ESTRO 36 Abstract Book

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S766 ESTRO 36 2017
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dose. For each of the treatment sites evaluated, the
computed dose typically showed closer agreement with
the Eclipse TPS calculation than the measured dose. This
study demonstrated that for the Prostate and Node
treatment site the average difference in gamma index
between the computed and measured dose was within -
0.51%. This was -1.22% and -2.02% for Head and neck and
Brain treatment sites respectively.
Conclusion
This result verified that the IBA Compass system is
sufficiently accurate and has been adopted for RapidArc
treatment plan verification based on either
measurements, computation or both.
EP-1452 Evaluation of a collapsed-cone algorithm in a
commercial software for in vivo volumetric dosimetry
J. Gimeno Olmos1, V. Carmona1, F. Lliso1, B. Ibanez-
Rosello1 , J. Bautista1, J. Bonaque1, J. Perez-Calatayud 1
1 Hospital Universitari i Politecnic la Fe, Radiotherapy
department, Valencia, Spain
Purpose or Objective
Dosimetry Check (DC) (Math Resolutions) commercial
software performs pre-treatment and transit EPID-based
dosimetry. It provides a verification of treatments, being
of interest due to the benefits of the in vivo volumetric
dosimetry, which guarantee treatment delivery and
anatomy constancy.
In this study, the performance of a newly introduced
collapsed-cone (CC) dose calculation algorithm is
evaluated, as compared with the currently available
pencil beam (PB) algorithm and with a conventional
Treatment Planning System (TPS) and ionisation chamber
measurements.
Material and Methods
The commercial version of DC (v.4.11) is only CE and FDA
cleared for PB algorithm. The CC algorithm is being used
as a beta version (v.5.1).
To test if the CC algorithm considers heterogeneities
correcty, measurements were done in the IMRT Thorax
Phantom (CIRS), which simulates a human thorax. It has
several inserts for ionisation chamber measurements.
Six plans were generated, similar to the already published
work for the PB commissioning (Phys Med 30: 954-9).
Three with the isocentre in the phantom centre (isocentre
A, tissue equivalent): (1) four open 10x10 cm static fields
in box configuration, (2) 10x10 cm rotational field, (3)
typical lung clinical treatment (patient A); and three
centred in the phantom’s left lung (isocentre B): (4) and
(5) equivalent to (1) and (2), (6) typical lung clinical
treatment (patient B).
The plans were delivered in a Clinac iX (Varian)
accelerator equipped with EPID aS1000, acquiring cine
images, which were then converted to fluence by DC to
finally calculate dose with PB and CC algorithms. The
plans were also calculated in the TPS Eclipse v.13.0
(Varian) with AAA and Acuros XB algorithms.
Calculated point doses were compared against ionisation
chamber measurements, performed in the isocentre for
each plan with a PinPoint chamber model 31006 (PTW).
DC dose distributions were also evaluated against TPS
(Acuros algoritm) using 3D gamma analysis (3% global/3
mm) for the structure defined by the 95% isodose.
Results
Results are shown in table 1. As expected, CC algorithm
improves PB results, mainly in isocentre B where the
heterogeneities have greater effects. For isocentre A, the
mean difference improves from 0.6% for PB to -0.2% for
CC, while for isocentre B, it improves from 6.5% to -0.8%.
A very significant improvement in the gamma analysis is
also observed. Figure 1 shows an example of dose
distribution.
It has to be mentioned that the calculation time for CC
algorithm is of the order of hours, making this algorithm
not yet suitable for routine patient verifications. An
improvement is expected by the manufacturer to allow
GPU calculations.
.Conclusion
The possibility of in vivo evaluation and the potentiality of
this new system have a very positive impact on improving
patient QA. CC algorithm provides much better results in
heterogeneous cases, but it is at the cost of a higher
computation time. Improvements are also required in the
integration of DC with the R&V system.
EP-1453 Modeling a carbon fiber couch in a
commercial Treatment Planning System
R. Gómez Pardos1, D. Navarro Jiménez1, A. Ramírez
Muñoz1, E. Ambroa Rey1, M. Colomer Truyols 1
1Consorci Sanitari de Terrassa CST, Radiotherapy,
Terrassa, Spain
Purpose or Objective
With the increased use of carbon fiber couch tops and the
raise of techniques like VMAT with considerable dose
delivered from posterior angles, currently their modeling
is strongly recommended (Report of AAPM Task Group
176).
The main objective of this work is to model the iBEAM®
evo Couchtop in the TPS Monaco. The second goal is to
assess the overall impact of not using the couch in VMAT
calculations comparing gamma passing rates with an
Octavius4D phantom (PTW, Freiburg, Germany).
Material and Methods
The modeling was made for an Elekta Synergy LINAC with
Agility head equipped with the iBEAM couch. The EasyCube
homogeneous phantom (Euromechanics Medical Gmbh,
Nuremberg, Germany) was placed centered on the couch
and aligned with the isocentre. The charge was measured
with a Farmer ionization chamber every 5 gantry degrees,
100 MU/field, 10x10 cm2 field size, for both 6 and 15 MV.
All the measurements were corrected by pressure and
temperature. The relative to zero gantry degree
attenuation was calculated for every gantry angle
analyzed. Previously the absolute dose at 0 gantry angle
was measured.
The couch was modeled by an outer shell of carbon fiber
(CF) and an inner part of foam (Foam). The same
measured fields were calculated in the Monaco TPS with
the Monte Carlo algorithm, 1% Statistical Uncertainty per
Control Point, 2 mm grid spacing, dose to water. Then the
relative electron density (rED) of both CF and Foam
volumes was adjusted iteratively to match the measured
and calculated dose attenuation in several angles.
Finally 36 VMAT patients of different pathologies
previously irradiated on the Octavius4D phantom were
compared with the calculated plans both with and without