Properties:
Thallium-201 (201Tl) is a SPECT radionuclide decaying in 72.9 hours with a gamma emission at 167 keV (10%) but also beta emissions at 316 keV (43.6%) and 481 keV (43.0%). Tenth value layer (TVL) is 4.7 cm for concrete and 1.1 mm for lead.
Manufacturing:
201Tl is produced in a cyclotron following the irradiation process [203Tl(p,3n)201Pb→201Tl] at an ideal energy of 28 MeV. The process is well controlled and routine production is performed at centers that run adequate cyclotrons.
For 201Tl, among several methods the most common production pathway consists of the 203Tl(p,3n)201Pb → EC (t½ = 9.4 h) → 201Tl process. The optimum energy range is Εp = 28 → 20 MeV. Similar to the 127I(p,5n)123Xe → (EC, β+) → 123I process, stable thallium is irradiated in order to obtain a thallium radioisotope. The 201Tl is selectively isolated via the 201Pb intermediate. Utilizing elaborate wet chemical processing, first 201Pb is separated from the irradiated target by co-precipitation. Thereafter, it is allowed to transform (for an optimum time of 32 h) whereby 201Tl grows in. This is removed from 201Pb via anion-exchange chromatography. The 201Tl isolated must be a 201Tl+ species for its sole medical application as a myocardial blood flow tracer.
Source and availability:
For historical reasons 201Tl was more easily accessible in Europe and became the preferred myocardial perfusion imaging agent in this area, while the US focused on 99mTc derivatives for the same indication. In the meantime, 99mTc-labeled compounds for MPI also became readily available in Europe, but there remains a culture of 201Tl use in Europe which made the 99Mo shortage issue less significant in this part of the world.
There are several stable and reliable sources (large cyclotrons) of 201Tl in Europe and less in the US although the existing US centers could produce much more 201Tl-Thallium Chloride if the demand would increase. All over the world, the use of 201Tl is declining, despite some advantages given to 201Tl by cardiologists. Manufacturers include IBA Molecular and Mallinckrodt (Curium) but not anymore Nordion (BWXT).
Derivatives:
There is one unique application for 201Tl which is cardiology. The chemistry and also the physical properties are not in favor of the development of other 201Tl-labeled derivatives.
Price:
201Tl is sold at around EUR 10–20 (US$ 13–26) per mCi depending on the amount (pharmaceutical grade). Doses used in patients range between 5 and 10 mCi, but recent studies have shown that good data can be obtained with doses between 2 and 3 mCi.
Issues:
◼ Long half-life and parallel beta emission (compared to 99mTc).
◼ The radiation burden to patients is higher than for 99mTc.
◼ Manufacturing sites are concentrated in Europe.
Comments:
There is an almost steady state use in cardiology (unique indication) mainly in Europe with some growing interest as a consequence of the 99Mo shortage, but this declined again since the shortage was solved. 201Tl ’s major but also quite unique application is in cardiology and it can easily be replaced by 99mTc-labeled compounds that are less expensive or, in some cases, with PET imaging agents (mainly 82Rb where available).