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