Relative dosimetry using active matrix flat-panel imager (AMFPI) technology

The first examination of the use of active matrix flat-panel arrays for dos imetry in radiotherapy is reported. Such arrays are under widespread develo pment for diagnostic and radiotherapy imaging. In the current study, an arr ay consisting of 512X512 pixels with a pixel pitch of 508 mu m giving an ar ea of 26X26 cm(2) has been used. Each pixel consists of a light sensitive a morphous silicon (a-Si:H) photodiode coupled to an a-Si:H thin-film transis tor. Data was obtained from the array using a dedicated electronics system allowing real-time data acquisition. In order to examine the potential of s uch arrays as quality assurance devices for radiotherapy beams, field profi le data at photon energies of 6 and 15 MV were obtained as a function of fi eld size and thickness of overlying absorbing material (solid water). Two d etection configurations using the array were considered: a configuration (s imilar to the imaging configuration) in which an overlying phosphor screen is used to convert incident radiation to visible light photons which are de tected by the photodiodes; and a configuration without the screen where rad iation is directly sensed by the photodiodes. Compared to relative dosimetr y data obtained with an ion chamber, data taken using the former configurat ion exhibited significant differences whereas data obtained using the latte r configuration was generally found to be in close agreement. Basic signal properties, which are pertinent to dosimetry, have been investigated throug h measurements of individual pixel response for fluoroscopic and radiograph ic array operation. For signal levels acquired within the first 25% of pixe l charge capacity, the degree of linear response with dose was found to be better than 99%. The independence of signal on dose rate was demonstrated b y means of stability of pixel response over the range of dose rates allowed by the radiation source (80-400 MU/min). Finally, excellent long-term stab ility in pixel response, extending over a 2 month period, was observed. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)02308- 1].