Biomedical Engineering Reference
In-Depth Information
Diagnosis (e.g., misdiagnosis, wrong histology, wrong staging)
Imaging (e.g., misinterpretation, spatial distortions, errors in
density)
Delineation of Volumes of Interest (e.g., incorrect tumor identi-
fication, incorrect normal tissue identification)
Prescription (e.g., dosage aims for the target volume and con-
straints on dose to normal tissues)
Development of a plan of treatment (e.g., choice of beam direction
and field shapes, dose algorithms, plan evaluation)
Patient handling (e.g., incorrect patient immobilization and/or
positioning, errors due to patient and organ motion, changes in
patient during therapy)
Treatment delivery (e.g., incorrect treatment machine configura-
tion, incorrect dose delivery)
One can never ignore the uncertainties; one must deal with them. To
do so, one must first have an appreciation of their causes and magni-
tudes, and of their consequences. Then, to the extent practicable, one
should attempt to reduce them to a therapeutically negligible level.
If this is not possible, one must adopt some strategy to allow for the
residual uncertainties in a manner that is likely to achieve the best
result for the patient, such as leaving a safety margin around the tumor
volume to allow for motion and alignment uncertainties.
U
NCERTAINTY AND
E
RROR
Physicists are generally comfortable with the proposition that they
routinely and unavoidably make errors in their measurements and
calculations
1
and are used to analyzing the likely magnitude of the
errors and employing tools such as error bars in graphs. Physicians,
on the other hand, generally dislike talk of error, perhaps partially for
medico-legal reasons, but also for psychological ones. The term
“uncertainty” is a bit more comfortable to them and, while I cannot
absolve physicians from the need to deal with error, I am happy to be
able to reassure them that the word “uncertainty” is the appropriate
term to use in characterizing it. For, while the terms “uncertainty”
1
Uncertainty analysis applies equally to measurements and calculations. In
what follows, I use the term “measurement” to refer to both.
