Thermal neutron flux mapping in a head phantom

Boron neutron capture therapy (BNCT) is a binary cancer treatment modality in which a boron-containing compound is preferentially loaded into a tumor, followed by irradiation by thermal neutrons. In accelerator-based BNCT, ne utrons are produced by charged particle-induced reactions such as Li-7(p, n )Be-7. For deeply seated brain tumors, epithermal (1 eV to 10 kev) neutrons are needed to penetrate the skull cap and subsequently thermalize at the t umor location. Cell damage in BNCT is caused by the high linear energy tran sfer (LET) products from the B-10(n, alpha)Li-7 reaction. Because the cross section for this reaction is of 1/upsilon character, the dose due to B-10 has essentially the same spatial distribution as the thermal neutron flux. A cylindrical acrylic head phantom (15.24 cm diameter by 21.59 cm length) h as been constructed to simulate the patient's head and neck, and acrylic sp acers of varying width allow placement of small (active sizes: 0.635 cm dia meter by 1.27 cm length and 1.5875 cm diameter by 2.54 cm length) BF3 propo rtional counters at nearly all radial and axial locations. Measurements of the thermal flux have also been benchmarked with gold and indium foils (bar e and cadmium covered), as well as MCNP simulations. Measurement of the the rmal neutron flux using these small BF3 counters is shown to be adequate fo r experimentally determining the spatial, variation of the B-10 dose in hea d phantoms for accelerator-based BNCT. (C) 1999 Elsevier Science B.V. All r ights reserved.