  Photons are exponentially attenuated in matter producing high doses close to
the surface. Therefore they are not well suited for the treatment of deep
seated tumours. Charged particles, in contrast, exhibit a sharp increase of
ionisation density close to the end of their range, the so-called Bragg-peak.
The depth of the Bragg-peak can be adjusted by varying the particle's energy.
In parallel with the large energy deposit the increase in biological
effectiveness for cell killing at the end of the range provides an ideal
scalpel for the surgeon effectively without touching the surface tissue.
Consequently proton therapy has gained a lot of ground for treating well
localized tumours. Even superior still are heavy ions, where the ionisation
pattern is increased by the square of their charge.
