On the need of ad hoc calibration factors for activity measurement of solutions containing radioactive microspheres

Abstract

Microspheres labelled with radioactive nuclides are widely used for TransArterial RadioEmbolization (TARE) in the treatment of patients with hepatocellular carcinoma (HCC) and/or hepatic metastases. Nowadays, three commercially available devices can be used: ⁹⁰Y-loaded glass (Theraspheres®, Boston Scientific, Marlborough, MA, USA) and resin microspheres (SirSpheres®, SIRTex medical, Woburn, MA, USA), or ¹⁶⁶Ho-microspheres (QuiremSpheres^TM, Quirem Medical, Deventer, The Netherlands). With reference to ⁹⁰Y-loaded glass and resin microspheres, recent studies (S. A. Graves et al., J. Nucl. Med. 23, 1131-1135 (2022); S. Gnesin et al., J. Nucl. Med. 64, 825-828 (2023)) highlighted the difference between the measured activity of commercial vials and the nominal one declared by the vendors. As discussed by L. Auditore et al. (J. Nucl. Med. 64, 1471-1477 (2023)), this discrepancy can be attributed to the procedure used to measure the activity of the vials at the vendor sites and to neglect the emission of Internal Bremsstrahlung (IB) photons among the radioactive decay particles emitted by ⁹⁰Y. The aim of this study was to investigate, by means of Monte Carlo simulations, the causes of such discrepancies, focusing on the geometrical factors affecting the activity measurement. In this study an activity calibrator was reproduced in Monte Carlo simulation environment and the measurement of the activity of a vial containing YCl₃ or ⁹⁰Y-labelled microspheres was simulated. With reference to the glass microspheres, the relative difference between the signal produced by the two solutions, YCl₃ and ⁹⁰Y-labelled microspheres, is 23.5%. This discrepancy reduces to −1.7% if the ⁹⁰Y-labelled microspheres are simulated as uniformly distributed in the vial. Moreover, results indicate that the geometry of the vial affects the results reducing the discrepancy from 23.5% to 16.6%. Similar results were obtained for ⁹⁰Y-resin microspheres for which the discrepancy with respect to the reference solution reduces from −15% to −8.6% when considering the microspheres homogeneously dispersed in the vial. Since the activity of a device employed for therapy has to be accurately known, this study highlights the need of updating the procedure of measuring the activity of such particulate solutions by using ad hoc calibration factors to consider the real vial geometry and distribution of the radioactive solute in the vial.