Jimenez Spang, F; (2014) Monte Carlo Study of the Dosimetry of Small-Photon Beams Using CMOS Active Pixel Sensors. Doctoral thesis , UCL (University College London).

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Abstract

Stereotactic radiosurgery is an increasingly common treatment modality that uses very small photon fields. This technique imposes high dosimetric standards and complexities that remain unsolved. In this work the dosimetric performance of CMOS active pixel sensors is presented for the measurement of small-photons beams. A novel CMOS active pixel sensor called Vanilla developed for scientific applications was used. The detector is an array of 520 × 520 pixels on a 25 μm pitch which allows up to six dynamically reconfigurable regions of interest (ROI) down to 6 × 6 pixels. Full frame readout of over 100 frame/s and a ROI frame rate of over 20000 frame/s are available. Dosimetric parameters measured with this sensor were compared with data collected with ionization chambers, film detectors and GEANT4 Monte Carlo simulations. The sensor performance for the measurement of cross-beam profiles was evaluated for field sizes of 0.5 × 0.5 cm2. The high spatial resolution achieved with this sensor allowed the accurate measurement of profiles from one single row of pixels. The problem of volume averaging is solved by the high spatial resolution provided by the sensor allowing for accurate measurements of beam penumbrae and field size under lateral electronic disequilibrium. Film width and penumbrae agreed within 2.1% and 1.8%, respectively, with film measurement and better than 1.0% with Monte Carlo calculations. Agreements with ionization chambers better than 1.0% were obtained when measuring tissue-phantom ratios. The data obtained from this imaging sensor can be easily analyzed to extract dosimetric information. The results presented in this work are promising for the development and implementation of CMOS active pixel sensors for dosimetry applications.

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