Potential therapeutic advantage of strongly forward-directed beams for boron neutron capture therapy

The delivery of sufficient thermal neutrons selectively to the tumor is one of key requirements for the success of boron neutron capture therapy (BNCT). The authors implemented the design study of forward-directed beams and presented their therapeutic advantage in terms of physical dose and tumor control probability. A reactor-based BNCT facility was simulated for two cylindrical collimators that yield forward-directed beams (Collimator I of 10 cm-diameter/20 cm-length and Collimator II of 14 cm-diameter and 15 cm-length). The total tumor dose and theoretical tumor control probability were calculated for the forward-directed beams and a beam of a linear angular distribution. The normal brain tolerance dose of 1500 cGy-Eq and ¹⁰B-concentrations of 7.5 ppm in the brain, and 30 ppm in the tumor were applied for these calculations. The average values of directional cosine (μ) at the irradiation port were determined to be 0.90 and 0.86 for Collimator I and II, respectively. The total dose averaged over a 4cm-diameter tumor in the brain was shown to be 6096 cGy-Eq for Collimator I, 6140 cGy-Eq for Collimator II, and 5237 cGy-Eq for the beam with the linear angular distribution. The theoretical tumor control probability was determined to be 0.93 for Collimator I, 0.95 for Collimator II, and 0.48 for the linear distribution. Strongly forward-directed beams of μ>0.8 can be generated using a long cylindrical collimator made of thermal neutron absorbers. The therapeutic advantage would be even more significant for deep-seated tumor treatments