Iodine-123
A radioactive form of iodine that decays quickly and gives off gamma rays, used to take a picture of the thyroid or, attached to a nerve-tracing molecule, of tumours of the adrenal gland and nervous system, before deciding whether a therapeutic dose of iodine-131 makes sense.
Overview
Iodine-123 has a 13.2-hour half-life and decays by electron capture with a dominant 159 keV gamma emission well suited to gamma-camera and SPECT imaging. Its main clinical roles are thyroid uptake and scintigraphy, and as the diagnostic imaging partner to iobenguane (MIBG): 123I-MIBG scintigraphy stages neuroblastoma and phaeochromocytoma/paraganglioma before or alongside treatment with the therapeutic 131I-MIBG.
It is cyclotron-produced, never reactor-produced, most commonly via 124Xe(p,2n)123Cs, which decays through 123Xe to 123I, or via the (p,pn) channel on the same enriched xenon-124 target; a 123Te(p,n)123I route on enriched tellurium is also used. Both routes need a medium-energy cyclotron (20-30 MeV) and an expensive enriched target, which keeps 123I costlier and less universally available than reactor- or generator-based isotopes, and its 13-hour half-life limits how far it can be shipped before it has decayed away.
Because iodine's biochemistry is essentially identical across its isotopes, 123I plays the same diagnostic-before-therapeutic role for iodine and MIBG imaging that 68Ga and 18F play for peptide and small-molecule theranostics.
- γ 159 keV (83%)
- electron capture
Theranostic partners
- Iodine-131 · A radioactive form of iodine. Because the thyroid gland naturally absorbs iodine, swallowing it lets the radiation destroy thyroid cells or thyroid cancer cells almost anywhere they have spread, with very little effect on the rest of the body.
A tracer originally built to look at brain dopamine cells in suspected Parkinson's disease. Because certain hormone-producing tumours use the same chemical machinery, doctors also use it, off-label, to find those tumours -- especially adrenal gland tumours called pheochromocytoma.
An injectable tracer that mimics a stress hormone and gets taken up by certain nerve-related tumours, such as pheochromocytoma and childhood neuroblastoma, so a scan afterwards can show where they are.
A radioactive tracer that mimics the body's own "fight or flight" nerve chemical and is taken up by the nerve endings in heart muscle. In people with heart failure, comparing how much of it the heart holds onto against the surrounding chest tissue gives a read on how damaged the heart's nerve supply is, which relates to future risk.
A brain scan that shows how many dopamine-releasing nerve endings remain in a part of the brain called the striatum, helping tell shaking from Parkinson's-type disease apart from other kinds of tremor and from Alzheimer's-type memory loss.
The original targeted radiotherapy: thyroid cells soak up iodine, so radioactive iodine destroys leftover thyroid tissue and metastases while sparing everything else.
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