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Performance of a direct‐detection active matrix flat panel dosimeter (AMFPD) for IMRT measurements
Author(s) -
Chen Yu,
Moran Jean M.,
Roberts Donald A.,
ElMohri Youcef,
Antonuk Larry E.,
Fraass Benedick A.
Publication year - 2007
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2805993
Subject(s) - dosimeter , dosimetry , photodiode , materials science , optics , active matrix , sensitivity (control systems) , scintillator , biasing , voltage , nuclear medicine , flat panel detector , ionization chamber , signal (programming language) , detector , optoelectronics , physics , thin film transistor , electronic engineering , medicine , computer science , ion , engineering , composite material , layer (electronics) , quantum mechanics , ionization , programming language
The dosimetric performance of a direct‐detection active matrix flat panel dosimeter (AMFPD) is reported for intensity modulated radiation therapy (IMRT) measurements. The AMFPD consists of a ‐ Si : H photodiodes and thin‐film transistors deposited on a glass substrate with no overlying scintillator screen or metal plate. The device is operated at 0.8 frames per second in a continuous acquisition or fluoroscopic mode. The effect of the applied bias voltage across the photodiodes on the response of the AMFPD was evaluated because this parameter affects dark signal, lag contributions, and pixel sensitivity. In addition, the AMPFD response was evaluated as a function of dose, dose rate, and energy, for static fields at 10 cm depth. In continuous acquisition mode, the AMFPD maintained a linear dose response( r 2 > 0. )up to at least 1040 cGy. In order to obtain reliable integrated dose results for IMRT fields, the effects of lag on the radiation signal were minimized by operating the system at the highest frame rate and at an appropriate reverse bias voltage. Segmental MLC and dynamic MLC IMRT fields were measured with the AMFPD, and the results were compared to film, using standard methods for reliable film dosimetry. Both AMFPD and film measurements were independently converted to dose in cGy. γ and χ values were calculated as indices of agreement. The results from the AMFPD were in excellent agreement with those from film. When 2 % ofD maxand 2 mm of distance to agreement were used as the criteria, 98 % of the region of interest (defined as the region where dose is greater than 5 % ofD max) satisfied| χ | ≤ 1 on average across the cases that were tested.