3164 The American Cancer Society estimates that 1.4 million people will be diagnosed with cancer in 2007. Men have a 45% probability of developing some form of cancer in their lifetime. Of men diagnosed with cancer, 1-in-3 will be prostate cancer. Current detection methods for prostate cancer involve determining the concentration of prostate specific antigen (PSA) in blood serum. Medications, certain herbs, and genetics can increase PSA levels, thus providing inaccurate results. Current treatment options for prostate cancer include surgery, irradiation, immunotherapy, hormone therapy and chemotherapy. Gold nanoparticles provide an attractive alternative to treating cancer by delivering high irradiation payloads. Gold-198 (Au-198) radiolabeled nanoparticles stabilized with gum Arabic, a natural additive used in everyday food products and thus non-toxic in-vivo , are described. These nanoparticles are rapidly produced by using the non-toxic, amino acid derived reducing agent tris-hydroxymethyl phosphine alanine (THPAL). The nuclear properties of Au-198 are well suited for radiotherapy (t 1/2 = 2.7 days, E βmax = 1.37 MeV). The 411 keV gamma emission (95%) allows tracking of the Au-198 radiolabeled nanoparticles using Single Photon Emission Computed Tomography (SPECT). The high thermal (98.6 barns) and epithermal (1550 barns) nuclear cross-section of natural, monoisotopic Au-197 allows for relatively rapid irradiation to achieve reasonable specific activity. If carrier-free radioactive Au is needed, Au-199 (t 1/2 = 3.1 days, E βmax = 0.45 MeV, 158 and 208 keV γ-rays) can be obtained from the β - decay of Pt-199 (t 1/2 = 30.9 min; produced from an enriched Pt-198 target). Three different routes of administering Au-198 radiolabeled gold nanoparticles (Au-198-GA-AuNP), prepared as described above, were compared using a PC-3 (human prostate cancer) xenograft tumor model in immunocompromised mice. Radiotracer uptake and retention were determined after 30 min, 1 h, 2 h, 4 h, and 24 h. Tumor uptake after 24 h was 1.79 + 0.32% D/g by intravenous (IV) injection, 5.75 + 1.49% D/g by intraperitoneal (IP) injection, and 87.73 + 37.61% D/g by intratumoral (IT) injection. The IV injection also showed high liver (65.2 + 2.30% D/g), lungs (17.9 + 4.92% D/g), and spleen (8.52 + 1.12% D/g) uptake over time. After IP injection, tumor uptake increased steadily over the time course. IT administration showed little to no leakage of Au-198-GA-AuNP from the tumor over the first 4 hr, while 60% of the injected dose was retained at 24 hr, indicating potential as a radiotherapeutic agent.