A large area MAPS-based upstream device for radiotherapy verification

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Radiotherapy is a cancer therapy in which ionising radiation is used to target cancerous tissue within the human body. The therapeutic aim is to irradiate cancerous tissue and create DNA strand breaks such that the cancerous tissue is unable to self-repair and subsequently dies. The primary therapeutic consideration is to minimise healthy tissue irradiation such that the probability of secondary cancers is minimised. External Beam Radiotherapy (EBRT) is a form of radiotherapy which uses a linear particle accelerator (LINAC) to generate an X-ray treatment beam to target cancerous tissue deep within the human body. Advanced forms of X-ray radiotherapy such as Intensity Modulated Radiotherapy (IMRT) utilises an advanced collimation device, known as a multileaf collimator (MLC) which is composed of thin abutting tungsten slats referred to as leaves to shape the X-ray treatment beam such that it conforms to the tumour geometry.

The precision of IMRT and other advanced EBRT’s is largely dependent upon an MLC’s ability to precisely shape the X-ray treatment beam. It is standard practise to calibrate MLC leaves to a tolerance of ±1 mm every 3 months. To maintain total dose errors below 2%, the verification tolerance should be set to a higher standard of ±0.3 mm. Pre-treatment verification, which is recommended for each treatment to minimise errors is time consuming, and is thus seldom performed for each individual patient, but instead for an acceptable sample of patients and/or treatment-deliveries. Incorporating a high-precision, real-time treatment monitoring device would allow MLC errors to be detected instantaneously and eliminate the need for pre-treatment verification as each treatment would be verified in real time. This would increase patient safety and treatment quality.

Monolithic Active-Pixel Sensors (MAPS) are thin silicon pixel sensors with a small pixel size. They therefore have low attenuation and can be used for high resolution measurements.
They can also be made to be radiation hard and can have a high speed readout. This makes MAPS an excellent candidate for real-time monitoring in radiotherapy. In this thesis a MAPS-based device is presented and shown to have excellent performance as a real-time, upstream treatment verification device. It is shown that the position of MLC leaves can be reconstructed with resolutions ranging between 62±6µm and 86±16µm depending on the leaf configuration using 0.15 sec of treatment data. An upstream full-scale large area MAPS device is tested and shown to have a clinically insignificant attenuation such that it does not change the delivered treatment when used as a monitoring device. Although the gap between the abutted sensors makes the leaf position resolution worse, it is still well within clinical tolerance. The device was shown to be radiation hard under proton irradiation. It can operate after radiation-induced proton damages of up to 20 kGy at 20°C, and up to 50 kGy and potentially beyond when cooled to -2.13±0.461°C or below. This is equivalent to ~ 2 years of clinical deployment.

The work in this thesis demonstrates the device known as the demonstrator is a clinically deployable solution as a real-time, upstream treatment verification device.
Date of Award21 Mar 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJaap J Velthuis (Supervisor) & Lana Beck (Supervisor)


  • CMOS
  • MAPS
  • Active-pixel sensors
  • Radiotherapy
  • Upstream
  • Radiotherapy verification
  • Radiotherapy monitoring
  • High-resolution radiotherapy
  • Real-time radiotherapy
  • MLC
  • Multileaf collimator
  • Lassena
  • Athena
  • Radiation damage
  • Radiation hardness
  • Pixel sensors
  • Radiotherapy concepts
  • Solid state detectors
  • X-ray detectors
  • Image reconstruction in medical imaging
  • radiation-hard detectors
  • image processing
  • data processing
  • detector alignment and calibration
  • medical imaging
  • IMRT
  • VMAT
  • transmission detector

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