Accurate radiation dosimetry is a requirement of radiation
oncology, diagnostic radiology and nuclear medicine. It is
necessary so as to satisfy the needs of patient safety, therapeutic
and diagnostic optimisation, and retrospective epidemiological
studies of the biological effects resulting from low absorbed doses
of ionising radiation. The radiation absorbed dose received by the
patient is the ultimate consequence of the transfer of kinetic
energy through collisions between energetic charged particles and
atoms of the tissue being traversed. Thus, the ability of the
medical physicist to both measure and calculate accurately patient
dosimetry demands a deep understanding of the physics of charged
particle interactions with matter. Interestingly, the physics of
charged particle energy loss has an almost exclusively theoretical
basis, thus necessitating an advanced theoretical understanding of
the subject in order to apply it appropriately to the clinical
regime.
Each year, about one-third of the world's population is exposed
to ionising radiation as a consequence of diagnostic or therapeutic
medical practice. The optimisation of the resulting radiation
absorbed dose received by the patient and the clinical outcome
sought, whether diagnostic or therapeutic, demands accuracy in the
evaluation of the radiation absorbed doses resulting from such
exposures. This requirement arrises primarily from two
broadly-encompassing factors: The requirement in radiation oncology
for a 5% or less uncertainty in the calculation and measurement of
absorbed dose so as to optimise the therapeutic ratio of the
probabilities of tumour control and normal tissue complications;
andThe establishment and further refinement of dose reference
levels used in diagnostic radiology and nuclear medicine to
minimise the amount of absorbed dose for a required degree of
diagnostic benefit.
The radiation absorbed dose is the outcome of energetic charged
particles decelerating and transferring their kinetic energy to
tissue. The calculation of this energy deposition, characterised by
the stopping power, is unique in that it is derived entirely from
theoretical principles. This dominant role of the associated theory
makes its understanding of fundamental to the calculation of the
radiation absorbed dose to the patient.
The theoretical development of charged particle energy
lossrecognised inmedical physics textbooksis in general limited to
basic derivations based upon classical theory, generally a
simplified form of the Bohr theory. More advanced descriptions of,
for example, the Bethe-Bloch quantum resultusually do not go beyond
the simplepresentationofthe result "without "full explanation of
the theoretical development of the theory and consideration of its
limitations, its dependencies upon the Born perturbation theory and
the various correction factors needed to correct for the failures
of that Born theory at higher orders. This is not appropriate for a
full understanding of the theory that its importance deserves. The
medical radiation physicist should be aware of the details of the
theoretical derivations of charged particle energy loss in order to
appreciate the levels of accuracy in tabular data provided in
reports and the calculation methodologies used in modern Monte
Carlo calculations of radiation dosimetry."
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