Applied Radiation and Isotopes 65 (2007) 318–327 A new internal pair production branching ratio of 90 Y: The development of a non-destructive assay for 90 Y and 90 Sr R.G. Selwyn à , R.J. Nickles, B.R. Thomadsen, L.A. DeWerd, J.A. Micka Department of Medical Physics, University of Wisconsin—Madison, Madison, WI 57306, USA Received 31 May 2006; received in revised form 31 July 2006; accepted 17 August 2006 Abstract 90 Y is utilized as a therapeutic radioisotope in radiolabeled monoclonal antibodies and in microspheres for targeted radiation therapy of the liver. Currently, the widely used dose calibrator assay of 90 Y can have uncertainties exceeding 710%. A non-destructive assay using spectroscopy is possible by reducing the currently published uncertainty (712%) in the internal pair production branching ratio for the 0 + –0 + transition of 90 Zr. A high-purity germanium detector was used to determine the branching ratio to be (31.8670.47)  10 6 . r 2006 Elsevier Ltd. All rights reserved. Keywords: 90 Y; 90 Zr; Positron; Assay; Activity; Branching ratio; Germanium; Spectroscopy; Annihilation; HPGe detector 1. Introduction 1.1. Medical use of 90 Y 90 Y is traditionally thought of as a pure beta emitter with a decay scheme shown in Fig. 1 (Lederer et al., 1978). The current medical uses of 90 Y include radiolabeled mono- clonal antibodies and labeled microspheres for targeted radiotherapy of the liver. Ibritumomab tiuxetan (Zevalin s ) is the first United States Food and Drug Administration (FDA) approved radiolabeled monoclonal antibody and is specific to treating patients with non-Hodgkin’s lymphoma (NHL). Other monoclonal treatments such as 90 Y-labeled chimeric anti-CEA monoclonal antibody for metastatic colorectal cancer (Oriuchi et al., 2005), Epratuzumab and Lym-1 for NHL (Oriuchi et al, 2005), murine HMFG1 for epithelial ovarian cancer (Verheijen et al., 2006), and others for breast and prostate cancer are in clinical trials. Also, glass microspheres (TheraSphere s , MDS Nordion) im- pregnated with 90 Y are approved by the FDA for the treatment of hepatocellular carcinoma (Wong et al., 2002), while resin microspheres (SIR-spheres s , SIRTeX Medical) labeled with 90 Y are approved for the treatment of metastatic hepatic cancer (Kennedy et al., 2004). In order to compare treatment outcomes, the injected activity should be known with a low uncertainty. The current assay method for 90 Y, however, can result in uncertainties as high as 710% (Valley et al., 2003). 1.2. 90 Y assay The accurate assay of 90 Y is beneficial for comparing treatment outcomes globally and for comparing theoretical dose calculations with experimental dose measurements. The activity of 90 Y is determined using either non- destructive or destructive assay methods. A widely accepted non-destructive assay of 90 Y uses reentrant ionization chambers or dose calibrators to detect brems- strahlung. The dose calibrator provides a convenient yet problematic non-destructive assay method. The brems- strahlung production is highly dependent on the source material, its container, and the chamber wall. The ionization current also depends on the probability of electron detection within the chamber, which varies with electron energy and individual dose calibrator construc- tion. Slight variations in the container wall thickness, solution volume, or location within the well can lead to an increase in the overall assay uncertainty when using the ARTICLE IN PRESS www.elsevier.com/locate/apradiso 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.08.009 à Corresponding author. Tel.: +1 608 262 7409; fax: +1 608 262 2413. E-mail address: selwyn@wisc.edu (R.G. Selwyn).