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Thorium-227: Potential for cancer therapy

Accurate determination of the activity and half-life of thorium-227

PTBnews 1.2020
Especially interesting for

nuclear medicine

radioactivity measurements

calibration laboratories

For the first time, PTB has succeeded in determining the specific activity of a thorium-227 solution very accurately by means of liquid scintillation counters. This means that traceable activity measurements of this radionuclide, which is being tested for radioimmunotherapy in nuclear medicine, are now possible. Also, the half-life of thorium-227 was determined by means of two measurement methods.

Decay chain of thorium-227. The particularity of thorium, which consists in thorium generating mainly alpha decay, is clearly visible here.

In nuclear medicine, alpha-emitting radionuclides offer interesting possibilities for cancer therapy. In spite of its relatively high energy, alpha radiation has a short particle range. If an alpha emitter is successfully introduced into the tumor tissue, it essentially destroys tumor cells, whereas strong damage to the healthy surrounding tissue is kept low. It is possible to use radioimmunotherapy to specifically introduce an alpha emitter into the tumor tissue. In radioimmunotherapy, the corresponding radionuclide is coupled with antibodies that will preferably target the diseased tissue. One of the radionuclides investigated for such a therapeutic approach is thorium-227.

At PTB, new methods have been developed to determine the activity of thorium- 227 accurately. A particularity of thorium-227 is that it is not in a radioactive equilibrium with its progenies. The activity of its progenies relative to the activity of thorium-227 changes over time. In many measuring devices, the measurement result (counting rate or ionization current) even rises first and only decreases again later. This particularity must be taken into account when determining activity. At PTB, activity has now been measured by means of liquid scintillation counting. In addition, new methods have been developed to calculate the probability of detection as a function of time. For the first time, a time-dependent correction has been applied to decay during the measurement period. For the activity concentration, a relative uncertainty of only 0.25 % has been achieved through these improvements.

The measurement data obtained by means of liquid scintillation counting and additional measurements carried out in an ionization chamber have also been used to determine the half-life of thorium- 227. The measurements at PTB took place over a period of almost half a year, which is a significant advantage compared to other experiments with considerably shorter measurement periods. The longer measurement periods reduce the uncertainty of the thorium-227 half-life, since, among other things, dependence on the half-life of radium-223 is reduced.

The results of both procedures are in good agreement, and by combining them, a half-life of 18.681(9) days is obtained.


Karsten Kossert
Department 6.1
Phone: +49 531 592-6140
Opens window for sending emailkarsten.kossert(at)ptb.de

Scientific publication

K. Kossert, O. Nähle: Determination of the activity and half-life of 227Th. Appl. Radiat. Isotop. 145, 12–18 (2019)