AbstractID: 8575 Title: Evaluating Light and Radiation Field Congruence with an EPID
EPID-based light and radiation field congruence was investigated, using a
commercially available EPID. Two methods were investigated. The first method
employed an EPID, a graticule, an EPID and the associated EPI software. The graticule
was mounted in the head of the gantry and the collimator jaws were adjusted
independently to bisect selected dots on the graticule (approximately a 14x14 cm2 light
field).
In the second method, the graticule was replaced by a block tray in which four
Sn/Pb wires, 1.27 mm in diameter, were arranged in the shape of a diamond. The light
field was aligned such that it fell directly on the corners of the diamond and the digital
jaw settings were recorded. This method was also applied to light fields defined by the
MLC and has been monitored past two weeks. The detected radiation field size was
compared to the digital jaw settings for each method and the light and radiation field
congruence was evaluated.
Results indicated the detected radiation field size agreed with the digital
collimator settings within ~ 1mm in X and 0.2 mm in Y. Light and radiation field
congruence tests were quantitatively difficult to assess with the graticule. For method
two, the differences were well below the allowed 2 mm or 1% of each side of the square
field. Application to light fields defined by a MLC revealed reproducibility of leaf
alignment and discrepancy between the light and radiation field consistent with zero.