Biomedical Engineering Reference
In-Depth Information
into one overall idea of “goodness.” Only then can one say that one is
better than another. An unambiguous way to rank objects is to assign
a numerical value to each of them. That is, to combine all their
aspects into a single judgment and express that judgment in the form
of a numerical “score.” The plan with the highest score will then be,
in the scorer's judgment, the best plan. Scoring has another
advantage. In addition to averting the logical impasse which ranking
alone can get one into, it also allows one to express the idea that two
plans are “not very different.” If their scores are quite similar, then
presumably they will be of quite similar goodness. One could say
that our logical problem of inconsistent rankings came about because
of uncertainties in scoring.
In manual treatment planning, quantitation of a score is not much
needed and is rarely, if ever, attempted. Rather, the planner is likely
to make pair-wise comparisons and live with the logical difficulties
that can result. He will understand that, when two plans are very
close, it is not critical which is ranked higher. However, in computer-
based optimization, quantitation is necessary. A computer can only
rank plans by giving each a score and then selecting the plan with the
highest score.
W
HAT IS
O
FTEN NOT IN THE
S
CORE
The optimization schemes used to date largely restrict themselves to
calculating the overall weights and fluence maps of each defined
beam. The beams themselves must, at the current level of the
technology, be pre-designed by the planner as regards to modality,
direction, and number
although some optimization schemes can also
constrain the total number of beams used from among a larger
−
number of pre-selected beams. So, the so-called optimization of
radiotherapy only optimizes a subset of the treatment variables. The
others have to be determined by the planner. The following discusses
some of the beam properties that are generally not included in the
optimization process - or, expressed more precisely, are not included
as variables in the score function.
Modality
The choice of modality (photons, electrons, protons, etc.) is usually
based on the planner's experience
−
and can be different for each
