806 / 1096
S790
ESTRO 36
_______________________________________________________________________________________________
Purpose or Objective
The aim of this study was to model a beam modulator
linear accelerator in Pinnacle v.14.0 treatment planning
system for intracranial stereotactic radiosurgery and
radiotherapy.
Material and Methods
Depth dose, beam profile and total scatter correction
factor data were collected for 6 MV photons of Elekta
Synergy Beam Modulator
TM
linear accelerator with 80
leaves each of 4 mm leaf pitch using unshielded IBA
stereotactic field diode for field sizes ranging from 0.8x0.8
cm
2
to 4x4 cm
2
and field sizes above 4x4 cm
2
up to 16x21
cm
2
using IBA CC04 pinpoint chamber. The measured data
were imported in to the photon physics module of Pinnacle
v.14.0 and physical accelerator head specific data such as
primary collimator, flattening filter, MLC were input in
addition to beam data measurements. The auto modeler
of Pinnacle TPS was iteratively used to adjust parameters
such as photon beam energy spectrum, Gaussian height
and width of the photon source that affect various regions
of the depth doses and beam profiles to match measured
data. Dose grids of 1 mm and 2.5 mm were used for beam
modelling of fields from 0.8x0.8 cm
2
up to an equivalent
square field of 6.7 cm
2
and above 6.7 cm
2
respectively. A
common photon energy spectrum did not prove sufficient
to achieve the required agreement between Pinnacle
calculated and measured depth doses and beam profiles
for the whole range of field sizes. This was overcome by a
split field model that employs field size specific beam
energy spectra, with higher relative weights of low energy
bins and lower relative weights of high energy bins for
small fields and vice-versa for field sizes larger than 6.7
cm
2
. The validity of the model was tested independently
using a Standard Imaging Exradin A26 chamber in LUCY
phantom for field sizes ranging from 0.8x0.8 cm
2
by
comparing calculated and measured absolute doses and
relative output factors.
Results
Optimization of photon beam energy spectrum specific to
small field sizes improved the agreement of depth doses
both in and beyond build-up region for the small fields.
Measured versus calculated absolute planned doses were
found to be within 1% for field sizes larger than 1.6x1.6
cm
2
and less than 2.5% for 0.8x0.8 cm
2
field. The
agreement between the measured and calculated relative
output factors were within 2% for field sizes larger than
1.6x1.6 cm
2
and less than 3.5% for 0.8x0.8 cm
2
fields.
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).