Abstract
The current trend in X-ray radiotherapy is to treat cancers that are in difficult locations in the body using beams with a complex intensity profile. Intensity-modulated radiotherapy (IMRT) is a treatment which improves the dose distribution to the tumor whilst reducing the dose to healthy tissue. Such treatments administer a larger dose per treatment fraction and hence require more complex methods to verify the accuracy of the treatment delivery. Measuring beam intensity fluctuations is difficult as the beam is heavily distorted after leaving the patient and transmission detectors will attenuate the beam and change the energy spectrum of the beam. Monolithic active pixel sensors (MAPSs) are ideal solid-state detectors to measure the 2-D beam profile of a radiotherapy beam upstream of the patient. MAPS sensors can be made very thin (~30 μm) with still very good signal-to-noise performance. This means that the beam would pass through the sensor virtually undisturbed (<; 1% attenuation). Pixel pitches of between 2 μm to 100 μm are commercially available. Large area devices (~15×15 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) have been produced. MAPS can be made radiation hard enough to be fully functional after a large number of fractions. All this makes MAPS a very realistic transmission detector candidate for beam monitoring upstream of the patient. A remaining challenge for thin, upstream sensors is that the detectors are sensitive to the signal of both therapeutic photons and electron contamination. Here, a method is presented to distinguish between the signal due to electrons and photons and thus provide real-time dosimetric information in very thin sensors that does not require Monte Carlo simulation of each linear accelerator treatment head.
Highlights
R ADIOTHERAPY is one of the most prevalent treatments for cancer, where 40% of cured cancers used radiotherapy treatments in the U.K. [1]
The use of higher dose rates has been made possible through the increased treatment precision afforded by intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)
The signal response is composed of the therapeutic photons and electrons generated predominantly by Compton scattering in the flattening filter and collimators in the linac head, as well as in the air
Summary
R ADIOTHERAPY is one of the most prevalent treatments for cancer, where 40% of cured cancers used radiotherapy treatments in the U.K. [1]. The use of higher dose rates has been made possible through the increased treatment precision afforded by intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). These modalities treat cancers that are in difficult locations in the body using megavoltage photon beams with a complex intensity profile. With many moving mechanical parts, tracking them to ensure they are in the right position is an additional safety measure, external to the linear accelerator’s (linac) own systems, to verify that the treatment is being performed correctly This real-time treatment monitoring is known as in-vivo dosimetry. In order to extract dose information, transmission detectors conventionally use a thick converter layer to generate signal from Compton scattering of the photon
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