Radiation physics for particle beam radiosurgery.

For the particles and energies considered suitable for radiosurgery, with increasing particle charge, the Bragg peak height reaches a maximum with helium and then decreases, the Bragg peak width narrows, the distal fall-off steepens, and the exit dose increases (Table 1). The helium-ion beam is superior to a proton beam because of the higher peak-plateau ratio, more rapid dose fall-off, and smaller beam deflection, and it suffers only in the modest exit dose. Comparison of the therapeutically useful parameters of these beams is complicated by the change in beam quality (LET) with depth. Considerations of RBE values, which change with the ion species and with depth of penetration, may alter the relative rankings based on one or more of these beam characterization values. For all these beams, the RBE increases with increasing LET. The effect for protons is small and occurs just at the end of range of the particles. Effective isodose distributions based on modeled beams have been reported for helium, carbon, and neon ions. These distributions include the effects of a varying RBE with changes in the beam quality (as measured by a dose-weighted LET) and the change in dose fraction size with depth (the dose per fraction is a function of the depth of penetration). These calculations suggest that the optimal charged-particle beam for radiosurgery might be carbon. Heavy charged-particle beams can produce dose distributions superior to those obtainable with photon or electron beams. In clinical trials, these dose distributions have proved to be useful for the treatment of human diseases, including neoplasia and life-threatening intracranial disorders.(ABSTRACT TRUNCATED AT 250 WORDS)