  The problem of variable cell survival probability along the spread-out Bragg
peak is one of the long standing problems in planning and optimisation of
ion-beam therapy. This problem is considered using the multiscale approach to
the physics of ion-beam therapy. The physical reasons for this problem are
analysed and understood on a quantitative level. A recipe of solution to this
problem is suggested using this approach. This recipe can be used in the design
of a novel treatment planning and optimisation based on fundamental science.
