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 (87mY). Yttrium-87, itself decays to a metastable state of 87Sr (87mSr). 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, 87mSr, is expected, in vivo. Although strontium is known to concentrate in bone, the electron emissions of 87mSr 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.