787 / 1096
S771
ESTRO 36
_______________________________________________________________________________________________
Conclusion
The application of different sensitometric curves and
correction methods caused errors in detected failed points
and passing rates in radiochromic film dosimetry. This can
lead misinterpret detected dose errors in quality
assurance. It is significant to apply the optimal
sensitometric curve with appropriate correction
methods, especially in modulated radiation fields for
reliable and accurate film dosimetry.
EP-1445 Performance evaluation of scintillators for
SiPM PET/MRI Brain Imaging
N. Campos Rivera
1
, B. Seitz
1
1
University of Glasgow, School of Physics & Astronomy,
Glasgow, United Kingdom
Purpose or Objective
The combination of PET and MRI has shown a great
potential to study the processes and progression of
diseases, such as cancer and Alzheimer’s, as well as to
control and observe novel treatments response. In the last
decade, whole-body hybrid systems have been
manufactured, such as the Biograph mMR by Siemens.
However, this apparatus tend to be very expensive and to
have limited performance on the neurological field. Thus,
a brain-dedicated portable PET insert with MRI
compatibility seems to be worth to investigate
Material and Methods
Recently, a SiPM with electronics integrated on cell level
has been developed: the Philips Digital Photon Counting
(PDPC). This device delivers a digital signal of the
detected photon counts as well as their timestamp,
making it a potential candidate for PET applications.
During this work, the performance of several scintillation
materials (GAGG, LYSO, LUAG and BGO) coupled with the
PDPC was studied. Each PDPC module has 64 channels of
3.2 x 3.8 millimeters, comprising 3200 cells of 59.4 x 64
micrometers and each scintillator crystals is 3 x 3 x 30
millimeters, polished on all faces with five of them
covered with a white reflective coating. Analysis on the
intrinsic performance of the SiPM (IV curves, temperature
dependence, crosstalk) and energy and time resolution for
every scintillator was carried out.
Results
Results showed that LYSO was the best candidate and so
detector blocks of this material were manufactured and
used to examine the behaviour of a full insert ring. The
experimental setup consisted on two 4x4 LYSO arrays
coupled to the PDPC. Each crystal of the array was 7 x 7 x
20 millimeters, fitting perfectly the size of the PDPC dies.
The detector blocks were placed facing each other and
separated by 30 mm with a Na-22 source in the middle and
rotating at a 30 degrees steps to imitate the geometry of
a
ring
of
the
PET
insert.
The outcome was used to create a GATE simulation that
explores the behaviour of this insert in the presence of
strong magnetic fields. Such simulation involves the design
of a ring device that moves along the patient axis and the
calculation of the detection efficiency varying the
geometry and speed of the apparatus.
Conclusion
Our findings show an adequate coupling of the PDPC with
the LYSO arrays and the simulation corroborates an
appropriate functionality of the detection system inside
an MRI machine. The next study phase involves the
construction of a prototype for real testing.
EP-1446 Can parallel plate ion chambers be used for
PDD measurements in FFF beams?
S. Vargas Castrillón
1
, F. Cutanda
2
1
Lanarkshire Beatson West of Scotland Cancer Centre,
Radiotherapy Physics, Airdrie, United Kingdom
2
Western General Hospital, Oncology Physics, Edinburgh,
United Kingdom
Purpose or Objective
The IAEA TRS-398 code of practice for radiation dosimetry
recommends measuring photon percentage depth-dose
(PDD) curves with plane-parallel ion chambers. This code
of practice was published before flattening-filter free
beams became widely used in clinical practice. This choice
of detector for PDD measurement should be re-assessed
for FFF beams, as the effect of recombination and polarity
factors, among others, could lead to differences. The
purpose of this work is to assess the use of plane parallel
ion chambers for PDD measurements in FFF beams, and to
compare them with other chamber types.
Material and Methods
Beams from Varian TrueBeam linacs, 6 FFF and 10 FFF
were used for this study. Depth dose curves (SSD=100cm)
were acquired with a PTW 31010 Semiflex, two small
volume chambers (Scanditronix-Wellhofer CC04 and PTW
31016 Pin Point 3D), PTW 34001 Roos, Scanditronix-
Wellhofer Roos and NACP 02 parallel plate chambers.
Measurements were carried out both in a PTW MP3 and an
IBA Bluephantom2 water tanks with PTW Tandem, PTW
Unidos E and CCU electrometers. PDD scans with plane
parallel ion chambers were acquired and corrected for ion
recombination. In order to apply this correction,
recombination factors were measured with the two
voltage technique for different depths and field sizes. The
effect of polarity was evaluated using both polarities for
measurements. Measurements with different detectors
were carried out for a set of field sizes, ranging from 5x5
to 40x40 cmxcm and SSD 100 cm SSD.
Results
It was found that parallel plate chambers show the closest
agreement between PDD curves acquired with different
polarities, being the differences below 0.1% for all depths
and 40x40 cmxcm.PDDs for one single polarity and
different ion chambers have been corrected for
recombination and compared. The largest difference in
PDD among different ion chambers, once corrected for
recombination, has been found for the Scanditronix Roos
chamber at 350 mm deep (excluding build up) for all field
sizes, which would amount to: 0.7% for 6 FFF and 0.6% for