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ABSTRACT: Here novel applications involving a Siemens EPID are presented:
determination of radiation central axis, and MLC leaf position
verification and calibration. The radiation central axis (cax) was
defined using portal images of a field at different collimator angles.
The location of the car in terms of pixel number on the imaging vidicon
camera was found to be stable within ±3 pixels (1.5 mm) over 2
weeks of repeated extensions/ retractions of the EPID. The variation in
the location of the cax as a function of gantry angle was found to be
repeatable for the same gantry angle to within I pixel. A number of edge
detection algorithms were tested: Sobel, Roberts, Prewitt, Laplace,
morphological and Canny. These algorithms were tested on simulated, EPID
and portal film images, and found to agree with each other to within 1
pixel. Using only a single one time calibration procedure, which
involved measuring the pixel dimension associated with the EPID at
isocenter, and the determination of a virtual machine coordinate system,
automated edge detection of the collimation boundary was successfully
carried out for both in air and phantom images, allowing for comparison
with intended leaf positions from the treatment planning system. The
location of the MLC leaves determined from the EPID images was found to
agree within 1 mm of the corresponding values measured using film and an
ionization chamber. Applications of this work include a fast
pre-treatment QA of MLC/cerrobend fields, dosimetric calibration of the
MLC, and improved quantification of linac QA
Engineering in Medicine and Biology Society, 2000. Proceedings of the 22nd Annual International Conference of the IEEE; 02/2000
