the toxicity profiles for
patients receiving RP planning with an adequate model.
EP-1529 Reducing total Monitor Units in RapidArc™
plans for prostate cancer
K. Armoogum
1
, M. Hadjicosti
1
1
Derby Hospitals NHS Trust, Department of
Radiotherapy, Derby, United Kingdom
Purpose or Objective
A retrospective planning study was performed on prostate
cancer RapidArc (RA) plans to evaluate the use of the ‘MU
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