Over the last 20 years, proton therapy has shown its many qualities and has become a really favorable treatment option for patients with certain types of cancer, showing considerable advantages over conventional x-ray therapy.
X-ray therapy and proton therapy are both radiotherapy techniques but the former uses photons to irradiate diseased tissues while the latter uses beams of energetic protons, which have very different physical properties.
Before they reach the tumor, both radiation types have to make their way through the patient’s skin and surrounding tissues. The photon, with no mass and no charge, is highly penetrating and delivers a dose throughout any volume of tissue irradiated. However, most of the radiation is delivered only 0,5 to 3 cm from the patient’s skin, depending on the energy it was initially given. It then gradually loses this energy until it reaches the target. As tumors are almost always in-depth located, the photon actively interacts with outer healthy cells and drops only a small remaining dose of ionizing radiation on the deeper diseased cells. Moreover, as photons are not all stopped by human tissue, they leave the patient’s body and continue to emit radiation (exit dose).
On the contrary, the proton is a heavy and charged particle that gradually loses its speed as it interacts with human tissue. It is easily controlled and delivers its maximum dose at a precise depth, which is determined by the amount of energy it was given by the cyclotron (via acceleration), and can go as far as 32 cm. The proton is very fast when it enters the patient’s body and deposits only a small dose on its way. The absorbed dose increases very gradually with greater depth and lower speed, suddenly rising to a peak when the proton is ultimately stopped. This is known as the Bragg peak. The behavior of the proton can be precisely determined and the beam can be directed so the Bragg peak occurs exactly within the tumor site. Immediately after this burst of energy, the proton completely stops to irradiate. Proton therapy therefore allows to target tumors inside the body, precisely localize the radiation dosage and spare the patient’s healthy cells, offering a much less invasive alternative to treat cancer.
- James M. METZ, “Proton Therapy”, Demos Medical, Radiation Medicine Rounds, vol. I, Issue 3, New York, 2010, p. 415.