The Dose Planning of BNCT for Brain Tumors

Summary

Radiation dose in BNCT and tumor cell survival relationship was studied to estimate the cure dose of tumors. SCCVII tumors, squamous cell carcinoma model of mouse, were used. The radiation doses were calculated from the¹⁰B-concentration measured by prompt γ—ray spectrometry and thermal neutron fluence. The radiation dose in Gy (D) vs. cell surviving fraction (SF) relationships were -InSF=−0.235+1.219_*D (γ=0.984), -InSF=0.0833+0.961_*D (y=0.990), and -1nSF=−0.013+1.141_*D (γ=0.991) for BPA, BSH and NCT alone, respectively. Tumor control probability (TCP) vs. D relationship was calculated according to the following equation: TCP=exp(−n) and n=N_*SF, where N was the initial number of clonogen. In calculation, N in 1 g tumor was postulated to be 10⁵ according to the reported data on several type human tumors. TCP90 (90% tumor control probability) doses were around 11 Gy and 14 Gy for BPA and BSH, respectively. This dose planning has been applied to BNCT for brain tumors since June 1993. Two representative cases were reported. The first was the patient of 57 years old female with glioblastoma in the left temporal lobe treated using BSH. The radiation dose delivered in the deepest tumor site by high LET particles (α and p) was 16.5 Gy. Three weeks later, complete clearance of the tumor was demonstrated by MRI. The second was the case of 16 years old boy with malignant astrocytoma treated by BPA. PET study revealed the accumulation of BPA in the tumor with tumor/blood ratio of 3.5, and blood ¹⁰B-concentration was 6.7 ppm following administration of BPA-fluctose complex (BPA, 8g=120 mg/kg). Tumor ¹⁰B- concentration was estimated to be 23.5ppm. The radiation dose delivered in the deepest tumor point by high LET particles was 9.7 Gy. Seven months after BNCT, MRI demonstrated complete regression of the tumor.