An improved neutron collimator for brain tumor irradiations in clinical boron neutron capture therapy

Abstract

To improve beam penetration into a head allowing the treatment of deeper seated tumors, two neutron collimators were built sequentially and tested for use in the clinical boron neutron capture therapy (BNCT) program at the epithermal neutron irradiation facility of the Brookhaven Medical Research Reactor. The collimators were constructed from lithium-impregnated polyethylene, which comprises Li2CO3 powder (approximately 93% enriched isotopic 6Li) uniformly dispersed in polyethylene to a total 6Li content of 7.0 wt. %. The first collimator is 7.6 cm thick with a conical cavity 16 cm in diameter on the reactor core side tapering to 8 cm facing the patient's head. The second collimator is 15.2 cm thick with a conical cavity 20 cm in diameter tapering to 12 cm. A clinical trial of BNCT for patients with malignant brain tumors is underway using the first collimator. Results of phantom dosimetry and Monte Carlo computations indicate that the new 15.2 cm thick collimator will improve the neutron beam penetration. Thus, the second collimator was made and will be used in an upcoming clinical trial. In-air and in-phantom mixed-field dosimetric measurements were compared to Monte Carlo computations for both collimators. The deeper penetration is achieved but at a sacrifice in beam intensity. In this report, a performance comparison of both collimators regarding various fluence rate and absorbed dose distributions in a head model is presented and discussed.

A physical dosimetry intercomparison for BNCT

“…Overall uncertainties, given throughout as one standard deviation, of 6.0% ͑BMRR͒ 4 and 7.4% ͑MIT͒ 6 are associated with these flux measurements. The BMRR flux values reported here appear systematically higher by between 6 and 11% depending upon depth than those previously reported by the group 4 ͑see Table I͒ but are within the stated uncertainties for two independent measurements. The apparent discrepancy between the two data sets, most obvious at shallow depths, is due to differences in the reporting methods adopted by the two groups.…”

“…The observed peak maximum is at a depth consistent with the 2.7 cm determined from model calculations of the beam. 4 Photon depth dose measurements performed with the graphite walled ionization chamber are shown in Fig. 3 and Table II.…”

“…The first of these was a solid PMMA ͑elemental composition by mass 8% H, 60% C and 32% N͒ cube 14ϫ14ϫ14 cm 3 that was routinely used for clinical dosimetry at BMRR. 4 The cube has a hole in the center of the front face that extends through to the back and runs parallel to the sides. The hole accommodates various PMMA rods of 1.59 cm diameter in which a series of slits are cut at depths of 3.5, 7.0 and 10.5 cm from the front surface of the phantom.…”

Development of a Neutron Energy-Biased in-Air Figure-of-Merit for Predicting in-Phantom BNCT Neutron Beam Characteristics

“…The modification to the Snyder phantom was the addition of a half centimeter layer of skin as used by Liu et al 7 Elemental compositions for brain, skin, and skull were taken from ICRU46. 8 The midpoint of this phantom is at 7.3cm.…”

A method for fast evaluation of neutron spectra for BNCT based on in‐phantom figure‐of‐merit calculation

“…Parallel beams of nearly monoenergetic neutrons, ranging from thermal up to 1 MeV, with equilogarithmic energy binning and ten bins per energy decade, were transported through a tissue volume simulating the human head. Snyder's head model 5 modified by Liu et al 6 was used. The elemental composition and densities in the scalp, the skull, and the brain were taken from Brooks et al 7 The head was irradiated bilaterally, from ear to ear, to treat a centrally located tumor, at 73 mm from the skin, of 1 cm 3 volume.…”