The various approaches in radionuclide dosimetry depend on the size an
d spatial relation of the sources and targets considered in conjunctio
n with the emission range of the radionuclide used. We present some of
the frequently reported computational techniques on the basis of the
source/target size. For whole organs, or for sources or targets bigger
than some centimetres, the acknowledged standard was introduced 30 ye
ars ago by the MIRD committee and is still being updated. That approac
h, based on the absorbed fraction concept, is mainly used for radiopro
tection purposes but has been updated to take into account the dosimet
ric challenge raised by therapeutic use of vectored radiopharmaceutica
ls. At this level, the most important computational effort is in the f
ield of photon dosimetry. On the millimetre scale, photons can often b
e disregarded, and beta or electron dosimetry is generally reported. H
eterogeneities at this level are mainly above the cell level, involvin
g groups of cell or a part of an organ. The dose distribution pattern
is often calculated by generalizing a point source dose distribution,
but direct calculation by Monte Carlo techniques is also frequently re
ported because it allows media of inhomogeneous density to be consider
ed. At the cell level, alpha and electron (low-range beta or Auger) ar
e the predominant emissions examined. Heterogeneities in the dose dist
ribution are taken into account, mainly to determine the mean dose at
the nucleus. At the DNA level, Auger electrons or alpha-particles are
considered from a microdosimetric point of view. These studies are oft
en connected with radiobiological experiments on radionuclide toxicity
.