Properties:
Iodine-125 (125I) is a gamma emitter at 35.5 keV (7%) with a half-life of 59.4 days. It also decays through an internally conversion process (93%) with emissions at 27.0 keV (113%) and 31.0 keV (26%). The mean therapeutic path length is 10 nm. Tenth value layer (TVL) is 8.3 mm for concrete and 0.1 mm for lead.
Manufacturing:
Two reactor irradiation routes are used to produce 125I, both starting with 124Xe, [124Xe(n,γ)125mXe]→125I and [124Xe(n,γ)125gXe]→125I. The irradiation of natural Xenon (containing only 0.096% of 124Xe) instead of using enriched 124Xe is a standard route, but requires long irradiation times which result in production of some contaminating 126I (half- life 13.1 days). 125I could also be produced in cyclotrons starting from 125Te via [125Te(d,2n)125I], although this route is rarely used because of the possible presence of 126I.
Source and availability:
The largest producers of 125I is Canadian, McMaster University’s Reactor. The second one, the National Research Universal (NRU) reactor (Nordion) did shut down by end of March 2018 and was definitively closed by end of 2018. However, this did not generate capacity limitation or potential shortage issue, due to the low amounts of 125I needed worldwide.
Derivatives:
125I was used as a clinical imaging agent in only very rare cases (125I-Fibrinogen for clot imaging has been discontinued). Due to its long half-life, it is often used in labeling of tracers for animal studies. The 125I-labeled products used in man are applied for ex vivo purpose (measurement of radioactivity concentration in fluids – urine, blood) at very low doses (less than 1 mCi). For example, 125I-Iothalamate (Glofil-125 from Iso-Tex Diagnostics) is used for evaluation of glomerular filtration. 125I-labeled products on the market include 125I-Serum Albumin and 125I-Sodium Iodide.
Price:
125I is considered inexpensive but mainly because it is also used in very small amounts (range of a few Euros per mCi).
Issues:
No special issue, but it is impossible to use this radionuclide as a tracer for imaging as a consequence of its too long half-life.
Comments:
The interesting Auger electron emission profile could open up future potential as a therapeutic for this radionuclide, despite the long half-life, providing that the therapeutic doses remain in the low range (about 5 mCi) which seems to be realistic if the vector is specific enough.For the time being, one has to consider that this radionuclide has and will have very limited interest in nuclear medicine.