Biomedical Engineering Reference
In-Depth Information
Nevertheless, to perform any type of optimization, one must compute
the best score one knows how to produce - just as, to treat a patient,
one can only base the plan on one's best judgment. This score should
combine elements, direct or indirect, that provide measures of:
the likelihood of local tumor control;
the likelihood of morbidity;
other aspects of the plan such as its complexity and feasibility.
(This important aspect of a plan, which planners take into account
subliminally when planning by hand, is generally not considered
in IMRT planning.)
It will not escape your notice that
these issues are exactly the same
issues that a planner faces during manual development of a plan
.
The difference is that, while the planner does the analysis in his head,
the computer must do the analysis by computation.
Unfortunately, many score functions are designed more for
computational convenience than for clinical appropriateness. The
deficiencies fall into two classes: (1) The parameters being optimized
are too crude - they may not include measures of dose correlated with
normal tissue volume although, as I have tried to emphasize, dose-
volume effects are very important. Or, (2) they are of a form (linear
or quadratic in the variables, for example) which simplifies or speeds
up the search process but has no medical basis. It is my opinion that
the reason that early attempts at optimization in the 1960s and 1970s
largely failed was because their score functions had almost no clinical
grounding and took no account of dose-volume effects.
Measures that describe a plan's impact
The following quantities, either singly or in some combination,
measure the impact of a plan on the patient. (
N.B
., the score is, of
course, based on the total overall dose, not the dose per beam.)
Tumor response
the difference between the minimum (or mean, or…) target volume
dose and the prescribed dose
the dose received or exceeded by 95% of the target volume
(D
95%
), which reflects the depth of any cold spots
the dose received or exceeded in 5% of the target volume (D
5%
),
which reflects the height of any hot spots
