PROTON THERAPY CENTER - Principles of Proton Therapy


Principles of Proton Therapy

Princip léčby

Principles of proton therapy

Proton therapy is based on the use of positively charged elementary particles of hydrogen atom nuclei – namely protons, that have a weight much higher than that of electrons. Protons are accelerated in a cyclotron to a speed equal to approximately half the speed of light. This also determines their energy, which reaches up to 230 MeV (mega-electron volts) and enables them to damage tumors up to a depth of approximately 30 cm. The protons are then targeted with a strong magnetic field into a very narrow beam (a “pencil beam”) and transferred with a high degree of accuracy via a 3D image to the malignant tumor. Energy is released during deceleration in the tumor tissue with subsequent ionization and damage to the DNA of the affected cell. If the damage is sufficient, the cell stops dividing (and growing) or dies immediately. 

The main benefit of the proton beam is the fact that the absolute greatest part of its energy is transferred to the area of the so-called 'Bragg peak', i.e. directly to the tumor, where it has maximum destructive effect. The beam of accelerated particles has a high energy level and is very accurately targeted. Most of the energy is transferred solely to tumor tissue. In comparison with current irradiation procedures, it saves healthy tissue in front of the tumor and it does not damage healthy tissue behind the tumor at all. Since the patient is irradiated in an isocentric system from all directions and the intensity of beams can be well modulated (IMPT), this method provides further reduction of adverse distribution

The figure shows depth distribution of energy for each type of ionizing radiation. While the photon curve demonstrates maximal loss of energy immediately under the skin and then continuous gradual loss (so that the healthy tissue in front of the tumor is irradiated with a much larger dose than the actual tumor), the proton depth curve shows that their energetic loss is small upon entry to the body, reaches its maximum at a specific depth, (the Bragg peak) where the transfer of energy reaches it's maximum (the target area of the tumor) and then decreases to zero, so that healthy tissue behind the tumor is not destroyed.

These physical properties of the proton beam – low entry dose, maximal dose of energy at the required depth and a zero exit dose – enable extremely precise modulation of dose distribution inside the patient's body and represent the main advantage of proton radiotherapy. Due to this feature it is possible to increase the dose directed at the tumor to a level above that which could be achieved using common, conventional techniques - and at the same time reduces the dose to surrounding tissues that are sensitive to the harmful effects of radiation.