IMRT and “OPTIMIZATION” - Radiation Oncology: A Physicist's-Eye View - page 205

O PTIMIZATION?

Voting the best piece of music

Every year in Boston − and, I am sure, in towns all around the world

− a local classical radio station asks its listeners to vote for the piece

of classical music that they consider to be the best. They get

thousands of responses from listeners who certainly know that the
question is nonsense. There are so many dimensions to a piece of
music, so many facets, and so many different occasions for which
different kinds of music might be preferred, that picking the “best” is,
at most, an amusing exercise. Optimization in this setting is simply
neither feasible nor useful.
Is treatment planning very different? There are, indeed, many
dimensions to the problem, and many of them are expressed in terms
that are unrelated to the way others are expressed. How shall one
combine a measure of the distribution of dose within the target
volume with the V 20Gy in a normal tissue? These are different facets

of the treatment, expressed in different physical units. This

conundrum makes the job of summarizing all these facets into a
single score, so that one plan can be ranked as the best, quite
daunting. There is one big difference, however. The patient must be
treated; one of the possible treatment plans must be selected; and that
plan will be selected because, in the planner's judgment, it is the best
that can be achieved in practice. A music director can pick this or
that piece of music, as whim directs, and there will generally be only
mild consequences if he misjudges his audience. A treatment planner
has no such freedom. This means that the exigencies of medical care
force optimization upon us, like it or not, whether or not the
optimization scheme is “optimal.”

The meaning of the term optimization

The term “optimization” has at least two rather different meanings
and this causes considerable confusion. In its vernacular sense

− that

is, the meaning intended in everyday speech it is the process of

finding the best possible solution to a particular problem. In the
mathematical sense , it is the process of finding values of the
independent variables that lead to an extremum of a score function.

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