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Introduction: Locoregional applications of beta emitting radiopharmaceuticals are known to suppress tumor growth by nonspecific ablation of tumors. While the current clinical schemes involve intraarterial injection of particulate radiopharmaceuticals, direct intratumoral injection is an attractive alternative. This study is designed to investigate the intratumoral therapeutic application of I-131 NaI which is readily available to most nuclear medicine clinics. Methods: 2 triplicate groups of female Fisher rats inoculated with 13762 tumor cells (100,000) in the right thigh received 2 oral doses of non-radioactive iodine (Lugol solution) for thryoid blockade 1 day before and 1 hour before injection. Doses of 0.5 or 2 mCi of I-131 NaI in saline were injected into the tumor site. Serial whole-body scintigraphic images were acquired up to 19 hours using a Seimens M-CAM. Organ distribution of I-131 were evaluated and compared with reference standards. Time activity curves were constructed and tumor growth rates were recorded up to 3 months (to date). Results: Clearance of I-131 from the injection sites was mainly through the circulation with biologic half-life of approximately 1 hour. There is also fast clearance of I-131 from the body and transient visualization of the stomach. No thyroid was identified indicating effective thyroid blockade. The group treated with 0.5 mCi showed only slightly slower growth compared with the control group with no treatment and were euthanized at about day 25 after injection. In 2 of the 3 rats treated with 2mCi I-131 NaI prolonged tumor suppression (>3 months) was observed with no growth retardation. Conclusion: Despite relatively fast clearance, direct injection of I-131 NaI into the tumor site may serve as an effective means to suppress tumor. Thyroid blockade using non-radioactive iodine did not adversely affect the effectiveness of I-131 NaI. The approach may be further applied to other radionuclides. Discussion: This example illustrates the advantages of locoregional injection of therapeutic radionuclides: the initial maximal exposure, lower systemic toxicity because of dilution and the use of nonradioactive and nontoxic drugs to decrease systemic toxicit(ies). This direct intratumoral injection approach may further be aided by recent developments of image-guidance for tumor localization and accurate placement of the radionuclides.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]