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Radiation Dosimetry: Why
Internal Emitters Are Different
An appraisal by Dr Philip Day, University of Manchester
When ionising radiation passes through biological tissue it generates
reactive chemical species, which may subsequently interact with crucial
biochemical processes and generate adverse biological effects. The science
of radiation dosimetry seeks to relate these biological outcomes to the
radiation energy absorbed by the tissues, the so-called absorbed dose.
These relationships are used to establish safety limits for radiation
dose, say for the medical use of X-rays, exposure of workers in nuclear
industries, the levels of radioactivity in food, etc. However, because
of the way that the methodology of radiation dosimetry has developed,
it is now recognised that there are a number of potential flaws in the
One of these arises because an individual may receive a dose of ionising
radiation either from a source external to the body - which may be nuclear
fallout, but could be an X-ray machine - or from a source within the body
- which would arise if a radioactive substance were ingested or inhaled.
The two types of irradiation sources are described as external and internal
emitters, respectively. It is the latter with which we are concerned here.
The potential flaws in radiation dosimetry to which I referred earlier
can now be more clearly understood. Firstly, much of the science relating
radiation dose to biological effect has been based on the effects of the
medical use of X-rays and the incidence of disease amongst the survivors
of the atomic bombs dropped on Hiroshima and Nagasaki in 1945.
These are both external sources of irradiation. Much less information
has come from the effects of internal emitters, and in these cases the
radiation doses received are often much less well defined. Secondly, radiation
sources can be of several types, but crucially one of these can only irradiate
the body if located inside it - these are the alpha emitters and, as described
below, the type of radiation they emit has a very short range only.
Thus, when located inside a person, the full radiation dose from an alpha
emitter will be received by the immediately surrounding tissue, but when
located externally, no dose will be received at all. Whilst this might
seem to render alpha emitters of lesser importance, this is not the case
as the intensity of radiation received from this type of radioactive source
is far greater, as indeed may be the biological effects.
To take all this into account, the standard methods of radiation dosimetry
allocate a quality factor to this type of radiation, which is held to
be 20 times more damaging than the radiation dose received from gamma
radiation, usually from external sources.
Unfortunately, things are probably not that simple. The biological effects
of the internal alpha emitters will depend crucially on where, precisely,
they are located, and the use of a single quality factor to encompass
all situations is far from ideal - in some cases, the appropriate factor
may be 20, or even less than 20, but in some cases an appropriate number
might be far greater. It is now realised by many that the logical basis
for the use of this quality factor approach is fundamentally flawed, and
that a radical re-appraisal to the evaluation of the radiation effects
of internal emitters may be needed.
There are 3 principal types of ionising radiation emitted by radioactive
substances, namely alpha, beta, and gamma, and these all have characteristic
- and radically different - properties. Alpha radiation arises when a
relatively heavy nucleus undergoes radioactive decay emitting relatively
heavy positively charged particles. It was shown early in the 20th century
by Earnest Rutherford, who discovered them, that these particles are identical
to helium nuclei, having relative mass and charge 4 and +2 (atomic units),
Although alpha particles are usually emitted with relatively high kinetic
energies (3 to 6 MeV per particle), because of their high mass the corresponding
velocity is relatively low. Travelling through the surrounding matter,
the massive, lumbering particles lose their energy and momentum continuously,
by multiple collisions with electrons in the surrounding atoms, without
deviating appreciably from a straight track. Alpha particles thus lose
their high energies over rather short distances - a few thousandths of
a millimetre - and for these reasons are classed as high linear energy
transfer radiation (high LET radiation).
This type of radiation (which also includes other charged nuclei and also
neutrons) gives rise to intense chemical ionisation and hence potentially
a large amount of biochemical damage over a relatively small volume. For
example, the typical energy of ionisation in a biological tissue is around
40 eV, so that a 4 MeV alpha particle will give rise to around 105 such
ionisations along a track length of at most 100 micrometres (Ám), producing
several thousand ion-pairs in each cell. Consequently, an alpha emitter
located internally will irradiate a small number of adjacent cells to
a high intensity, killing some and causing major biochemical changes in
Radioactive decay by beta radiation also consist in the emission of charged
particles, in this case electrons of relative charge -1 and very low mass
(around one the thousandth that of an alpha particle). Beta particle kinetic
energies generally range up to around 1 to 2 MeV, but because of their
low mass, to achieve this energy beta particles start at relatively high
However, when a beta particle undergoes a collision with an atomic electron
- which has the same mass as itself - the beta particle may well be deflected
from its initial track direction, and the momentum (and energy) transferred
depends greatly on the angle of impact - rather like a snooker ball colliding
with a loose pack of other snooker balls. In general, a beta particle
will lose its energy over far longer distance than an alpha particle of
the same initial kinetic energy.
For this reason, beta radiation is categorised as being low linear energy
transfer (low LET) radiation and in passing through tissues, the number
of ionisations generated within a given distance - the density of ionisation
- is much lower than for alpha radiation, and the track is not straight
and may have many branches.
Thus, in contrast to alpha radiation, beta radiation will penetrate longer
distances - maybe a few mm - in tissue and can be of importance as an
external radiation source, although mainly to the skin or any other tissues
in immediate contact. Because the density of ionisation is much lower,
the number of ion pairs generated per cell is far less than for alpha
radiation, and the biological effects may be qualitatively, as well as
Gamma radiation contrasts strongly with both alpha and beta particle
radiation as described above. Firstly, gamma radiation is true electromagnetic
radiation, consisting of bundles of energy (photons), which lose energy
to matter in a very different manner to the collisions described for alpha
and beta particles.
Secondly, gamma radiation is highly penetrating and important both as
a source of external and of internal radiation. Gamma photons do not leave
a "track" of ionisation in the same way as alpha or beta particles. Instead,
they behave on a probabilistic basis - that is, they have a certain (rather
low) probability of being absorbed at any particular point along their
track, but if a gamma photon is absorbed, its entire energy is absorbed
at the one point and converted into a high energy electron - a so-called
photoelectron - in practice indistinguishable from a beta particle. A
beam of gamma rays - that is, many photons - will, of course, leave a
ionisation path because a vast number of photoelectrons will be released
at random intervals along the track.
In summary then, gamma radiation will pass through material over quite
long distances, and is of primary importance as an external irradiation
source, although of course a gamma emitter absorbed internally will also
irradiate the body in which it is absorbed. Alpha radiation is absorbed
over very short distances and is important only as an internal emitter.
Beta radiation is intermediate in these respects. In conventional dosimetry,
radiation doses are averaged over relatively large volumes of similar
tissue, for example whole organs, and herein lies the problem. Because
alpha particles are absorbed over short distances, they generate a very
high density of ionisation and hence chemical damage along their tracks.
Consequently, an alpha emitter and to a lesser extent a beta emitter,
located internally, will irradiate relatively small numbers of adjacent
cells to a high intensity, causing major biochemical changes in localised
This contrasts with the conventional approach, appropriate to external
sources of radiation, in which uniformity of dose, and consequently of
biological damage, is assumed. These problems have, of course, long been
recognised by radiobiologists, and has led to the development of the science
of microdosimetry, in which short range effects of the type described
are taken specifically into account.
However, the prediction of radiation damage from internal emitters also
requires other factors - biological and biochemical - to be taken into
account. Because radiation induced changes from alpha emitters are highly
localised, the exact location of the alpha source within the tissue, or
even within the cell, becomes an important factor.
Not all alpha emitters react biochemically in the same way, and as a consequence
different elements will end up at different locations: maybe in different
organs, maybe in different cells in the same organ, or maybe at different
locations in the same cell. For example, when bone into which plutonium
or radium has been incorporated is examined by autoradiography, a photographic
technique which identifies the positions of the radiating atoms, the pattern
of uptake is very different: radium is found to locate generally throughout
the bone mass, whereas plutonium is located particularly at the growing
Thus, for the same amount of each radioactive element (in terms of amount
of radioactivity present) the doses to the bone surface and to the bone
interior will be radically different for the two elements and consequently
the risks for the initiation of different types of cancer may well be
different. However, standard dosimetry would not distinguish between the
two cases: for the same amount of radioactivity the average radiation
dose over the whole bone will be the same in each case, and the predicted
(but not the actual) cancer risks would be the same for each element.
Thus, when considering biological outcome - for example, the initiation
of a particular form of cancer - not only the gross structural difference
between the radiation damage generated by alpha, beta and gamma radiation
must be taken into account, but also - particularly in the case of alpha
emitters - the molecular location of the radiating atoms. For these reasons,
the biological effects of radiation from alpha, and to a lesser extent
beta, emitters, absorbed internally, cannot reasonably be equated with
the damage generated by gamma emitters, sited externally, even with the
application of a quality factor to account for the difference in effectiveness.
A more fundamental approach, and one taking both the chemistry and biology
into account, needs to be adopted.
There is, of course, nothing new in these claims, nor indeed to the rationale
I have advanced in support. Radiation dosimetry is, essentially, a pragmatic
science and was originally devised as a rough and ready means to protect
workers in the nuclear industry. As the scientific understanding of the
affects of ionising radiation have advanced, and as the potential applications
of dosimetry have multiplied, so have the basic flaws in the methodology
become more apparent.
At each stage, emerging defects have been - in part - remedied by applying
"factors" (for example, the radiation quality factors) to compensate for
potential problems. However, there comes a stage at which the whole edifice
becomes so complex, the exceptions so numerous, and the adjusting factors
so apparently arbitrary, that the whole logical structure for the science
becomes open to question. This stage may now have been reached.
Finally, the question: does it matter? Is this merely an academic quibble,
nice to put right maybe, but of no practical consequence? The answer is
firmly no, for several reasons. Quite simply, that the application of
current methods of dosimetry may lead to the wrong conclusions with essentially
practical consequences: the allowed levels of certain radionuclides in
food, the levels of discharge allowed from a certain industrial site,
the need or otherwise to evacuate an area after a nuclear accident.
There are cases where standard dosimetry has led to startling conclusions,
which might not have been reached had a more scientifically rational approach
been adopted. Clusters of leukemia or other cancers which have no apparent
cause, and for which ionising radiation has been categorically dismissed
as a potential cause, might take on a different aspect if radiation doses
and radiation effects were evaluated taking the microdosimetry into account.
It is important to recognise that this would not necessarily be the outcome
of such a re-evaluation; but, whatever the outcome, it would certainly
be more convincing for being based on a rational and scientific analysis
rather than by the application of a number of empirical rules.
Dr Philip Day is Reader in Chemistry at the University of Manchester,
and has had research interests in environmental radioactivity for nearly
30 years. His Ph.D. thesis was on the chemistry of uranium, and a major
continuing research interest has been the chemistry - inorganic, biological,
environmental and analytical - of the actinide elements uranium, neptunium,
plutonium and americium. He has also had continuing concerns for the risks
to human health from the release of artificial radioactivity to the wider
environment, and particularly for the ways in which risks to the wider
population have been and are being assessed.
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Page last updated: 6th December 2002