ESTRO 36 Abstract Book

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S408 ESTRO 36 2017
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Conclusion
All together these results strongly support the fact that an
accurate dosimetry needs to be performed before an
experiment but also to cautiously follow all the defined
parameters for one condition of irradiation to avoid errors
in the dose delivered on the sample and to be able to
properly compare and interpret experiments.
PO-0778 New Razor silicon diode for Cyber Knife small
beam relative dosimetry: a multi-site evaluation
S. Russo 1, L. Masi2, P.R. Dicarolo3, R. Doro2, E. De
Martin4, M.L. Fumagalli4, A.S. Martinotti5, A. Bergantin 5,
E. Rondi6 , S. Vigorito6 , P. Mancosu7
1 Aziend USL Toscana Centro, Fisica Sanitaria, Florence,
Italy
2 IFCA, Radiotherapy, Firenze, Italy
3 Medical Physycs, Meyer Children’s University Hospital-,
Florence, Italy
4 Istituto Besta, Radiotherapy, Milano, Italy
5 C.D.I., Radiotherapy, Milano, Italy
6 I. E. O., Radiotherapy, Milano, Italy
7 Humanitas Research Hospital, Medical Physics Unit of
Radiation Oncology, Milan, Italy
Purpose or Objective
The aim of this work was to evaluate the suitability of a
new unshielded p-type silicon diode (Razor, IBA
Dosimetry, Germany) for relative small beams dosimetry
over different CyberKnife systems.
Material and Methods
Output Factors (OFs) measurements with Razor detector
were performed by four Italian Radiotherapy Centers
equipped with CyberKnife units for field sizes ranging from
5 to 60 mm, defined by fixed circular collimators. Setup
conditions were 80 cm source to detector distance and 1.5
cm depth in water. Measurements were repeated by each
center with a PTW-60017 diode. Monte Carlo correction
factors reported in literature were applied to PTW-60017
measured data and corrected values were considered as a
reference.
Crossplane and inplane dose profiles ranging from 5-60 cm
fixed collimators were measured by Razor detector at a
depth of 10 cm in water and SSD 70 cm. The effective field
size (EFS), defined as EFS=, where A and B correspond to
the in- and cross-line FWHM, were calculated. Penumbra
20%- 80% was also evaluated.
This work has been conducted in the framework of the
Italian Association of Medical Physics (AIFM) SBRT working
group.
Results
Razor OFs measured for fixed collimators in the four
enrolled centers showed a variability (relative range)
decreasing from 1.2% to 0.4% for field sizes from 7.5 to 60
mm and equal to 2.2% for the smallest cone. The
variability obtained for OF measured by PTW-60017 was
analogous: lower than 1 % for field sizes from 7.5 to 60
mm and equal to 3.5% for the smallest diameter.
For field sizes down to 7.5 mm Razor measured OFs were
lower than PTW-60017 uncorrected measured values.
Relative differences between Razor OFs and Monte Carlo
corrected PTW-60017 data were below 1% for 60-10 mm
cone sizes and within 2 % for 7.5 mm field size over all
centers. For the smallest collimator differences ranging
from to 2.5% to 6% were observed among centers. Average
values and SD of OFs measured by Razor and PTW-60017
diode (MC corrected and not) are shown in figure.
Nominal field size NFS, effective field size EFS and
penumbra Razor measurements averaged over the four
CyberKnife centers are reported in table. Maximum
difference between NFS and EFS was about 6% for 5 mm
field size. Penumbra values were lower than 3 mm for field
sizes up to 15 mm.
Conclusion
Conclusions: CyberKnife OFs measured by Razor showed a
high consistency among different centers and a
comparable variability to data obtained by PTW-60017
routine detector. Comparison between Razor OFs and
PTW-60017 measurements corrected by Monte Carlo
indicated that correction factors for Razor should be
smaller than for PTW-60017 down to 7.5 mm field size. EFS
and penumbra measured over the four centers showed a
good consistency confirming Razor as a good candidate for
small beam relative dosimetry.
PO-0779 New robotic phantom for evaluation of
imaging and radiotherapy of moving structures
H. Arenbeck1, L. Eichert1, G. Hürtgen 2, K. Gester2, I.
Brück2, N. Escobar-Corral2, M. Fleckenstein1, A. Stahl3,
M.J. Eble2
1Boll Automation GmbH, Research and Development,
Kleinwallstadt, Germany
2RWTH Aachen University Hospital, Radiooncology and
Radiotherapy, Aachen, Germany
3RWTH Aachen University, III. Institute of Physics B,
Aachen, Germany
Purpose or Objective
Four dimensional radiotherapy processes that allow an
adaptation to intrafractional motion require increased
accuracy of dose application while displaying increased
technological and procedural complexity and thus
multiplied sources of error. Consequentially, novel 4D
phantoms are required that feature anthropomorphic
structure and motion. In this work, a prototype of such
phantom, which is fit for long term clinical service, is
presented.
Material and Methods
The modular phantom architecture allows different static
and moving human equivalent structures and dose
measurement devices to be placed into the irradiated
region. A new kind of parallel robot generates freely
programmable motion in all Cartesian directions. The
whole system is portable and features similar extension as
a human. Concept, kinematics, construction and software
of a previously presented evaluation model have been
fundamentally refined.