The potential radio-immunotherapeutic α-emitter 227Th – part I: Standardisation via primary liquid scintillation techniques and decay progeny ingrowth measurements
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Abstract
Thorium-227 is a potential therapeutic radionuclide for applications in targeted α-radioimmunotherapy
for the treatment of various types of cancer. To provide nuclear medicine departments
involved in Phase I clinical trials traceability to the SI unit of radioactivity (Bq),
a standardisation of a radiochemically pure 227Th aqueous solution has been performed
at the National Physical Laboratory. This was achieved via two primary liquid scintillation
(LS) techniques -4π(LS)-γ digital coincidence counting (DCC) and 4π LS counting. These
absolute techniques were supported by the indirect determination of the 227Th activity
via the measurement of the ingrowth and decay rate of the decay progeny by both ionisations
chambers and high purity germanium (HPGe) gamma-ray spectrometry. The results of the
primary techniques were found to be consistent, both with each other (zeta score = 1.1)
and to the decay progeny ingrowth measurements. An activity per unit mass of 20.726
(51) kBq g-1 was determined for the solution. A procedure has been developed that
provided an effective separation of the 227Th from its decay progeny, which was shown
by the effective time zero of the 227Th-223Ra nuclear chronometer measured by HPGe
gamma-ray spectrometry.