Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

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test parameters (RPE, APE, rms RPE, rms APE) against the Elekta values in
the case of IMRT treatments. The analysis of variance of the short- and long-
term data showed that the stripe test is statistically significant to test the
leaf-positioning reproducibility.
Conclusion: The stripe test is a useful method to test the MLC perfor-
mance because it allows the quantitative evaluation of accuracy on leaf-
positioning and it is not time-consuming. However, for the use of this test
to control MLC accuracy in an IMRT QA program, it is necessary to choose
different tolerance levels against the ones suggested by the manufacturer.
A.175
PERSPECTIVES OF USING AN INTEGRAL QUANTITY FOR REFERENCE
DOSIMETRY OF SMALL PHOTON BEAMS
M. Pimpinella * A.S. Guerra C. Caporali V. De Coste L. Silvi
A. Petrucci S. Barile a Istituto Nazionale di Metrologia delle Radiazioni
Ionizzanti, ENEA-INMRI, Roma, Italy; b U.O. Fisica Sanitaria, Azienda Ospedaliera
S. Filippo Neri, Roma, Italy
Introduction: Assessment of absorbed dose to water at a point (Dw) in
photon beams with very small field size is still affected by quite large un-
certainty despite the effort devoted in the last decade to develop suitable
dosimetric methods. An alternative approach to the issue of dosimetry in
small beams is to change the reference quantity from dose at a point to
an integral dose. Hence, in this work the use of the integral quantity dose-
area product (DAP) is explored.
Materials and methods: The integral of Dw over an area larger than the
beam size (DAPw) has been considered for field diameter below 2 cm. The
parameters affecting the DAPw value were identified and studied by Monte
Carlo (MC) calculation. Two commercial large-area p-p ionization cham-
bers (ICs) with different cavity diameters were used for measurements. 6 MV
and 10 MV stereotactic photon beams with diameters of 2.0, 1.5 and 1.25 cm
produced by a Varian DHX accelerator were considered.
Results: Both MC and experimental results highlighted the need of chamber-
specific correction factors to obtain DAPw values independent of the air
cavity diameter. Correction factors to appropriately scale DAPw value from
an integration area to another were determined by MC simulations. For a
given area, the ratio of DAPw at 20 and 10 cm depth in water (DAPR20,10)
was found to be fairly independent of beam size while variations with beam
energy were similar to that of TPR20,10. Then, if a reference integration
area is defined, use of DAPw to characterize the accelerator output and
DAPR20,10 to specify the beam quality is practicable for small photon beams.
Conclusions: The results of this work show that, in order to use DAPw as
reference quantity, the first step is to agree upon a reference area for in-
tegrating the dose. Then, commercial ICs can provide reliable DAPw values
with the help of MC calculations. Work is ongoing to extend the present
results to other beam sizes and shapes as well as to different types of
accelerators.
A.176
CVD DIAMOND SCHOTTKY PHOTODIODE IN AWIRELESS CONFIGURATION
FOR IN-VIVO DOSIMETRY APPLICATION
M. Pimpinella A.S. Guerra V. De Coste M. Marinelli G. Prestopin
C. Verona A. Tonnetti G. Verona-Rinat M.D. Falc P. Bagal a Istituto
Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI, Roma, Italy;
b INFN – Dipartimento di Ingegneria Industriale, Università di Roma Tor Vergata,
Roma, Italy; c Department of Radiation Oncology, Università di Chieti SS.
Annunziata Hospital, Chieti, Italy
Introduction: In-vivo dosimetry should be the ultimate step of a treat-
ment planning verification programme allowing to check the whole
dosimetry procedure including occurrence of possible human errors.
However, to benefit from in-vivo dosimetry detectors that combine good
accuracy, precision and ease of use are required. To meet this demand a
new diamond detector in a wireless configuration has been developed and
tested.
Materials and methods: The synthetic single crystal diamond Schottky pho-
todiode developed at Rome Tor Vergata University has been used for
fabricating a wireless detector for offline absorbed dose measurements in
high energy photon and electron beams. Various prototypes have been built
and preliminary tests, mainly signal stability and temperature depen-
dence, have been carried out in order to choose the most appropriate device
hardware components. Then a basic dosimetric characterization of the se-
lected prototype has been performed and the detector features compared
with those of the cabled diamond detector commercially available from
PTW as microDiamond.
Results: The new detector has a planar geometry with dimensions of
5.5 mm × 14 mm × 3 mm. It is equipped with an adaptor for connection to
a commercial electrometer for the readout after irradiation. A careful se-
lection of the dosimeter components allowed to reduce within 0.5% effects
due to the temperature during irradiation (up to 38 °C) and the time elapsed
between irradiation and readout (up to 30 minutes). The detector showed
signal stability of 0.5%, response reproducibility within 1%, fairly good lin-
earity with dose in the range of 1–12 Gy and negligible dose rate dependence
up to 5 Gy/min.
Conclusions: The wireless detector showed dosimetric performances in
general slightly worse than those of the cabled microDiamond; neverthe-
less, it appears to be a convenient dosimeter for in-vivo application, providing
absorbed dose measurements with an uncertainty compatible with a tol-
erance level of 5%.
A.177
ESTABLISHMENT AT ENEA-INMRI OF A NEW ABSORBED DOSE TO WATER
PRIMARY STANDARD FOR MEDIUM-ENERGY X-RAY BEAMS
M. Pinto *, M. Pimpinella, M. D’Arienzo, A.S. Guerra. Istituto Nazionale di
Metrologia delle Radiazioni Ionizzanti (ENEA-INMRI), Roma, Italy
Introduction: Within the framework of the EURAMET-funded project
“MetrExtRT” , ENEA-INMRI has recently built
a novel miniaturized graphite calorimeter which, in a water phantom, re-
alizes the quantity absorbed dose to water, Dw, in medium-energy filtered
x-ray beams.
Materials and methods: The three graphite bodies of the new calorime-
ter are separated by 0.5 mm gaps which are evacuated using a high-
vacuum pumping system. To realize the unit of Dw, a conversion of the
calorimetric measurement is made using Monte Carlo simulations: the mea-
sured absorbed dose to graphite, averaged over the volume occupied by
the 21 mm dia calorimeter’s graphite core, is converted to the point-
quantity Dw at the reference depth in water of 2 g cm−2.
Results: In the irradiation set-up currently available at ENEA-INMRI, a com-
bined standard uncertainty of 1.9% has been obtained for the measurement
of Dw in medium energy x-rays with generating potentials of 180 kV or
250 kV. Calorimetric measurements of Dw at ENEA-INMRI are under val-
idation, within the ongoing international I)-S13 comparison
with three other European metro-logical institutes. Although the uncer-
tainty in the calorimetric determination of Dw is significantly lower than
the about 3% uncertainty of Dw determinations traceable to air-kerma
primary standards (IAEA TRS 277, AAPM TG 61), work is advancing, at ENEA-
INMRI toward the reduction of the current uncertainty figure of 1.9%. This
includes advancing the Monte Carlo simulation of x-ray beams and in-
creasing the dose rate at the measurement point.
Conclusions: The novel water–graphite calorimeter built at ENEA-INMRI
represents a unique measuring instrument of its kind, as no other calo-
rimeter exists with this design at present. After validation of its performance,
in the international I)-S13 comparison, a new national cali-
bration service will be provided traceable to the newly commissioned
calorimetric Dw standard (IAEA TRS 398).
e52 Abstracts/Physica Medica 32 (2016) e1–e70