Ridge filter design and optimization for the broad-beam three-dimensional irradiation system for heavy-ion radiotherapy

The broad-beam three-dimensional irradiation system under development at Na tional Institute of Radiological Sciences (NIRS) requires a small ridge fil ter to spread the initially monoenergetic heavy-ion beam to a small spread- out Bragg peak (SOBP). A large SOBP covering the target volume is then achi eved by a superposition of differently weighted and displaced small SOBPs. Two approaches were studied for the definition of a suitable ridge filter a nd experimental verifications were performed. Both approaches show a good a greement between the calculated and measured dose and lead to a good homoge neity of the biological dose in the target. However, the ridge filter desig n that produces a Gaussian-shaped spectrum of the particle ranges was found to be more robust to small errors and uncertainties in the beam applicatio n. Furthermore, an optimization procedure for two fields was applied to com pensate for the missing dose from the fragmentation tail for the case of a simple-geometry target. The optimized biological dose distributions show th at a very good homogeneity is achievable in the target. (C) 2000 American A ssociation of Physicists in Medicine.