Purpose: Yttrium-86 has been proposed for use as a quantitative positron emission tomography imaging agent to determine the in vivo distribution of therapeutic pharmaceuticals labeled with yttrium-90, a pure beta minus emitter. This study identifies, and proposes a solution for, an artifact, which interferes with quantitation.Procedures: Yttrium-86 is a 14.7-hour halflife positron emitter (33% abundance) with multiple high energy gamma rays in cascade. Phantom measurements with a GE Advance PET scanner using standard attenuation and scatter corrections, demonstrated anomalous apparent activity in inactive higher density regions.Results: Apparent activity up to 30% of the surrounding true activity was observed in a bone equivalent material. Even higher activities were observed if the scatter correction was omitted. This phenomenon was determined to result from the effect of attenuation correction on true coincidences between one gamma ray and a second gamma ray or annihilation photon.Conclusion: A simple additional correction based on sinogram tail subtraction improves accuracy significantly.