The characterisation of MP512 for 2D/3D dose reconstruction for small beams

Stereotactic radiation therapy such as SRS and SBRT utilise multiple beams delivery with a small radiation field and high dose gradients. A quality assurance tool with high stability and linearity of radiation response that can map a 2D dose read out in real time and with a high spatial resolution is needed to accurately verify two-dimensional (2D) pre-treatment dose distributions. This thesis describes two 2D monolithic diode arrays based on different silicon substrates called MagicPlate-512 (MP512) designed and developed by the Centre for Medical Radiation Physics to verify small field dosimetry. The first substrate is based on bulk p-type silicon known as MP512-Bulk and the other is based on the high resistivity of a thin epitaxial layer known as MP512-EPI. MP512 allows real time 2D dose mapping with a high spatial and temporal resolution. Both detector arrays consist of 512 0.5x0.5 mm2 active pixels with a 2 mm pitch that covers an area of 52x52 mm2. The angular response of MP512-Bulk as well as its correction factor were investigated for various field sizes and photon energies. It showed a maximum variation of relative angular response normalised to an incidence beam angle zero at a beam angle of 90° of approximately 18.5±0.5% and 15.5±0.5% for 6 MV and 10 MV photon beams. These results indicate that the angular response is sensitive to the energy, whereas the variation of angular response is less affected by field size. The packaging and intrinsic asymmetry of the monolithic silicon detector array structure are the major elements that affect the angular dependence of an MP512-Bulk

The effect of an air gap on a 2D monolithic silicon detector for relative dosimetry

Purpose: To evaluate the impact of an air gap on the Magic Plate (MP512) response and optimize this gap for relative dosimetry in photon and electron beams. Materials and methods: MP512 is a 2D monolithic silicon detector manufactured on a p-Type substrate. The array consists of 512 pixels with 0.5 x 0.5 mm2 size and 2 mm pitch with an overall dimension of 52 x 52 mm2. The signal ratio (SR) as a function of beam size and the percentage were measured with MP512 in 6 MV and 10 MV photon beams. The enhanced dynamic wedge (EDW) beam profile measurements were performed for 6 MV photon beams. In this work the signal ratio is defined as the ratio of central axis MP512 reading for field sizes ranging from 0.5 x 0.5 cm2 to 10 x 10 cm2 and for the reference square field of side 10 cm at a depth of 10 cm in solid water phantom. The measurements were performed with an air gap immediately above the detector array of 0.5, 1.0, 1.2, 2.0 and 2.6 mm, respectively. The PDD was measured for field sizes 2x 2 cm2, 5x 5 cm2 and 10x 10 cm2 by scanning the MP512 from the depth of 0.5 cm to 10 cm. The beam profiles were measured for Varian linac enhanced dynamic wedge (EDW) angles of 15, 45 and 60 for field size 5x 5 cm2. The PDD for 6, 12 and 20 MeV electron beams were performed for a standard applicator providing 10x 10 cm2 field size. Results: The signal ratio measured with MP512 reduces with increasing air gap above the detector. The strongest effect of the air gap size was observed for small fields of 0.5x 0.5 cm2 and 1x 1 cm2 while the effect was negligible within ± 2% (1 standard deviation) for field sizes larger than 4x 4 cm2. The signal ratio measured with MP512 with air gaps of 0.5 mm and 1.2 mm showed a good agreement with signal ratio measured with the EBT3 film (within ± 2%) and MOSkinTM for 6 MV and 10 MV, respectively. Similar results were observed for the PDD measurement for field size 5x 5 cm2 and 10x 10 cm2. The PDD measured with M512 was in good agreement with Markus Ionization chamber (IC) within ± 1.6% (1 standard deviation) for 6 MV and ± 1.5% (1 standard deviation) for 10 MV. The PDD discrepancy for 2x 2 cm2 was within ± 3% of the EBT3 for both photon energies. The EDW dose profile matched well with the EBT3 for the air gap of 0.5 mm within ± 2% (1 standard deviation) for all wedge angles. The PDD measured by electron beams demonstrated no significant effect of the air gap size above MP512 for all energies. The results showed similar variations (within ± 3%) compared to Markus IC for both 0.5 mm and 2.6 mm gap. Conclusion: The MP512 diode array was demonstrated to be suitable as an in-phantom dosimeter for QA in small radiation treatment fields. The study shows that air gap size has a significant effect on small field photon beam dosimetry due to a loss of electronic equilibrium. The small air gaps of 0.5 mm and 1.2 mm were the best air gaps for 6 MV and 10 MV, respectively. The effect of the air gap in electron beam fields is not significant due to the fact that an electronic equilibrium is fully established

Optimisation of output factor measurements using the Magic Plate 512 silicon dosimeter array in small megavoltage photon fields

We evaluate the impact of an air gap and optimization of this air gap for the MP512 silicon detector array when operated in dosimetry mode for small photon field measurements in solid water. We present output factor measurements for 6MV and 10 MV photon beams with the square field sizes ranging from 0.5 to 10 cm2. The size of the air gap above the MP512 detector was changed from 0.5, 1.0, 1.2, 2.0 and 2.6 mm. We compare the output factors measurements of the MP512 with EBT3 film and the MOSkin dosimeter. For the two photon energies investigated, we find that the output factor measured by the MP512 reduce with increasing air gap and reducing of field size. The reduction in output factor is most pronounced for the 0.5 and 1 cm2 field sizes. The air gap of 0.5 mm and 1.2 mm showed good agreement with the EBT3 film and MOSkin output factor for 6 and 10 MV photon fields, respectively. The negligible effect on dosimetry for the field sizes larger than 4x4 cm2 demonstrates that the electronic disequilibrium caused by small air gap only influences the dosimetry measurements for small fields. The study shows that the output factor reduction is enhanced by increasing of air gap and demonstrates that the optimal air gap for the MP512 at 6 and 10 MV photon fields is 0.5mm

The angular dependence of a two dimensional monolithic detector array for dosimetry in small radiation fields

The purpose of this study is to investigate the directional dependence of a two dimensional monolithic detector array (M512) under 6 MV photon irradiation and to evaluate the effect of field size on angular dependence. Square fields of sizes: 3x3 cm2 and 10x10 cm2 were measured at the iso-centre of a cylindrical phantom. Beam angles with incidences from 00- 1800 in increments of 150 were used to investigate the central pixel angular response of M512, normalized to the pixel response for normal (0°) beam incidence. The angular response of the detector was compared to the response of EBT3 radiochromic film in the identical geometric orientation. The maximum angular dependence was observed at the angle 90°±15° to be -18.62% and -17.70% for the field sizes 3x3 cm2 and 10x10 cm2, respectively. The angular dependence of M512 showed no significant difference between field sizes of 3x3 cm2 and 10x10 cm2 (p\u3e0.05). The maximum dose difference measured by the central pixel of M512 and EBT3 for all angles are -20% for 3x3 cm2 field size and -18.58% for the 10x10 cm2 field. The diode array\u27s size and packaging effects the angular response of the detector. The angular correction factor is necessary to apply to increase accuracy in dosimetry for arc treatment delivery

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

Quality assurance of VMAT on flattened and flattening filter-free accelerators using a high spatial resolution detector

© 2020 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine Purpose: This study investigated the use of high spatial resolution solid-state detectors (DUO and Octa) combined with an inclinometer for machine-based quality assurance (QA) of Volumetric Modulated Arc Therapy (VMAT) with flattened and flattening filter-free beams. Method: The proposed system was inserted in the accessory tray of the gantry head of a Varian 21iX Clinac and a Truebeam linear accelerator. Mutual dependence of the dose rate (DR) and gantry speed (GS) was assessed using the standard Varian customer acceptance plan (CAP). The multi-leaf collimator (MLC) leaf speed was evaluated under static gantry conditions in directions parallel and orthogonal to gravity as well as under dynamic gantry conditions. Measurements were compared to machine log files. Results: DR and GS as a function of gantry angle were reconstructed using the DUO/inclinometer and in agreement to within 1% with the machine log files in the sectors of constant DR and GS. The MLC leaf speeds agreed with the nominal speeds and those extracted from the machine log files to within 0.03 cm s−1. The effect of gravity on the leaf motion was only observed when the leaves traveled faster than the nominal maximum velocity stated by the vendor. Under dynamic gantry conditions, MLC leaf speeds ranging between 0.33 and 1.42 cm s−1 were evaluated. Comparing the average MLC leaf speeds with the machine log files found differences between 0.9% and 5.7%, with the largest discrepancy occurring under conditions of fastest leaf velocity, lowest DR and lowest detector signal. Conclusions: The investigation on the use of solid-state detectors in combination with an inclinometer has demonstrated the capability to provide efficient and independent verification of DR, GS, and MLC leaf speed during dynamic VMAT delivery. Good agreement with machine log files suggests the detector/inclinometer system is a useful tool for machine-specific VMAT QA

Two-dimensional solid-state array detectors: A technique for in vivo dose verification in a variable effective area

Purpose:We introduce a technique that employs a 2D detector in transmission mode (TM) to verify dose maps at a depth of dmaxin Solid Water. TM measurements, when taken at a different surface‐to‐detector distance (SDD), allow for the area at dmax(in which the dose map is calculated) to be adjusted.Methods:We considered the detector prototype MP512 (an array of 512 diode‐sensitive volumes, 2 mm spatial resolution). Measurements in transmission mode were taken at SDDs in the range from 0.3 to 24 cm. Dose mode (DM) measurements were made at dmaxin Solid Water. We considered radiation fields in the range from 2x2cm2to 10x10 cm2, produced by 6 MV flattened photon beams;we derived a relationship between DM and TM measurements as a function of SDD and field size. The relationship was used to calculate, from TM measurements at 4 and 24 cm SDD, dose maps at dmaxin fields of 1x1cm 2 and 4x4cm2, and in IMRT fields. Calculations were cross‐checked (gamma analysis) with the treatment planning system and with measurements (MP512,films, ionization chamber).Results:In the square fields, calculations agreed with measurements to within±2.36%. In the IMRT fields, using acceptance criteria of 3%/3 mm, 2%/2 mm, 1%/1 mm, calculations had respective gamma passing rates greater than 96.89%,90.50%, 62.20% (for a 4 cm SSD); and greater than 97.22%, 93.80%, 59.00% (for a24 cm SSD). Lower rates (1%/1 mm criterion) can be explained by submillimeter misalignments, dose averaging in calculations, noise artifacts in film dosimetry. Conclusions:It is possible to perform TM measurements at the SSD which produces the best fit between the area at dmaxin which the dose map is calculated and the size of the monitored target

Technical Note: Angular dependence of a 2D monolithic silicon diode array for small field dosimetry

Purpose This study aims to investigate the 2D monolithic silicon diode array size of 52 × 52 mm2 (MP512) angular response. An angular correction method has been developed that improves the accuracy of dose measurement in a small field. Methods The MP512 was placed at the center of a cylindrical phantom, irradiated using 6 MV and 10 MV photons and incrementing the incidence of the beam angle in 15° steps from 0° to 180°, and then in 1° steps between 85° and 95°. The MP512 response was characterized for square field sizes varying between 1 × 1 cm2 and 10 × 10 cm2. The angular correction factor was obtained as the ratio of MP512 response to EBT3 film measured doses as a function of the incidence angle (Ɵ) and was normalized at 0° incidence angle. Beam profiles of the corrected MP512 responses were compared with the EBT3 responses to verify the effectiveness of the method adopted. Results The intrinsic angular dependence of the MP512 shows maximum relative deviation from the response normalized to 0° of 18.5 ± 0.5% and 15.5 ± 0.5% for 6 MV and 10 MV, respectively, demonstrating that the angular response is sensitive to the energy. In contrast, the variation of angular response is less affected by field size. Comparison of cross-plane profiles measured by the corrected MP512 and EBT3 shows an agreement within ±2% for all field sizes when the beams irradiated the array at 0°, 45°, 135°, and 180° angles of incidence from the normal to the detector plane. At 90° incidence, corresponding to a depth dose measurement, up to a 6% discrepancy was observed for a 1 × 1 cm2 field of 6 MV. Conclusion An angular correction factor can be adopted for small field sizes. Measurements discrepancies could be encountered when irradiating with very small fields parallel to the detector plane. Using this approach, the MP512 is shown to be a suitable detector for 2D dose mapping of small field size photon beams