A novel method for accurate activity measurements of emerging 227Th-labelled conjugates

Aims: Activity measurements for radiopharmaceuticals using commercially available ionisation chamber based systems are commonplace and in the majority of cases the procedure used is widely known and simple to implement. Recent interest in the use of 227Th-labelled conjugates has led to a demand for knowledge of the measurement characteristics of this radionuclide. Unlike conventional radionuclides, 227Th is measured at intervals following separation from its decay progeny (T0¬) which means that careful thought must be given to the equilibrium status of the solution at the measurement time (T-T0). The response of conventional calibrators depends on the relative ratios of the decay progeny, therefore the aim of this study was to develop a method to determine a suitable dial setting and time dependent correction factor. Materials and Methods: The Bateman equations account for the temporal dependence of the number of radioactive atoms in a decay chain and are required to model the decay series of 227Th and 223Ra. 227Th decays to 223Ra with half-life of 18.68 days, which in turn decays into a series of 5 alpha and 2 beta emitting7 different radionuclides. In the case of ?pure? 223Ra, the system equilibrates within about 7 hours. From this point, the total activity is approximately 6.0072 times the 223Ra activity and decaying with the half-life of 223Ra. A similar series of calculations on the decay series of ?pure? 227Th leads to a more complex situation whereby the total activity of the system increases to more than 3.2 times the initial starting activity of 227Th. After approximately 20 days, the system contains more of the decay progeny than 227Th. In order to determine dial settings for individual calibrators a series of readings was taken of a 227Th source as a function of time. Using some fundamental information such as the activity of the 227Th in the geometry of interest at T0, the dial setting for the calibrator for 223Ra in the same geometry, and predicted dial settings for each of the 227Th decay progeny, a dial setting for 227Th may be determined. Results and Conclusion: TwoA methods to obtain accurate activity measurements for 227Th activities wereas developed for a range of commercially available radionuclide calibrators. This method requires prior knowledge of the calibrator dial factor for 223Ra, as well as a standardised source of 227Th, at an accurately known chemical separation time of the 227Th from its progeny. This methodology will provide clinical sites with accurate measurements of activity for patients with cancer treated with 227Th-labelled conjugates.