Dose Difference and Distance to Agreement

Dose Difference and Distance (courtesy Yeo et al Procedural Method for Film Dosimetry, Medical Physics Publishing)
to Agreement Analysis
There exist other methods of analysis that account
for the limitation of alignment. Van Dyk et al. (1993)
introduced the idea of dividing the evaluation into
two groups depending on the magnitude of dose gradient:
high- and low-gradient regions each with a
different acceptance criterion. The idea is based on
the fact that dose difference in a high-dose-gradient
region can be extremely higher than that in a lowerdose-
gradient region because of imperfect
alignment. This approach may provide exceedingly
simplistic analysis for an IMRT field, where a
diverse degree of a dose gradient typically exists.
To overcome this limitation, therefore, the simultaneous
use of a distance-to-agreement (DTA) and
a percent dose difference (DD) is proposed. These
parameters can help evaluate the agreement of the
two distributions in terms of misalignment and
difference, respectively. DTA is defined as the nearest
distance from a point of a reference dose to the
point of the same amount of dose on the compared
(or quarried) dose distribution. If the former is
selected in the measured distribution, then select the
latter in the calculated distribution. DTA, thus, is an
indicator of how good the alignment of the two distributions
is, provided that the difference is zero. The
percent dose difference is defined as the difference
in percent, implicitly assuming that the alignment of
the two distributions is perfect. In reality, as the dose
difference as well as the misalignment contribute to
the difference of the two clinical distributions, use of
the two independent parameters together will be
necessary. By providing an acceptance criterion,
respectively for a dose difference and a DTA, the
acceptability of the comparison can be determined.