Speaker
Description
Introduction:
Interest in the element terbium (Tb) for medical application has grown recently [1]. Four Tb isotopes have been identified with the potential to provide unique theragnostic treatment strategies which combine cancer therapy with diagnostic imaging. The isotopes $^{155}$Tb and $^{152}$Tb can provide SPECT and PET imaging respectively [2], whilst $^{161}$Tb can be used for beta− therapy [3] and $^{149}$Tb for alpha therapy [4][5]. Using a combination of these isotopes as labels for radio-pharmaceuticals can provide both pre-therapy diagnostic imaging and post-therapy dosimetry and treatment optimisation using the same delivery vector. In order to validate the use of these isotopes for patient treatments extensive pre-clinical studies [1] are required to provide the foundation for future clinical trials.
The determination of administered activity, traceable to a primary standard of radioactivity is essential for all radio-pharmaceuticals. Accurate nuclear data measurements combined with a primary activity standardisation underpin the clinical use of any radioisotopes.
Methods:
Samples of $^{155}$Tb were collected with the prototype MEDICIS collection chamber at ISOLDE. At NPL, pseudo-isobaric $^{139}$Ce impurities have removed from the dissolved target using ion-exchange and extraction chromatography separation procedures. A new primary activity standardisation was performed using digital coincidence counting [6] and liquid scintillation techniques. Calibration factors for the NPL secondary standard ionisation chamber were also determined. Gamma spectrometry measurements of the $^{155}$Tb decay scheme and half-life were also performed.
Results:
After purification of the sample a detection limit for $^{139}$Ce of < 0.021 % is reported. An absolute activity standardisation for $^{155}$Tb will be reported. Revised gamma-ray intensities for transitions in $^{155}$Tb are reported with significant variations from the ENSDF evaluation [7]. A new half-life measurement is also reported.
The impact of these revised measurements on the clinical use of $^{155}$Tb will be highlighted.
[1] C. Muller et al, J. Nucl. Med. 53, 1951-1959 (2012)
[2] C. Muller et al, Nucl. Med. and Biol. 41, e58-e65 (2014)
[3] S. Lehenberger et al, Nucl. Med. and Biol. 38, 917-924 (2011)
[4] G.J. Beyer et al, EJNMMI 31, 547-554 (2004)
[5] C. Muller et al, Pharmaceuticals 7, 353-365 (2014)
[6] J. Keightley and T. S. Park, Metrologia 44, S32 (2007)
[7] R. A. Meyer et al, Phys. Rev. C 13(4), 2466 (1976)
