Characterisation of a cobalt-60 small-beam animal irradiator using a realtime silicon pixelated detector

The paper presents a study performed by the Centre for Medical Radiation Physics (CMRP) using a high spatial and temporal resolution silicon pixelated detector named MagicPlate- 512. The study focuses on the characterisation of three pencil beams from a low-dose rate, 6 TBq, cobalt-60 source, in terms of percentage depth dose, beam profiles, output factor and shutter timing. Where applicable, the findings were verified against radiochromic EBT3 film and ionization chambers. It was found that the results of the MagicPlate-512 and film agreed within 0.9 mm for penumbra and full-width at half-maximum measurements of the beam profiles, and within 0.75% for percentage depth dose study. The dose rate of the cobalt-60 source was determined to be (10.65±0.03) cGy/min at 1.5 cm depth in Solid Water. A significant asymmetry of the small pencil beam profile was found, which is due to the irregular machining of the small collimator. The average source shutter speed was calculated to be 26 cm/s. The study demonstrates that the MagicPlate-512 dosimetry system, developed at CMRP, is capable of beam characterisation even in cases of very low dose rate sources

In vivo real-time rectal wall dosimetry for prostate radiotherapy

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 realtime 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 μm. 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

Today\u27s monolithic silicon array detector for small field dosimetry: The Octa

The dosimetry of small photon beams is challenging due to detector position uncertainties, dose averaging and lack of electronic equilibrium. Currently only few, single detectors are suitable for measurements in this context, and none is ideal. This study reports on the dosimetric characterization of small fields collimated by fixed cones, performed by a novel 2D monolithic silicon array detector, the Octa

On monolithic silicon array detectors for small-field photon beam dosimetry

Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Submillimeter accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications. 2D monolithic silicon array detectors can provide high spatial-resolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode-SVs are arranged with a sub-millimeter pitch along 4 intersecting orthogonal linear arrays. We report on the experimental and numerical characterization (performed with Sentaurus Workbench within the Synopsys framework) of two Octa detectors, manufactured respectively on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their large-area monolithic silicon wafers is compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency and clinical performance in the case of a small radiation field delivered with a flattening filter free beam

A novel high-resolution 2D silicon array detector for small field dosimetry with FFF photon beams

Purpose: Flattening filter free (FFF) beams are increasingly being considered for stereotactic radiotherapy (SRT). For the first time, the performance of a monolithic silicon array detector under 6 and 10 MV FFF beams was evaluated. The dosimeter, named Octa and designed by the Centre for Medical Radiation Physics (CMRP), was tested also under flattened beams for comparison. Methods: Output factors (OFs), percentage depth-dose (PDD), dose profiles (DPs) and dose per pulse (DPP) dependence were investigated. Results were benchmarked against commercially available detectors for small field dosimetry. Results: The dosimeter was shown to be a \u27correction-free\u27 silicon array detector for OFs and PDD measurements for all the beam qualities investigated. Measured OFs were accurate within 3% and PDD values within 2% compared against the benchmarks. Cross-plane, in-plane and diagonal DPs were measured simultaneously with high spatial resolution (0.3 mm) and real time read-out. A DPP dependence (24% at 0.021 mGy/pulse relative to 0.278 mGy/pulse) was found and could be easily corrected for in the case of machine specific quality assurance applications. Conclusions: Results were consistent with those for monolithic silicon array detectors designed by the CMRP and previously characterized under flattened beams only, supporting the robustn ess of this technology for relative dosimetry for a wide range of beam qualities and dose per pulses. In contrast to its predecessors, the design of the Octa offers an exhaustive high-resolution 2D dose map characterization, making it a unique real-time radiation detector for small field dosimetry for field sizes up to 3 cm side

New 3D Silicon detectors for dosimetry in Microbeam Radiation Therapy

Microbeam Radiation Therapy (MRT) involves the use of a spatially fractionated beam of synchrotron generated X-rays to treat tumours. MRT treatment is delivered via an array of high dose \u27peaks\u27 separated by low dose \u27valleys\u27. A good Peak to Valley Dose Ratio (PVDR) is an important indicator of successful treatment outcomes. MRT dosimetry requires a radiation hard detector with high spatial resolution, large dynamic range, which is ideally real-time and tissue equivalent. We have developed a Silicon Strip Detector (SSD) and very recently, a new 3D MESA SSD to meet the very stringent requirements of MRT dosimetry. We have compared these detectors through the characterisation of the MRT radiation field at the Australian Synchrotron Imaging and Medical Beamline. The EPI SSD was able to measure the microbeam profiles and PVDRs, however the effective spatial resolution was limited by the detector alignment options available at the time. The geometry of the new 3D MESA SSD is less sensitive to this alignment restriction was able to measure the microbeam profiles within 2 ¿m of that expected. The 3D MESA SSD measured PVDRs were possibly affected by undesired and slow charge collection outside the sensitive volume and additional scattering from the device substrate

Impact of a monolithic silicon detector operating in transmission mode on clinical photon beams

Purpose To investigate the effect on surface dose, as a function of different field sizes and distances from the solid water phantom to transmission detector (Dsd), of using the monolithic silicon detector MP512T in transmission mode. Methods The influence of operating the MP512T in transmission mode on the surface dose of a phantom for SSD 100cm was evaluated by using a Markus IC. The MP512T was fixed to an adjustable stand holder and was positioned at different Dsd, ranging from 0.3 to 24 cm. For each Dsd, measurements were carried out for irradiation field sizes of 5 × 5cm2, 8 × 8 cm2 and 10 × 10 cm2. Measurements were obtained under two different operational setups, (i) with the MP512T face-up and (ii) with the MP512T face-down. In addition, the transmission factors for the MP512T and the printed circuit board were only evaluated using a Farmer IC. Results For all Dsd and all field sizes, the MP512T led to the surface dose increasing by less than 25% when in the beam. For Dsd \u3e18 cm the surface dose increase is less than 5%, and negligible for field size 5 × 5 cm2. The difference in the surface dose perturbation for the MP512T operating face up or operating face down is negligible (sd and field sizes. Conclusion The study demonstrated that positioning the MP512T in air between the Linac head and the phantom produced negligible perturbation of the surface dose for Dsd \u3e18 cm, and was completely transparent for 6 MV photon beams

2D monolithic silicon-diode array detectors in megavoltage photon beams: does the fabrication technology matter? A medical physicist\u27s perspective

A family of prototype 2D monolithic silicon-diode array detectors (MP512, Duo, Octa) has been proposed by the Centre for Medical Radiation Physics, University of Wollongong (Australia) for relative dosimetry in small megavoltage photon beams. These detectors, which differ in the topology of their 512 sensitive volumes, were originally fabricated on bulk p-type substrates. More recently, they have also been fabricated on epitaxial p-type substrates. In the literature, their performance has been individually characterized for quality assurance (QA) applications. The present study directly assessed and compared that of a MP512-bulk and that of a MP512-epitaxial in terms of radiation hardness, long-term stability, response linearity with dose, dose per pulse and angular dependence. Their measurements of output factors, off-axis ratios and percentage depth doses in square radiation fields collimated by the jaws and produced by 6 MV and 10 MV flattened photon beams were then benchmarked against those by commercially available detectors. The present investigation was aimed at establishing, from a medical physicist\u27s perspective, how the bulk and epitaxial fabrication technologies would affect the implementation of the MP512s into a QA protocol. Based on results, the MP512-epitaxial would offer superior radiation hardness, long-term stability and achievable uniformity and reproducibility of the response across the 2D active area