Biomedical Engineering Reference
In-Depth Information
placed at the point of interest, whose calibration, together with the
electrometer with which it is read out, is traceable to a primary
standards laboratory. In the United States the National Institute of
Standards and Technology uses a water calorimeter - an instrument
that measures the heat deposited in a unit mass of material - as its
standard for absorbed dose in water from a
60
Co beam.
The question arises: In what medium should the measurement be
made? Exposed to the same radiation, different materials receive
slightly different doses at a given point. By convention, in radio-
therapy, the dose is reported as though the medium were water.
What form does the deposited energy take? We have already
sufficiently addressed the interactions of radiation with matter to be
able to answer this question. In the end,
all the energy appears either
as heat, or in the form of chemical changes resulting from ionizations
.
In practice, by far the greatest proportion of the deposited energy - at
least 96% - appears as heat. Anyone who has touched or leant
against the head of a
60
Co therapy machine knows well the warmth
generated in the shielding surrounding the source.
5
Of course, it is the
chemical changes that lead to tissue damage, and I always find it a bit
surprising that so small a fraction of the radiation's energy loss is
therapeutically effective.
The experience of a single incident photon
Consider a 4 MeV photon
6
which impinges upon a patient - say,
laterally in the brain where it potentially can pass through perhaps
14 cm of tissue and bone.
Question:
what is the most probable thing that will happen to that
photon?
Answer:
absolutely nothing!
generally small temperature and pressure correction factor. (See, also,
Chapter 10.)
5
A patient exposed to therapeutic doses of radiation (e.g., 2 Gy) experiences
a warming of his or her irradiated tissues. However, the temperature rise is
of the order of some 5
10
−4
degrees and is imperceptible. (Actually, the
temperature rise will be even less since this number is based on calculation
in a static situation and, in practice, much of the heat is likely to be carried
away by blood flow)
6
The reason for picking 4 MeV will become clear shortly.
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