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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. Lin
1
, H. Liu
1
, X. Liang
1
, A. Lin
1
, P. Ahn
1
, H. Zhai
1
, M.
Kirk
1
, A. Kassaee
1
, J. McDonough
1
, S. Both
2
1
University of Pennsylvania, Radiation Oncology,
Philadelphia, USA
2
Memorial 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.
Material and Methods
10 HN oropharynx consecutive patients treated with PBS
PT underwent prospective evaluation computer
tomography scans (eCTs) during their course of treatment
(average 4 eCTs per patient). The robustness of the
treatment plans containing two-posterior oblique(PO) PBS
fields (2PBS), contingency VMAT plans (2 arcs) and
retrospectively generated 3-field PBS plans (3PBS) was
evaluated against anatomy changes and residual setup
uncertainties via evaluation plans generated on eCTs.
3PBS plan matched two PO fields (same to 2PBS) with an
anterior field at thyroid notch level in order to treat the
lower neck nodal target. Plan robustness was assessed
based on the accumulated dose through deformable image
and dose registration between treatment and evaluation
plans using VelocityAI. The D98% dosimetric indicator of
target coverage and OARs planning constraints were used
to evaluate the plan robustness. Changes over 5% in target
coverage, excessive cord dose and/or clinical decision
triggered proton replan or the use of the VMAT
contingency plan.
Results
The average change of D98% in the accumulated plans for
2PBS, 3PBS and VMAT were:4.1%±4.8%, -0.1%±0.8% and -
2%±3.2% for low risk CTV, -1.7±1.8%, -0.5±0.8% and -
0.73±1.2% for high risk CTV, -0.2%±0.2%, -0.1%±0.1% and
-0.4%±1.3% for gross CTV respectively. The main source of
coverage loss at low risk CTV level for 2PBS was found to
be due to variable soft tissue deformation of the posterior
neck for elderly or short neck patients leading to
replanning for 2 out10 patients. OARs robustness was
maintained within planning constraints.
Conclusion
2PBS plans were not consistently robust relative to target
coverage due to variable folding neck tissue, and
therefore it should be cautiously employed for elderly and
short neck patients. 3PBS was proved to be consistently
robust, similarly with VMAT.
PO-0828 Stereotactic body radiotherapy (SBRT) for
localised prostate cancer on the MR-Linac
A. Pathmanathan
1
, A. Mitchell
2
, K. Thomas
3
, D.
Henderson
2
, S. Nill
1
, U. Oelfke
1
, R. Huddart
1
, N. Van As
2
,
A. Tree
2
1
Institute of Cancer Research, Radiotherapy and Imaging,
London, United Kingdom
2
The Royal Marsden NHS Foundation Trust, Department
of Radiotherapy, London, United Kingdom
3
The Royal Marsden NHS Foundation Trust, Statistics
Unit, London, United Kingdom
Purpose or Objective
As the estimated alpha-beta ratio for prostate cancer is
low (1), moderate hypofractionation has been shown to be
isoeffective (2). The MR-Linac (MRL) combines an MR
scanner and linac allowing intrafractional tracking of the
target (3). However, dose distributions are affected by the
magnetic field (4). The first Elekta system MRL (1.5T/
7MV) will deliver step-and-shoot intensity modulated
radiotherapy (IMRT), a technique rarely used for
stereotactic body radiotherapy (SBRT). This planning study
assesses whether adequate dose distributions for MRL-
based prostate SBRT are possible with comparison to non-
MRL based planning techniques: IMRT, volumetric
modulated arc therapy (VMAT) and CyberKnife.
Material and Methods
Using planning CT scans acquired for ten patients with
localised prostate cancer, clinical target volume (CTV)
was defined as prostate plus proximal 1cm of seminal
vesicles. The planning target volume (PTV) was created by
addition of a 5mm isotropic margin, except 3mm
posteriorly. For the MRL, 5, 7 and 9-field step-and-shoot
IMRT plans were created to deliver 36.25Gy in 5 fractions
to the PTV with an integrated dose of 40Gy in 5 fractions
to the CTV using Monaco 5.19 (research version, Elekta AB,
Stockholm, Sweden). Non-MRL comparison plans included: