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detector was positioned in the dose minimum
below. Finally, the detector in air instead of water with
a PMMA cap fitted on top. A range of typically used
detectors were analyzed, namely a microDiamond, a
PinPoint ionization chamber, an EDGE diode, as well as
three shielded and three unshielded diode detectors. EBT3
Gafchromic film served as reference. Measurements were
carried out on a PRIMUS linac at a photon beam quality of
6 MV with field sizes between 0.8 and 10 cm.
Responses in the blocked field and the PMMA setup were
combined to calculate the response in the open square
fields. The results were interpolated to a general matrix
from which responses in any field could be calculated.
Examples of such fields were measured for comparison.
Results
A higher detector overresponse and increasing detector to
detector differences were observed when the primary
beam was blocked out, whereas almost identical response
was seen for all detectors in the primary beam. A
combination of the responses in those two setups in a
detector-dependent ratio reproduced the values obtained
in the open field geometry with less than 1% deviation for
all detectors studied and all quadratic field sizes. For
rectangular and offset fields the agreement is still almost
within 1 %. Only when the detector was close to the field
edge larger deviations occurred (fig. 2).
Conclusion
Detector responses in open fields could be calculated from
the response to scatter and in the primary beam with 1%
agreement in all studied square fields and for all studied
detectors. The calculation was extended to rectangular
and non-symmetric fields yielding results in agreement
with the measurements for a wide range of fields. This
method suggests a way to calculate correction factors for
arbitrary fields.
PO-0776 Thermoluminescence Characteristics of
Fabricated Ge Doped Optical Fibre for Radiotherapy
Dosimetry
M.S. Ahmad Fadzil
1
, N. Tamchek
2
, N.M. Ung
3
, A. Ariffin
4
,
N. Abdullah
5
, D.A. Bradley
6
, N. Mohd Noor
1
1
Universiti Putra Malaysia, Department of Imaging-
Faculty of Medicine and Health Sciences, Serdang,
Malaysia
2
Universiti Putra Malaysia, Department of Physics-
Faculty of Science, Serdang, Malaysia
3
University of Malaya, Clinical Oncology Unit- Faculty of
Medicine, Kuala Lumpur, Malaysia
4
University of Malaya, Department of Physics- Faculty of
Science, Kuala Lumpur, Malaysia
5
Malaysia Nuclear Agency, Medical Physics Group,
Kajang, Malaysia
6
University of Surrey, Department of Physics, Guildford,
United Kingdom
Purpose or Objective
The dosimetry characteristics of the fabricated
Germanium doped cylindrical fibre (CF) and flat fibres (FF)
were evaluated including linearity, repeatability, energy
dependence, dose rate dependence, angular dependence
and fading. The TL kinetic parameters of Ge-doped CF and
FF have been investigated using the computerized glow
curve deconvolution analysis.
Material and Methods
A screening process was carried out for the optical fibres
with 6 MV photon beam. Optical fibres with sensitivity out
of ±5% mean sensitivity value were discarded in order to
ensure the selected dosimeters have constant response.
The dosimetry characteristics tests were performed using
6 MV and 10 MV photon beams. The dosimeters were
irradiated with doses ranging from 1 Gy to 3 Gy with
several dose rates (100 MU/min up to 500 MU/min).
Fabricated perspex phantoms were used to study the
angular dependency of the optical fibres. Fading rate was
studied for 55 days post irradiation. The glow curves were
analyzed with a curve fitting computer program known as
WinGCF. The glow curves were deconvoluted into five
individual peaks in order to figure out the kinetic
parameters such as the maximum peak temperature
(
T
max
), peak integral (
PI
), activation energy (
E
a
) and
frequency factor.
Results
The screening result revealed that the coefficient of
variation was observed to be less than ±10% for CF and FF.
Both CF and FF were found to be linear with r
2
more than
0.99 over the entire dose range explored for both 6 MV and
10 MV photon beams. These fibres provide consistent
reading within ±5% over five repeated measurements. The
signal lost was higher in FF (57%) compare to CF (38%) after
55 days of irradiation. Both fibres also offer dose rate- and
angular independence. The glow curve for both CF and FF
consist of 5 individual glow peaks. The peak height
increased with increasing irradiation dose. The T
max
of the
glow peaks (P1 to P5) is consistent over the dose range
used
.
Peak 1 has the lowest E
a
which lies between 0.544
to 0.636 eV and 0.632 to 0.720 eV for CF and FF
respectively, indicating the shallow electron traps. The
results also revealed that the PI for both of the fibres will
increase as the dose increase. The Ge-doped CF and FF
demonstrated a constant glow curves shape with
increasing dose. As the dose increases, area under the
glow curve increases, suggesting an increasing number of
electrons released from its traps.
Conclusion
CF demonstrated greater TL signal compared to FF. In
order to employ these optical fibres in absorbed dose
measurement, correction factors for energy dependence
and fading should be applied. The Ge-doped CF and FF
demonstrated the second-order kinetic model to the high
temperature half of the curve is slightly broader than the
low temperature half which suggest the possibility of
strong electron retrapping. The evaluation on Ge doped
optical fibres showed a highly favourable TL
characteristics exhibited by CF and FF indicate a great
potential in radiotherapy postal dose audit.
PO-0777 Importance of dosimetry formalism for cells
irradiation on a SARRP and consequences for RBE
M. Dos Santos
1
, V. Paget
2
, M. Ben Kacem
2
, F. Trompier
3
,
M. Benadjaoud
1
, A. François
2
, O. Guipaud
2
, M.