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Abstract
Although intensity modulated radiotherapy (IMRT) is a step forward in comparison to
conventional, static beam delivery, quality assurance is more complex and labour intensive,
demanding detailed two-dimensional dosimetric verification. Regardless of the technique
used for measuring the dose distribution, what is essential to the implementation
of routine verification of IMRT fields is the efficient and accurate comparison of
the measured versus desired dose distribution. In order to achieve a fast, yet accurate
quantitative measure of the correspondence between measured and calculated dose, the
theoretical concept of the gamma evaluation method presented by Low et al. (Med. Phys.,
25 (1998) 656) was converted into a calculation algorithm, taking into account practical
considerations related to the discrete nature of the data.
A filter cascade of multiple levels was designed to obtain fast and accurate comparison
of the two dose distributions under evaluation. The actual comparison consists of
classification into accepted or rejected datapoints with respect to user-defined acceptance
criteria (dose difference and distance to agreement). The presented algorithm was
tested on dosimetric images calculated and/or acquired by means of a liquid filled
portal imaging device during the course of intensity modulated treatments of prostate
cancer, including pre-treatment verification as well as verification during treatment.
To assess its ability to intercept possible errors in dose delivery, clinically relevant
errors were deliberately introduced into the dose distributions.
The developed gamma filter method proves successful in the efficient comparison of
calculated versus measured IMRT dose distribution. Secondly, intercomparison of dosimetric
images acquired during different treatment sessions illustrate its potential to highlight
variations in the dosimetric images. The simulated errors were unmistakably intercepted.
The readily obtained gamma evaluation images are an easy tool for quality control
of IMRT fields. To reduce the artefacts related to the discrete nature and limited
resolution of the data, a fast and accurate filter cascade was developed, offering
the possibility to use the gamma method for day to day evaluation of patient dosimetric
portal images with or without comparison to a predicted portal dose distribution.