A Novel Approach to Contamination Suppression in Transmission Detectors for Radiotherapy

Beck, L.; Velthuis, J. J.; Page, Ryan; Hugtenburg, Richard P.; Sio, Chiara De; Pritchard, J. L.

doi:10.1109/trpms.2020.2995059

Cited by 5 publications

(4 citation statements)

References 23 publications

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“…Here we present a method for dosimetry using a very thin layer of conversion material in strips of two thicknesses, producing a grating pattern (International patent granted 2018), as shown in figure 1, to maintain a very low attenuation. This has been shown to work as a proof-of-principle experiment previously in Beck et al (2020). It is key to pick a type of material with appropriate thickness to minimally change the signal from the electron contamination, whilst causing a larger change in the photon signal, between thicker and thinner strips in the grating.…”

Section: Introductionmentioning

confidence: 93%

“…The distribution of energy deposited by electrons in X and Y will be due to a combination of the angular distribution of the electrons and the flux at each point. It has been shown in Beck et al (2020) (figures 13 and 14) that the angle of incidence of the electrons impacts the signal generated at the edges of the grating, putting a limit on the pitch of the grating.…”

Section: Phasementioning

confidence: 99%

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Electron contamination suppression in transmission detectors for radiotherapy

Beck,

De Sio,

Hugtenburg

et al. 2023

Phys. Med. Biol.

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Objective: Higher energy and intensity radiotherapy beams are being used, in part, due to the increased spatial accuracy of treatments. However, higher intensity beams can result in a larger total dose error, motivating the increasing need for real-time dose monitoring. We are developing a thin, real-time upstream Monolithic Active Pixel Sensor based system for beam monitoring with excellent precision on measuring the beam shape. Here we present a method to additionally provide dosimetry by adding thin conversion material in strips to the surface of the detector, a grating structure. Approach: By modulating the thickness of the conversion material to minimally disturb the contamination electron signal while enhancing the photon signal, the difference in these signals can be used to extract a photon-only signal, and hence dose. The simulation software Gate, based on Geant4, is utilised to study whether well functioning gratings can be better made from aluminium or copper and to optimise the thickness of a copper grating. Main results: It is possible to enhance the photon signal by a factor 6.7 (7.7) compared to the bare sensor for a 5.8 (6.7) MV beam, without modulation of the signal due to beam electrons. Significance: The grating can be used to perform dosimetry in real-time using a thin upstream detector.

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Section: Introductionmentioning

confidence: 93%

Section: Phasementioning

confidence: 99%

Electron contamination suppression in transmission detectors for radiotherapy

Beck,

De Sio,

Hugtenburg

et al. 2023

Phys. Med. Biol.

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“…The concept was patented in [ 20 , 21 ]. Dosimetry with the MAPS was shown in [ 22 , 23 ] and patented in [ 24 ].…”

Section: Introductionmentioning

confidence: 99%

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy

Velthuis

Pritchard

et al. 2023

Sensors

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Intensity-modulated radiotherapy is a widely used technique for accurately targeting cancerous tumours in difficult locations using dynamically shaped beams. This is ideally accompanied by real-time independent verification. Monolithic active pixel sensors are a viable candidate for providing upstream beam monitoring during treatment. We have already demonstrated that a Monolithic Active Pixel Sensor (MAPS)-based system can fulfill all clinical requirements except for the minimum required size. Here, we report the performance of a large-scale demonstrator system consisting of a matrix of 2 × 2 sensors, which is large enough to cover almost all radiotherapy treatment fields when affixed to the shadow tray of the LINAC head. When building a matrix structure, a small dead area is inevitable. Here, we report that with a newly developed position algorithm, leaf positions can be reconstructed over the entire range with a position resolution of below ∼200 μm in the centre of the sensor, which worsens to just below 300 μm in the middle of the gap between two sensors. A leaf position resolution below 300 μm results in a dose error below 2%, which is good enough for clinical deployment.

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“…Real-time verification is therefore desirable. We are developing a large area CMOS MAPS system based on the Athena, a further development of the LASSENA [3], placed upstream of the patient to verify the treatment in real-time by measuring leaf positions [4][5][6] and dose [7]. The sensor measures 12 × 14 cm 2 , with a 3T pixel architecture and 50 μm pixel pitch.…”

Section: Introductionmentioning

confidence: 99%

Complex field verification using a large area CMOS MAPS upstream in radiotherapy

Pritchard¹,

Velthuis²,

Beck³

et al. 2022

J. Inst.

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A multileaf collimator (MLC) is an integral component in modern radiotherapy machines as it dynamically shapes the photon field used for patient treatment. Currently, the MLC leaves which collimate the treatment field are mechanically calibrated to ±1 mm every 3 months and during pre-treatment calibration are calibrated to the mechanically set leaf positions. Leaf drift can occur between calibration dates and hence exceed the ±1 mm tolerance. Pre-treatment verification, increases LINAC usage time so is seldom performed for each individual patient treatment, but instead for an acceptable sample of patients and/or treatment fractions. Independent real-time treatment verification is therefore desirable. We are developing a large area CMOS MAPS upstream of the patient to monitor MLC leaf positions for real-time treatment verification. CMOS MAPS are radiation hard for photon and electron irradiation, have high readout speeds and low attenuation which makes them an ideal upstream radiation detector for radiotherapy. Previously, we reported on leaf position reconstruction for single leaves using the Lassena, a 12 × 14 cm2, three side buttable MAPS suitable for clinical deployment. Sobel operator based methods were used for edge reconstruction. It was shown that the correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 60.6 ± 8 and 109 ± 12 μm for a single central leaf with leaf extensions ranging from 1 to 35 mm using 0.3 sec of treatment beam time at 400 MU/min. Here, we report on leaf edge reconstruction using updated methods for complex leaf configurations, as occur in clinical use. Results show that leaf positions can be reconstructed with resolutions of 62 ± 6 μm for single leaves and 86 ± 16 μm for adjacent leaves at the isocenter using 0.15 sec at 400 MU/min of treatment beam. These resolutions are significantly better than current calibration standards.

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A Novel Approach to Contamination Suppression in Transmission Detectors for Radiotherapy

Cited by 5 publications

References 23 publications

Electron contamination suppression in transmission detectors for radiotherapy

Electron contamination suppression in transmission detectors for radiotherapy

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy

Complex field verification using a large area CMOS MAPS upstream in radiotherapy

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