Therapy of myeloma tumor xenografts with radioiodine. (A) Myeloma tumor xenografts expressing hNIS and treated with a single dose of ¹³¹I (1 mCi [37 MBq] intraperitoneally) regressed completely within 2 weeks of therapy. Tumor xenografts not expressing hNIS continued to grow, as did untreated control tumors. (B) NIS with ¹³¹I has a significant bystander effect. Mixed tumors were established in SCID mice and all were treated with a single dose of ¹³¹I (1 mCi, [37 MBq] intraperitoneally). All tumors in which 50% to 100% of the cells expressed NIS regressed completely, while tumors with only 10% of the cells expressing NIS were slowed transiently. Vertical arrows indicate time of therapy with radioiodine.