Yttrium-90 biodistribution by yttrium-87 imaging: A theoretical feasibility analysis

Yttrium-90 is widely used in radioimmunotherapy but does not emit photons for imaging. Indium-111, a chemically similar atom has been used instead as a tracer for the distribution of ⁹⁰Y-labeled antibodies. Recent advances in gamma camera technology have made it possible to image at energies higher than 364 keV. This work provides a theoretical analysis of the feasibility of using ⁸⁷Y (485 keV, 92.2% yield) as a tracer for in vivo imaging of ⁹⁰Y-labeled antibodies. Yttrium-87 may be produced by the ⁸⁷Sr(p,n)⁸⁷Y reaction in a cyclotron and has a 3.3 day half-life. This reaction also yields a metastable state of ⁸⁷Y ((87m)Y). Yttrium-87, itself decays to a metastable state of ⁸⁷Sr ((87m)Sr). The level of these contaminants and their anticipated impact on the utility of ⁸⁷Y as a tracer for ⁹⁰Y are examined theoretically. Dosimetry is performed to assess the absorbed dose associated with using ⁸⁷Y. A 100 h delay following the end of bombardment of an isotopically enriched ⁸⁷Sr target reduces the activity of the metastable state of ⁸⁷Y in the product by 10- to 15-fold, with greater delays resulting in a further reduction. A rapid equilibration between ⁸⁷Y and its daughter, (87m)Sr, is expected, in vivo. Although strontium is known to concentrate in bone, the electron emissions of (87m)Sr are short range, thereby making possible biologic effects highly position dependent; the photon emissions allow for independent imaging which may be used to perform dosimetry and thereby directly assess potential toxicity. ⁸⁷Y merits further consideration as an imaging tracer for ⁹⁰Y.