AN IMPROVED NEUTRON COLLIMATOR FOR BRAIN-TUMOR IRRADIATIONS IN CLINICAL BORON NEUTRON-CAPTURE THERAPY

To improve beam penetration into a head allowing the treatment of deep er seated tumors, two neutron collimators were built sequentially and tested for use in the clinical boron neutron capture therapy (BNCT) pr ogram at the epithermal neutron irradiation facility of the Brookhaven Medical Research Reactor. The collimators were constructed from lithi um-impregnated polyethylene, which comprises Li2CO3 powder (approximat e to 93% enriched isotopic Li-6) uniformly dispersed in polyethylene t o a total Li-6 content of 7.0 wt.%. The first collimator is 7.6 cm thi ck with a conical cavity 16 cm in diameter on the reactor core side ta pering 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 u nderway using the first collimator. Results of phantom dosimetry and M onte Carlo computations indicate that the new 15.2 cm thick collimator will improve the neutron beam penetration. Thus, the second collimato r 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 ach ieved but at a sacrifice in beam intensity. In this report, a performa nce comparison of both collimators regarding various fluence rate and absorbed dose distributions in a head model is presented and discussed . (C) 1996 American Association of Physicists in Medicine.