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ESTRO 36 2017
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Conclusion
A split field model was generated for the whole range of
field sizes of a beam modulator linear accelerator from
0.8x0.8 cm
2
to 16x21 cm
2
using field size dependent
photon beam energy spectra. The model was successfully
validated independently and was found have a good
agreement with measured doses and relative output
factors.
EP-1479 Gamma 3D analysis for VMAT treatments
using two detector arrays
E.M. Ambroa Rey
1
, D. Navarro Jiménez
1
, A. Ramirez
Muñoz
1
, R. Gómez Pardos
1
, D. Amat de los Angeles
1
, A.
Gibert Serrano
1
, A. López Muñoz
1
, M. Parcerisa Torné
1
,
M. Colomer Truyols
1
1
Consorci Sanitari de Terrassa, Medical Physics Unit-
Radiation Oncology Department, Terrassa, Spain
Purpose or Objective
The development of advanced radiation therapy
techniques, such as volumetric modulated arc therapy
(VMAT), requires a patient-specific pre-treatment quality
assurance (QA). Two-dimensional array detectors are
widely used for dose distribution verifications and the 3D
gamma index is one of the metrics which have been
extensively used for clinical routine patient specific QA.
The aim of this study is to evaluate the 3D gamma index
for different VMAT plans, such as head and neck (H&N) and
prostate, with the Octavius 4D system using two 2D-arrays
(PTW Octavius4D 1500 and PTW Octavius4D 729).
Material and Methods
Fourteen H&N and ten prostate VMAT plans were created
and their respective QA plans were developed using
Monaco 5.1 treatment planning system and delivered by
an Elekta Synergy Linac equipped with an Agility 160 MLC
system. The cylindrical phantom Octavius 4D was used to
measure the dose distribution.
The 2D-array Octavius 729 consists of 729 vented
ionization chambers arranged in a 27x27 matrix with a
spatial resolution of 10mm.The chamber volume is
0.125cm
3
. The Octavius 1500 array has the same layout
and dimensions but with 1405 ionization chambers with a
chamber volume of
0.06cm
3
.
In order to reconstruct and analyze the measured 3D dose
from each plan, the PTW VeriSoft 6.2 patient plan
verification software was used and a volumetric 3D gamma
index analysis (max dose of calculated volume, suppress
dose below 10% of max dose of calculated volume) for both
3%3mm and 2%2mm criteria was performed to compare
and evaluate the measured and calculated doses for both
arrays.
Results
Table I summarizes the results for both cases. The mean
pass rate of global 3D gamma index for all prostate cases
was superior to 99% with 3%3mm and 95% with 2%2mm
criteria. The Octavius1500 achieves higher results for both
criteria. The mean difference was 2.9% for the gamma
2%2mm
and
0.6%
for
the
3%3mm.
For the H&N cases, the mean passing rate was lower than
prostate cases. Similarly, the Octavius1500 obtain better
results for both criteria. The mean difference was 4.2% for
the gamma 2%2mm and 0.8% for the 3%3mm.
Expectedly, the greatest differences between Octavius
1500 and 729 are shown in the gamma 2%2mm criteria.
Figure 1 shows a comparison between the gamma 2%2mm
for both detectors for a H&N case.
Conclusion
As we expected, the Octavius 1500 achieves a better
result for pre-treatment VMAT plan verification and the
results are more remarkable for the gamma 2%2mm
criteria. In addition, plans with a higher complexity such
H&N can benefit from the superior results of the Octavius
1500.
Moreover, the Octavius 1500 present the possibility to
increase the spatial sampling frequency and the coverage
of a dose distribution by merging two measurements. We
can conclude that Octavius 1500 outperformed Octavius
729 for VMAT pre-treatment QA.
EP-1480 Patient-specific QA for CyberKnife MLC plans
using Monte Carlo