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Kiev, Ukraine

Lerch M.L.F.,University of Wollongong | Petasecca M.,University of Wollongong | Cullen A.,University of Wollongong | Hamad A.,University of Wollongong | And 5 more authors.
Radiation Measurements | Year: 2011

Intensive synchrotron X-ray microbeams form an integral part of microbeam radiation therapy (MRT). MRT is a novel radiation medicine modality being developed for inoperable and otherwise untreatable brain tumours. The extremely high dose rate (∼20 kGy/s), laterally fractionated radiation field and steep dose gradients utilized in this therapy make real-time dosimetry a significant challenge. In order for this treatment to advance to the clinical trial stage of development real-time dosimetry systems must be developed. This paper demonstrates the capabilities of a new dosimetry system based on an epitaxial silicon detector. The system combines high spatial resolution and real-time readout and we have measured the lateral dose profile of the MRT radiation field which incorporates 59 X-ray microbeams. All microbeam peaks and valley regions between two microbeams are clearly resolved. The measured detector response at any point is reproducible to within 0.5% after scaling for the known synchrotron storage ring beam current lifetime. The variation of the lateral dose profile at different depths in a PMMA phantom has been measured with the results compared to those from Penelope Monte Carlo simulations. The trend in the measured response with depth agrees with the simulation data (within the experimental variation of the central five microbeams peaks and valleys measured). However the measured peak-to-valley ratio response is a factor of 4.5 ± 0.1 times lower than that expected. The disagreement was further investigated and shown to be contributed to by charge recombination effects at the low bias voltages used. © 2011 Elsevier Ltd. All rights reserved. Source

Hardcastle N.,University of Wollongong | Hardcastle N.,University of Wisconsin - Madison | Cutajar D.L.,University of Wollongong | Metcalfe P.E.,University of Wollongong | And 5 more authors.
Physics in Medicine and Biology | Year: 2010

Rectal balloons are used in external beam prostate radiotherapy to provide reproducible anatomy and rectal dose reductions. This is an investigation into the combination of a MOSFET radiation detector with a rectal balloon for real-time in vivo rectal wall dosimetry. The MOSFET used in the study is a radiation detector that provides a water equivalent depth of measurement of 70 m. Two MOSFETs were combined in a face-to-face orientation. The reproducibility, sensitivity and angular dependence were measured for the dual MOSFET in a 6 MV photon beam. The dual MOSFET was combined with a rectal balloon and irradiated with hypothetical prostate treatments in a phantom. The anterior rectal wall dose was measured in real time and compared with the planning system calculated dose. The dual MOSFET showed angular dependence within ±2.5% in the azimuth and +2.5%/-4% in the polar axes. When compared with an ion chamber measurement in a phantom, the dual MOSFET agreed within 2.5% for a range of radiation path lengths and incident angles. The dual MOSFET had reproducible sensitivity for fraction sizes of 2-10 Gy. For the hypothetical prostate treatments the measured anterior rectal wall dose was 2.6 and 3.2% lower than the calculated dose for 3DCRT and IMRT plans. This was expected due to limitations of the dose calculation method used at the balloon cavity interface. A dual MOSFET combined with a commercial rectal balloon was shown to provide reproducible measurements of the anterior rectal wall dose in real time. The measured anterior rectal wall dose agreed with the expected dose from the treatment plan for 3DCRT and IMRT plans. The dual MOSFET could be read out in real time during the irradiation, providing the capability for real-time dose monitoring of the rectal wall dose during treatment. © 2010 Institute of Physics and Engineering in Medicine. Source

Petasecca M.,University of Wollongong | Petasecca M.,Illawarra Health Medical Research Institute | Alhujaili S.,University of Wollongong | Aldosari A.H.,University of Wollongong | And 15 more authors.
Medical Physics | Year: 2015

Purpose: In this work, the "edgeless" silicon detector technology is investigated, in combination with an innovative packaging solution, to manufacture silicon detectors with negligible angular response. The new diode is also characterized as a dosimeter for radiotherapy with the aim to verify its suitability as a single detector for in vivo dosimetry as well as large area 2D array that does not require angular correction to their response. Methods: For the characterisation of the "edgeless-drop-in" detector technology, a set of samples have been manufactured with different sensitive areas (1×1 and 0.5×0.5 mm2) and different thicknesses (0.1 and 0.5 mm) in four different combinations of top and peripheral p-n junction fabricated on p-type and n-type silicon substrates. The diode probes were tested in terms of percentage depth dose (PDD), dose rate, and linearity and compared to ion chambers. Measurements of the output factor have been compared to film. The angular response of the diodes probes has been tested in a cylindrical PMMA phantom, rotated with bidirectional accuracy of 0.25 under 10×10 cm2 6 MV Linac photon beam. The radiation hardness has been investigated as well as the effect of radiation damage on the angular and dose rate response of the diode probes when irradiated with photons from a Co-60 gamma source up to dose of 40 kGy. Results: The PDDs measured by the edgeless detectors show an agreement with the data obtained using ion chambers within ±2%. The output factor measured with the smallest area edgeless diodes (0.5×0.5 mm2-0.1 and 0.5 mm thick) matches EBT3 film to within 2% for square field size from 10 to 0.5 cm side equivalent distance. The dose rate dependence in a dose per pulse range of 0.9×10-5-2.7×10-4 Gy/pulse was less than -7% and +300% for diodes fabricated on p-type and n-type substrates, respectively. The edgeless diodes fabricated on the p-type substrate demonstrated degradation of the response as a function of the irradiation dose within 5%-15%, while diodes on the n-type substrate show a variation of approximately 30% after 40 kGy. The angular response of all probes is minimal (within 2%) but the N on N and P on P configurations show the best performances with an angular dependence of ±1.0% between 0 and 180 in the transversal direction. In this configuration, the space charge region of the passive diode extends from the behind and sidewall toward the anode on the top providing beneficial electric field distribution in the peripheral area of the diode. Such performance has also been tested after irradiation by Co-60 up to 40 kGy with no measurable change in angular response. Conclusions: A new edgeless-drop-in silicon diode fabrication and packaging technology has been used to develop detectors that show no significant angular dependence in their response for dosimetry in radiation therapy. From the characterisation of the diodes, proposed in a wide range of different geometries and configurations, the authors recommend the P-on-P detectors in conjunction with "drop in" packaging technology as the candidate for further development as single diode probe or 2D diode array for dosimetry in radiotherapy. © 2015 American Association of Physicists in Medicine. Source

Petasecca M.,University of Wollongong | Cullen A.,University of Wollongong | Fuduli I.,University of Wollongong | Espinoza A.,University of Wollongong | And 9 more authors.
Journal of Instrumentation | Year: 2012

Microbeam Radiation Therapy (MRT) is a radiation treatment technique under development for inoperable brain tumors. MRT is based on the use of a synchrotron generated X-ray beam with an extremely high dose rate (∼ 20 kGy/sec), striated into an array of X-ray micro-blades. In order to advance to clinical trials, a real-time dosimeter with excellent spatial resolution must be developed for absolute dosimetry. The design of a real-time dosimeter for such a radiation scenario represents a significant challenge due to the high photon flux and vertically striated radiation field, leading to very steep lateral dose gradients. This article analyses the striated radiation field in the context of the requirements for temporal dosimetric measurements and presents the architecture of a new dosimetry system based on the use of silicon detectors and fast data acquisition electronic interface. The combined system demonstrates micrometer spatial resolution and microsecond real time readout with accurate sensitivity and linearity over five orders of magnitude of input signal. The system will therefore be suitable patient treatment plan verification and may also be expanded for in-vivo beam monitoring for patient safety during the treatment. © 2012 IOP Publishing Ltd and Sissa Medialab srl. Source

Aldosari A.H.,University of Wollongong | Petasecca M.,University of Wollongong | Espinoza A.,University of Wollongong | Newall M.,University of Wollongong | And 11 more authors.
Medical Physics | Year: 2014

Purpose: Silicon diode arrays are commonly implemented in radiation therapy quality assurance applications as they have a number of advantages including: real time operation (compared to the film) and high spatial resolution, large dynamic range and small size (compared to ionizing chambers). Most diode arrays have detector pitch that is too coarse for routine use in small field applications. The goal of this work is to characterize the two-dimensional monolithic silicon diode array named "MagicPlate-512" (MP512) designed for QA in stereotactic body radiation therapy (SBRT) and stereotactic radio surgery (SRS). Methods: MP512 is a silicon monolithic detector manufactured on ap-type substrate. An array contains of 512 pixels with size 0.5 × 0.5 mm2 and pitch 2 mm with an overall dimension of 52 × 52 mm 2. The MP512 monolithic detector is wire bonded on a printed circuit board 0.5 mm thick and covered by a thin layer of raisin to preserve the silicon detector from moisture and chemical contamination and to protect the bonding wires. Characterization of the silicon monolithic diode array response was performed, and included pixels response uniformity, dose linearity, percent depth dose, output factor, and beam profiling for beam sizes relevant to SBRT and SRS and depth dose response in comparison with ionization chamber. Results: MP512 shows a good dose linearity (R2 = 0.998) and repeatability within 0.2%. The measured depth dose response for field size of 10 × 10 cm2 agreed to within 1.3%, when compared to a CC13 ionization chamber for depths in PMMA up to 30 cm. The output factor of a 6 MV Varian 2100EX medical linac beam measured by MP512 at the isocenter agrees to within 2% when compared to PTW diamond, Scanditronix point EDD-2 diode and MOSkin detectors for field sizes down to 1 × 1 cm2. An over response of 4% was observed for square beam size smaller than 1 cm when compared to EBT3 films, while the beam profiles (FWHM) of MP512 match to within 2% the data measured by radiochromic film. Conclusions: The response of the 2D detector array, MP512, has been evaluated. The properties of the array demonstrated suitability for use as in phantom dosimeter for QA in SRS and SBRT. Although MP512 matches film measurements down to 1 × 1 cm2 well, it showed a discrepancy of 4% in the determination of output factors of beams smaller than 0.5 × 0.5 cm2 due to the field perturbation generated by the large amount of silicon surrounding the central diode. MP512 is highly capable of measuring beam size (FWHM) and has a discrepancy of less than 1.3% when compared to EBT3 film. A reduction in the detector pitch to less than 2 mm would improve the penumbra reconstruction accuracy at the cost readout electronics complexity. © 2014 American Association of Physicists in Medicine. Source

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