Early investigations of therapeutic radiolabeled antibodies for B-cell lymphoma were performed with iodinated antibodies (Press et al, 1989; Kaminski et al, 1993; Wahl et al, 1994; Press et al, 1995). Currently, two treatment regimens have been approved for treatment of relapsed/refractory and transformed follicular lymphoma. The Bexxar Treatment Regimen includes murine anti-CD20 antibody Tositumomab iodinated with Iodine I 131. The Main anti-tumor effect comes from high energy beta particles emitted by I 131. In addition, I 131 is emitting also low energy gamma radiation, allowing for gamma camera measurements and calculation of individualized dose of radioactive tracer for a given patient during therapeutic phase (see below). In Zevalin, a murine anti-CD20 antibody ibritumomab is covalently linked to 90Yttrium (90Y), which is a pure beta emitter. Lack of gamma radiation from 90Y does not allow for dosimetric assessments and a fixed dose of Zevalin is used for all patients based on their weight. This can possibly result in either under- or overdosing of some patients.