Macro- and microdistributions of boron drug for boron neutron capture therapy in an animal model.

BACKGROUND The boron concentration (BC) in the blood, rather than in normal tissue, is often used as the reference to calculate the BC in tumor for boron neutron capture therapy (BNCT). The aims of this study were to justify whether BC in the blood is equal to that of normal tissue, and to verify the macro- and microdistributions of boron in tumor. MATERIALS AND METHODS BALB/c nude mice bearing SAS human oral carcinoma xenografts were intravenously injected with 400 mg/kg of boronophenylalanine (BPA). Macro- and microdistributions of boron in the tumor were assayed with (18)F-fluoro-L-boronophenylalanine-fructose (FBPA-Fr)/micro-positron-emission tomography (PET) and alpha track autoradiography, respectively. RESULTS The BCs assayed from the blood, normal tissue and tumor varied even on sampling at the same time points post-BPA administration. The ratio of BC in normal tissue to that in blood, i.e. N/B ratio, remains about 1.31 at 30 to 45 min post-BPA administration. Furthermore, (18)F-FBPA-Fr/micro-PET imaging and autoradiography also showed heterogeneous boron distribution in the tumor. CONCLUSION The heterogeneous distribution of boron in the tumor is a limiting factor for the precise calculation of BC in the tumor. Here we suggest that the N/B ratio could be used to calculate the true BC in the tumor and in normal tissue for BNCT. (18)F-FBPA-Fr/PET imaging is useful to justify the N/B ratio for BNCT treatment.