11.5. Microdosimetry 649
11.5Microdosimetry..............................
Microdosimetry is a rapidly evolving technique that is being used to measure the
stochastic distribution of energy deposited by radiation in microscopic sites. It has
been found to be an extremely valuable tool in assessing energy transfer to very
small volumes, such as cellular and subcellular structures. For example, in radiation
therapy one is often interested in determining the distribution of energy transfer
inside single cancer cells so that the cells could be effectively targeted. Conventional
dosimetry does not provide such information since it deals with macroscopic descrip-
tion of energy transfer in large volumes. Microdosimetry has, in fact, been mostly
developed for dealing with such issues in radiation therapy.
The biggest issue faced in microdosimetry is the statistical fluctuation involved
in the energy transfer process. In conventional dosimetry this is not of much prob-
lems since there the number of particles interacting is so large that the statistical
fluctuations are much smaller than the systematics of the measurement process. To
understand this further the reader is pointed to the fact that the energy transfer can
be considered a Poisson process. The statistical fluctuations of a Poisson process
are equal to the square root of the number of interacting particles or
√
N.IfNis
small the fluctuations are large and so is the measurement uncertainty.
11.5.AMicrodosimetricQuantities
The quantities used in microdosimetry are somewhat different than the ones used
in conventional dosimetry. This section is therefore devoted to the discussion of the
most commonly used microdosimetric quantities.
A.1 LinearEnergyTransferandDose...............
In microdosimetry one is interested in measuring or estimating the distribution of
energy transfer along a particle’s track. Earlier in this chapter and also in chapter
2 we introduced a term, the linear energy transfer orLET, to characterize how the
particles loose energy along their track. This quantity can be used in microdosimetry,
but one should be cautioned that it actually is a macroscopic quantity. Let us first
understand howLETcan be used to characterize the absorbed dose.
The absorbed dose is related directly to the linear energy transfer through the
relation
D=1
ρLtΦ ̄, (11.5.1)whereΦ represents the average fluence of particles passing through the medium of ̄
densityρ.Ltrepresents the track averageLET, which can be computed from
Lt=∫
Lf(L)dL, (11.5.2)wheref(L) is the normalized probability distribution function forLET and the
integration is carried out over the whole track length. Since we are essentially taking
the weighted average ofLETat each point along the track, theLtis referred to as
the track averageLET. One can similarly define thedose averageLETthrough
Ld=∫
LD(L)dL, (11.5.3)