521 / 1096
S506
ESTRO 36
_______________________________________________________________________________________________
practice without any guidance. Responses were evaluated
by the EAG and used to inform on best practice and
identify centres where additional support was required.
Results are given here for PTV margins used for metastatic
disease. PTV margins are particularly important when
treating multiple mets as they can increase the volume of
normal brain irradiated and the commissioning criteria
requires the total treated volume to be below 20cc, so the
choice of PTV margins can impact patient eligibility for
treatment.
Results
All 20 centres responded to the questionnaire with one
centre excluded as they were in process of changing
equipment. Responses are summarized in Table 1.
The most common platform used was the Elekta
Gammaknife system. A variety of linacs were used, the
majority of those used for the commissioning were
specialised units (e.g. Novalis, Truebeam STx) or had been
adapted (e.g. fixed cones) for SRS treatments.
Centres used a 0, 1 or 2mm margin for brain mets. All
Gammaknife centres used a 0mm margin, but other
platforms varied depending on the centre as seen in Figure
1. Only four centres used a 2mm expansion for treating
brain mets, three of these were non specialised linacs.
Conclusion
There is significant variation in the equipment used to
treat SRS nationally. A 0mm PTV expansion was the most
common for SRS treatment regardless of platform.
Gammaknife centres were consistent with their PTV
margins, which is based on historical practice but other
platforms varied depending on the centre. No system has
an end to end accuracy of 0mm, however many centres
are choosing to use, which may lead to under-coverage of
the target.
Following feedback, centres using non-specialised
equipment are planning to acquire either stereotactic
linacs or upgrades such as the Apex head, with some
frameless users acquiring Exactrac systems to reduce
uncertainty in patient positioning. These will facilitate
margin reduction at centres using 2mm PTV expansions, in
line with the ≤1mm recommended by the EAG.
PO-0914 Helium Beam Radiography System based on
pixelized semiconductor detectors
T. Gehrke
1
, G. Arico
1
, S. Berke
2
, J. Jakubek
3
, M.
Martisikova
4
1
Heidelberg University Hospital, Radiation Oncology and
Radiation Therapy, Heidelberg, Germany
2
German Cancer Research Center, Medical Physics in
Radiation Therapy, Heidelberg, Germany
3
Czech Technical University in Prague, Institute of
Experimental and Applied Physics, Prague, Czech
Republic
4
German Cancer Research Center DKFZ, Department of
Medical Physics, Heidelberg, Germany
Purpose or Objective
(1) Purpose: In highly conformal radiotherapy, like ion
beam radiotherapy, inter- and intrafractional monitoring
of the target is desirable. Due to the steepness of the
rising part of the Bragg curve, ion beam radiography can
in principle provide high resolution of the traversed tissue
thickness. Ion beam radiography is furthermore attractive
due to its potential to measure the stopping power of the
tissue directly. However, currently there is no detection
system for clinical imaging of patients. Helium ions as the
imaging modality provide the advantages of decreased
multiple scattering in comparison to protons and lower
biological effectiveness than the carbon ions.
Material and Methods
(2) Methods: Plastic phantoms contain ning 1mm deep
step-like inhomogeneities were imaged with helium ion
beams at the Heidelberg Ion Beam Therapy facility in
Germany. To register the radiation, a system of 5 parallel
layers, based on the semiconductor pixelized detector
Timepix, which was developed by the Medipix
Collaboration at CERN, was used. Two layers in front of
the phantom enabled us to measure the position and
direction of incoming helium ions. Another pair of
detection layers, located behind the phantom, registered
the outgoing particles and an additional layer was used to
measure their energy loss and to identify the ion type.
Synchronization of all the five detector layers enabled us
to associate the outgoing particles to the incoming ones.
To build the image of the phantom, we used the measured
information about the transversal position of the incoming
and outgoing particle, their direction and type (He or H).
Results
(3) Results: With this system we imaged a 1 mm step in a
160 mm thick PMMA phantom. Spatial resolution below 2
mm was reached when the inhomogeneity was located in
the phantom, while resolution below 1 mm was achieved
in the cases where the step was located at the front or at
the end of the phantom. Hereby we have shown that the
information about flight direction of the incoming and
outgoing ion, together with the capability to identify them
and thus select solely helium ions, enables to improve the
spatial resolution by a factor of more than three.
Conclusion
(4) Conclusion: We have shown experimentally that helium
beam radiography reaches in simple phantoms spatial
resolution in the region which is attractive for highly
conformal radiotherapy. In the presentation the results
obtained with helium beams as the imaging modality will
be compared to proton-based imaging.
PO-0915 Performance study of a prototype straight-
through linac delivery system with an EPID assembly
R. Scheuermann
1
, C. Kennedy
1
, D. Mihailidis
1
, J. Metz
1
1
University of Pennsylvania, Radiation Oncology,
Philadelphia, USA
Purpose or Objective
To study and expand the use of the Machine Performance
Check (MPC) tool in monitoring the continuous operational
performance of a prototype delivery system composed of
a straight-through-linac and an in-line MV portal imaging
panel (Proof-of-Concept).
Material and Methods
The MPC, as implemented in TrueBeam (TB), is an
integrated self-check tool that assures that critical
machine performance are within specifications, e.g.
mechanical accuracy and radiation output. As adapted to
the prototype straight-through linac delivering 6X-FFF
(filter-free) beam, the automated tests are based on its