IMPROVED APPARATUS FOR NEUTRON-CAPTURE THERAPY OF RAT-BRAIN TUMORS

Purpose: The assembly for irradiating tumors in the rat brain at the t hermal neutron beam port of the Brookhaven Medical Research Reactor wa s redesigned to lower the average whole-body dose from different compo nents of concomitant radiation without changing the thermal neutron fl uence at the brain tumor. Methods and Materials: At present, the tumor -bearing rat is positioned in a rat holder that functions as a whole-b ody radiation shield. A 2.54 cm-thick collimator with a centered conic al aperture, 6 cm diameter tapering to 2 cm diameter, is used to restr ict the size of the thermal neutron field. Using the present holder an d collimator as a baseline design, Monte Carlo calculations and mixed- field dosimetry were used to assess new designs. Results: The computat ions indicate that a 0.5 cm-thick plate, made of (Li2CO3)-Li-6 dispers ed in polyethylene (Li-poly), instead of the existing rat holder, will reduce the whole-body radiation dose. Other computations show that a 10.16 cm-thick (4 inches) Li-poly collimator, having a centered conica l aperture of 12 cm diameter tapering to 2 cm diameter, would further reduce the whole-body dose. Conclusion: The proposed irradiation appar atus of tumors in the rat brain, although requiring a 2.3-fold longer irradiation time, would reduce the average whole-body dose to less tha n half of that from the existing irradiation assembly.