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 theranostic 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 and auger electron 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 trails.
Accurate determination of administered activity, traceable to a primary standard of radioactivity is essential for all radio-pharmaceuticals. Molecular radiotherapy (MRT) is a cancer therapy technique in which radioactive pharmaceuticals are administered to deliver a lethal radiation dose to malignant cells whilst sparing surrounding healthy tissue. Calculations of absorbed dose for MRT require the determination of the specific distribution of radioisotopes within the body - primarily from SPECT imaging. Clinical SPECT camera systems are typically optimised for imaging with 99mTc (Eγ = 140 keV), in contrast the decay of $^{155}$Tb produces gamma rays of 87 keV, 105 keV, 180 keV and 262 keV. Establishing optimised SPECT imaging protocols for $^{155}$Tb with a clinical camera system is a crucial first step in demonstrating the efficacy of $^{155}$Tb for clinical applications.
Methods
Samples of $^{155}$Tb were collected at the ISOLDE GLM beamline. The samples were imaged using a GE Discovery 670 SPECT/CT camera at The Christie NHS Foundation Trust (Manchester, UK). Data was acquired in 'list-mode' enabling retrospective projected SPECT images to be produced from scan data after the initial samples had decayed. Experimental SPECT/CT data was combined with a Monte Carlo simulation of the Discovery 670 SPECT camera [6] to optimise the energy windows and collimators used to acquire $^{155}$Tb SPECT images. An additional sample of $^{155}$Tb has been characterised for purity and activity using high precision gamma-ray spectroscopy at the UK National Physical Laboratory.
Results
Preliminary results will be presented on the characterisation of $^{155}$Tb samples produced at CERN-ISOLDE. These results will be discussed in the context of providing a primary standard of activity for this isotope.
The first isotope-specific photopeak and scatter imaging energy windows for $^{155}$Tb will be presented. The effect of applying these energy windows, calculated using a minimization method applied to data from a full MC simulated SPECT acquisition, to clinical SPECT/CT imaging will be discussed.
References
[1] C. Müller et al, J. Nucl. Med. 53, 1951-1959 (2012)
[2] C. Müller 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. Müller et al, Pharmaceuticals 7, 353-365 (2014)
[6] A.P. Robinson et al, Phys. Med. Bio 61, 5107-5127 (2016)
