AbstractID: 9382 Title: EPID Based Quality Assurance for Collimation Verification in External Beam
Radiotherapy
A methodology for automatic EPID based quality assurance (QA) for multileaf
collimation (MLC) verification (i.e. leaf position and collimator angle) is presented. The
clinical implementation consists of 3 features. (1) EPID image acquisition is done
independently of treatment verification software (TVS) and linac. Images are acquired
continuously regardless of ‘beam on’ condition, enabling an image sequence (consisting
of blank, incomplete and full exposure images) to be continuously and asynchronously
acquired without operator intervention. (2) An automatic image classifier is used to group
the images based on collimation for comparison to the treatment planning fields. The
classifier is self-organizing, allowing images acquired at time of first treatment to be
grouped without input from the TVS. Images from subsequent fractions are automatically
sorted into the initial groupings. Area and centroid position are used as indices. For
similarity measure, cross-correlation, entropy and newly formulated ‘weighted
Hausdorff’ distance are considered. (3) Leaf position is computed using a novel
‘supervised Laplacian of Gaussian’ edge detector. Collimation angle is determined using
a ‘dual orientation transform’ algorithm (hybridized truncated Hough and Radon
transforms). Both algorithms are not significantly affected by patient anatomy, and
accuracies of 0.5mm and <0.5º are obtained. This methodology was clinically
implemented, and used for pre-treatment verification of MLC and micro-MLC fields for
both conformal radiotherapy and IMRT. With the resulting software, one can
automatically obtain precise collimation verification of beam segments associated with a
modulated field. For 60 fields, image acquisition and analysis each required 10min using
a 2.0GHz PC.