ESTRO 36 Abstract Book

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S444 ESTRO 36
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errors and to blindness relative to their potential
degeneracies. The CI for M2 equals 88%, but 90% CI can be
achieved for M2 by extending slightly the lateral PTV
margin to encompass 92% of possible beam positions and
98% of possible ranges, leading to a 425% volume, thus still
better than M3.
Conclusion
The best tradeoff between robustness and optimality was
achieved through random sampling of all errors limited by
the lateral conventional PTV margin and a large margin for
the possible proton ranges.
PO-0826 Evaluation of the new InCise MLC for
Cyberknife stereotactic radiotherapy
C. Limoges1, J. Bellec1, N. Delaby 1, M. Perdrieux1, F.
Jouyaux 1, E. Nouhaud2, I. Lecouillard 2, E. Chajon2, R. De
Crevoisier2,3,4, E. Le Prisé2, C. Lafond1,3,4
1 Centre Eugène Marquis, Medical Physics Department,
Rennes, France
2 Centre Eugène Marquis, Radiation Oncology
Department, Rennes, France
3 INSERM, U 1099, Rennes, France
4 University of Rennes1, LTSI, Rennes, France
Purpose or Objective
The aim of this study was to evaluate treatment planning
performances of the new InCise multileaf collimator (MLC)
with reference to the Iris variable circular aperture
collimator for intracranial and extracranial Cyberknife
stereotactic radiotherapy.
Material and Methods
The study was performed on a Cyberknife M6 v10.6
(Accuray). A total of 50 cases including 10 brain
metastases, 10 acoustic neuromas, 10 liver targets, 10
spinal metastases and 10 prostate cases were
investigated. For each case, two treatment plans were
generated with TPS Multiplan v5.3 (Accuray): one plan
using the InCise MLC v2 associated with the Finite Size
Pencil Beam (FSPB) dose calculation algorithm and one
plan using the Iris collimator associated with RayTracing
(RT) or MonteCarlo (MC) dose calculation algorithm. Dose
was prescribed near the 80 % isodose and normalized to
obtain the same PTV coverage at ± 0.5 % for both plans.
The comparison was performed in terms of dose
distribution and efficiency by reporting OARs DVH, Baltas’
conformal index (COIN), Paddick’s gradient index (GI),
ICRU homogeneity index (HI), integral dose to normal
tissue (NTID), number of monitor units (MU) and treatment
time.
For both collimators, accuracy of dose calculation within
heterogeneity was evaluated by delivering a typical lung
treatment plan on a QUASAR Respiratory phantom (Modus
Medical Inc) including a lung target insert. Calculated dose
was compared with delivered dose measured by
Gafchromic EBT3 films (Ashland) using a gamma index
analysis with a local dose criteria of 3 % and a distance-to-
agreement criteria of 2 mm.
Results
Results are summarized in table 1. Compared to Iris plans,
MLC plans did not produce significant differences in terms
of OARs sparing and dose conformality except for acoustic
neuroma for which COIN was degraded by 20 % with MLC.
Dose gradient was improved by using the MLC with a GI
mean reduction of 18 %. MLC allowed a slight improvement
of PTV dose homogeneity for brain metastasis and liver
targets and lead to a NTID reduction for extra-cranial
treatments. Except for liver targets, MLC plans delivered
less MU than Iris plans with a mean reduction of 25 %. MLC
plans lead to a treatment time reduction of 28 % in
average compared to Iris plans.
The comparison between calculated and measured dose in
lung phantom showed a gamma passing rate of 51.6 %,
45.5 % and 98.7 % for FSPB MLC plan, RT Iris plan and MC
Iris plan respectively.
Conclusion
The use of the InCise MLC for Cyberknife st ereotactic
radiotherapy allows a significant reduction of MU and
treatment time compared to Iris collimator while
maintaining a high degree of conformality and a steep
dose gradient. However, circular collimators should be
still preferred for treatment of small targets like acoustic
neuromas due to their smaller field size capability. The
use of the InCise MLC for lung targets treatment should
not be recommended currently due to the absence of a
type B dose calculation algorithm.
PO-0827 Robustness Evaluation of Head and Neck
Treatment with Proton Pencil Beam Scanning
Technique
H. Lin1, H. Liu1, X. Liang1, A. Lin1, P. Ahn1, H. Zhai1, M.
Kirk 1, A. Kassaee1, J. McDonough1, S. Both2
1University of Pennsylvania, Radiation Oncology,
Philadelphia, USA
2Memorial Sloan Kettering Cancer Center, Medical
Physics, New York, USA
Purpose or Objective
To evaluate the treatment robustness of two novel pencil
beam scanning proton therapy (PBS PT) beam
arrangements relative to volumetric arc therapy (VMAT)
for oropharynx head and neck (HN) cancer patients.