Why is use of radioactive Abs conjugates?

Radiolabeled monoclonal antibodies — Entire Ig molecule or Fab or (Fab’)2: binds Ag — Radioisotope responsible for cell toxicity (high intensity, low action radius) — Ex: radiolabeled anti-CD20 (Zevalin) for treating Non-Hodgkin lymphoma (NHL) cases that are resistant to purified anti-CD20 — May be used in “multistep targeting”: Ab- streptavidin then biotinylated radioactive ligand (clinical trials)