Radionuclide impurities

[jtowson@nucmed.rpa.cs.nsw.gov.au (Jocelyn Towson)]

Some thoughts on radiopharmaceutical impurities and why they might be
undesirable.
- Dosimetry. Diagnostic radionuclides are selected for low radiation dose,
and it is not surprising that contaminants in small abundance can increase
the dose disproportionately.  Especially true if the contaminants are of
longer half life and even more so in sick patients.  Pathology may increase
residence times in various organs, eg impaired renal clearance,
hyperthyroidism.   A long physical half life then has more impact.  Vaguely
remember alarming suggestion many years ago of high dose to the spinal cord
from ytterbium-169 (32 day half life) DTPA administered by lumbar puncture
if cerebrospinal fluid flow was low.
- Chemistry.  Isotopic contaminants (eg Tc-99 in Tc-99m), stable or not,
will compete in radiochemical labelling reactions and lower the useful
specific activity. Important in Ceretec, iodinations etc.
- Image quality.  Contaminants with high energy gamma photons, even in
small abundance, could degrade gamma camera images by down scatter and
septal penetration.  Contaminant photons if in the photopeak would be just
as much of a nuisance.  Only lower energy photons could be entirely
discriminated out.

A variation of 10% or so in dispensed activity (and therefore dose) of the
primary radionuclide is usually considered acceptable for nuclear medicine
diagostic and therapeutic procedures.    My feeling is that this leniency
shouldn't be extended to contaminants, for which the impurity level should
be set low enough to take account of the possible increased dose in sick
patients.
I'm not aware how our regulators set a limit on radionuclidic impurities. I
guess each case is considered on its merits.  BTW, the first time I heard
it, it struck me as odd that diagnostic radiopharmaceuticals come under the
umbrella of "Therapeutic Goods".

Jocelyn Towson
Radiation Safety Officer
Royal Prince Alfred Hospital
Sydney
jtowson@nucmed.rpa.cs.nsw.gov.au