Protons accelerated to 60% of the speed of light (180,000 km/s, 250 MeV kinetic energy) by cyclotrons and synchrotrons penetrate approximately 38 cm into the body. Initially, they transfer relatively small amounts of energy to the traversed molecular electron clouds (meaning there is a low degree of ionization). However, they undergo braking in the process (see Figure 1). The slower the particles become, the greater is the linear energy transfer and the braking effect. This leads to an "energy explosion" at the end of the particle's path, i.e. a characteristic tissue depth of 1-4 mm for monoenergetic particles, which is known as the Bragg peak. In contrast to photons, there is a lower dosage, not a higher dosage, in front of the tumour when proton radiation is used. The tissue behind the tumour remains unexposed. This physical phenomenon makes it possible to focus the in-depth dosage "three-dimensionally" onto the tumour and to do so with absolute precision by determining the depth of the Bragg peak through modulation of the particle velocity, thus beneficially reversing the ratio of useful radiation to damaging radiation. The Bragg peak, however, is so sharply defined that it must be spread in the tumour by varying the particle speed.  

Figure 2 shows the resulting dosage distribution for a large tumour. Also, in such cases the reduction of upstream dosage is maintained while no radiation is deposited downstream of the tumour. Figure 4 presents a direct comparison of the local dosage distribution for photon beams versus photon beams based on Figure 3. At the end of their penetration depth at the Bragg peak, protons, in penetrating the tissue, release similar amounts of energy to molecules as photons do, at least with respect to the hydrogen present in cellular water. The effect is the same in both cases: the loss of electrons from molecules. The tissue subsequently "forgets" the cause of the electron loss and the resulting ionization, whether protons or photons. The ionization, which is identical for both radiation types but is more effectively targeted in the case of protons, acts as a cellular toxin, as illustrated in Figure 5. Thanks to the identical biological effects of the two radiation types, it is possible to draw on the entire body of empirical clinical knowledge regarding x-rays and thus apply the clinical experience in x-ray dosing to the use of protons. The use of proton beams instead of x-rays allows the therapeutic tumour dosage, which is limited by side effects, to be increased while simultaneously reducing the dosage deposited in healthy tissue.

In clinical practice, this proton beam control, which is no longer two-dimensional (through lateral bundling) but is three dimensional, has reduced the harmful dosage deposited in healthy tissue by approximately 43% to 78%, depending on the tumour geometry. Figures 6, 7, 8, 9 and 10 present comparisons of dosage distributions in the same patients. The left-hand image in each figure shows the conventional photon radiation actually received by the patients. The middle image shows the dosage distribution using advanced intensity-modulated radiotherapy (IMRT), while the right-hand image shows the exposure which would have been possible with proton therapy. The fine yellow line within each image indicates the boundary of the target area, meaning the tumour, while the colours correspond to the local dosage delivered.