ESTRO 36 Abstract Book

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S809 ESTRO 36
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Purpose or Objective
This work aims to accurately quantify the attenuation and
skin dose increase for 6 MV photon beams of an Elekta
Compact linear accelerator transmitted through the
Elekta iBEAM Standard carbon fiber couchtop and related
immobilisation devices. A study of this combination of
couchtop and linac has not been reported. Other novel
aspects of this work include the use of Monte Carlo (MC)
simulation in conjunction with thin-buildup diode
measurements for better estimation of the clinically-
relevant dose to skin basal cell layer, as well as putting
the results into context by direct comparison of PDDs in
the buildup region and further depths with a typical Co-60
treatment unit utilizing a ‘tennis racket’ type couch
without a Mylar sheet (Theratron Phoenix).
Material and Methods
Manufacturer-supplied information was used to add an MC
model of the couchtop to an existing detailed model of the
linac head. Beam attenuation by the couchtop was
simulated and measured using an ionisation chamber both
in air and in a water-equivalent cylindrical phantom at
gantry angles 125°, 135°, 150°, 165° and 180° for field
sizes 5×5 cm2, 8×8 cm2, 10×10 cm 2, 15×8 cm2 and 20×8 cm2.
Also beam attenuations of the head-and-neck (H&N)
extension and BreastSTEP boards were measured for an
8×8 cm2 field. The effect on skin dose was studied by
measurement of percentage depth dose (PDD) in the
buildup regions of 180° gantry beams of both linac and Co-
60 units, using an electron diode in a Perspex slab
phantom for 5×5 cm2 , 10×10 cm2 and 20×20 cm 2 field sizes,
as well as the corresponding linac MC simulations.
Results
The simulated and measured couchtop attenuation results
agreed to within 0.4%, which further validated the MC
model. The highest couchtop attenuation (7.6%) was
measured at 135° gantry and 5×5 cm2 field size. The
attenuation values of the H&N extension and breast boards
at 180° gantry angle were 6.9% and 6.7%, respectively. MC
results showed that the couchtop increased dose at
various depths of basal cell layer (0.1-0.4 mm) by 55.3%-
63.2%. The measured dose increase at 0.4 mm depth
ranged between 60.6% and 74.6% with field sizes 20×20
cm 2 to 5×5 cm2, the corresponding Co-60 unit increase for
a 10×10 cm2 field being 18.1%. To directly compare two
prescribed treatment beams, when the PDDs were
normalized at 10 cm depth for a 10×10 cm2 field, although
dose to subcutaneous tissues was always higher with the
Co-60 unit, it produced an at least 49.7% lower skin basal
layer dose.
Conclusion
The beam attenuation values should be applied in
treatment planning. The obtained skin dose results
support and explain the higher observed skin effects in
patients treated on the Compact unit compared to those
previously treated on the Co-60 unit with similar 180°
gantry angle beams. Modifying the treatment techniques
to reduce the fraction of the dose delivered through the
couchtop and/or the use of a ‘tennis racket’ type carbon
fiber couchtop should be considered.
EP-1509 Small fields defined by jaw or MLC: evaluation
of MU estimation by AAA and Acuros algorithms
F. Lobefalo1, A. Fogliata1, G. Reggiori1, A. Stravato1, S.
Tomatis 1, M. Scorsetti2 , L. Cozzi2
1 Humanitas Research Hospital and Cancer Center,
Radiation Oncology, Milan-Rozzano, Italy
2 Humanitas Cancer Center and Humanitas University,
Radiation Oncology, Milan-Rozzano, Italy
Purpose or Objective
The small field output factor measurements are studied in
literature, covering the aspects of lack of charged particle
equilibrium, the partial occlusion of the finite source, and
the detector’s volume and response. However, the related
accuracy of the MU calculation from dose calculation
algorithms has not been investigated with similar
intensity. Aim of the present work is the evaluation of the
MU calculation accuracy for small fields generated by jaw
or MLC for two photon dose calculation algorithms in the
Eclipse system (Varian): AAA and Acuros. Simple static
beam geometries were chosen in order to better estimate
the accuracy with no additional biases. Flattening filter
free beams (6 and 10 MV) and and flattened 6MV were
evaluated.
Material and Methods
Single point output factor measurement were acquired
with a PTW microDiamond detector for 6MV, 6 and 10MV
unflattened beams generated by a Varian TrueBeamSTx
equipped with HD-MLC. Since the greatest
indetermination of the measurement accuracy resides in
the detector sensitivity correction factors for detector,
different corrections, field size dependent, were applied
according to different publications on the used detector.
Fields defined by jaw or MLC apertures were set; jaw-
defined: 0.6x0.6, 0.8x0.8, 1x1, 2x2, 3x3, 4x4, 5x5 and
10x10 cm2; MLC-defined: 0.5x0.5 cm2 to the maximum
field defined by the jaw, with 0.5 cm stepping, and jaws
set to: 2x2, 3x3, 4x4, 5x5 and 10x10 cm2. MU calculation
was obtained with 1 mm grid in a virtual waterphantom
for the same fields, for AAA and Acuros algorithms
implemented in the Varian Eclipse treatment planning
system (version 13.6). Configuration parameters as the
effective spot size (ESS) and the dosimetric leaf gap (DLG)
were varied to find the best parameter setting.
Differences between calculated and measured doses were
analyzed.
Results
Agreement better than 0.5% was found for field sizes equal
to or larger than 2x2 cm2. In the following the results are
given for the two extreme detector sensitivity correction
factors, with the second value in brackets. A dose
overestimation was present for smaller jaw-defined fields,
with the best agreement, over all the energies, of 1.6
(0.5)% and 4.6 (3.5)% for a 1x1 cm2 field calculated by AAA
and Acuros, respectively, for a configuration with EES=1
mm for X, Y directions for AAA, and EES=1.5, 0 mm for X,
Y direction for Acuros. Conversely, a calculated dose
underestimation was found for small MLC-defined fields,
with the best agreement averaged over all the energies,
of -3.9 (-4.9)% and 0.2 (-0.8)% for a 1x1 cm2 field
calculated by AAA and Acuros, respectively, for a
configuration with EES=0 mm for both directions, both
algorithms.
Conclusion
For optimal setting applied in the algorithm configuration
phase, the agreement of Acuros calculations with
measurements could achieve the 3 (6)% for MLC-defined
fields as small as 0.5x0.5cm 2. Similar agreement was found
for AAA for fields as small as 1x1 cm2.
EP-1510 Dosimetric characterisation of stereotactic
cones by means of MC simulations
A. Nevelsky1, E. Borzov1, S. Daniel 1, R. Bar-Deroma1
1Rambam Medical Center, Oncology, Haifa, Israel
Purpose or Objective
The objective of this work was to employ an MC model of
6MV FFF beam from the ELEKTA VersaHD linac to perform
dosimetric investigation of the new ELEKTA stereotactic
cones.
Material and Methods
The BEAMnrc code was used to create detailed model of
the linac head and stereotactic cones for the 6MV FFF
beam based on the manufacturer data supplied by Elekta.
MC simulation with the BEAMnrc code generated the
phase-space file which was used in the DOSXYZnrc code to