IRRADIATION OF MIXED BEAM AND DESIGN OF SPREAD-OUT BRAGG PEAK FOR HEAVY-ION RADIOTHERAPY

Data on cellular inactivation resulting from mixed irradiation with ch arged-particle beams of different linear energy transfer (LET) are nee ded to design a spread-out Bragg peak (SOBP) for heavy-ion radiotherap y, The present study was designed to study the relationship between th e physical (LET) and biological (cell killing) properties by using dif ferent monoenergetic beams of He-3, He-4 and C-12 ions (12 and 18.5 Me V/nucleon) and to attempt to apply the experimental data in the design of the SOBP (3 cm width) with a 135 MeV/nucleon carbon beam. Experime ntal studies of the physical and biological measurements using sequent ially combined irradiation were carried out to establish a close relat ionship between LET and cell inactivation. The results indicated that the dose-cell survival relationship for the combined high- and low-LET beams could be described by a linear-quadratic (LQ) model, in which n ew coefficients alpha and beta for the combined irradiation were obtai ned in terms of dose-averaged alpha and root beta for the single irrad iation with monoenergetic beams, Based on the relationship obtained, t he actual SOBP designed for giving a uniform biological effect at 3 cm depth was tested with the 135 MeV/nucleon carbon beam, The results of measurements of both physical (LET) and biological (90% level of cell killing, etc.) properties clearly demonstrated that the SOBP successf ully and satisfactorily retained its high dose localization and unifor m depth distribution of the biological effect, Based on the applicatio n of these results, more useful refinement and development can be expe cted for the heavy-ion radiotherapy currently under way at the Nationa l Institute of Radiological Sciences, Japan. (C) 1997 by Radiation Res earch Society