Preliminary Treatment Planning and Dosimetry for a Clinical Trial of Neutron Capture Therapy using a Fission Converter Epithermal Neutron Beam

A Phase I/II clinical trial of neutron capture therapy (NCT) was conducted at Harvard–MIT using a fission converter epithermal neutron beam. This epithermal neutron beam has nearly ideal performance characteristics (high intensity and purity) and is well-suited for clinical use. Six glioblastoma multiforme (GBM) patients were treated with NCT by infusion of the tumor-selective amino acid boronophenylalanine-fructose (BPA-F) at a dose of 14.0 g/m2 body surface area over 90 min followed by irradiation with epithermal neutrons. Treatments were planned using NCTPlan and an accelerated version of the Monte Carlo radiation transport code MCNP 4B. Treatments were delivered in two fractions with two or three fields. Field order was reversed between fractions to equalize the average blood boron concentration between fields. The initial dose in the dose escalation study was 7.0 RBE Gy, prescribed as the mean dose to the whole brain volume. This prescription dose was increased by 10% to 7.7 RBE Gy in the second cohort of patients. A pharmacokinetic model was used to predict the blood boron concentration for determination of the required beam monitor units with good accuracy; differences between prescribed and delivered doses were 1.5% or less. Estimates of average tumor doses ranged from 33.7 to 83.4 RBE Gy (median 57.8 RBE Gy), a substantial improvement over our previous trial where the median value of the average tumor dose was 25.8 RBE Gy.