Shizuoka Cancer Center
What is proton
Proton-beam therapy is a form of radiotherapy used to treat cancer (generic term for malignant tumors, sarcomas, etc.). You may think about X-rays and gamma rays when it comes to radioactive rays, but it is a proton beam that replaces them to be used for this therapy. Detailed information about what a proton beam is and why it is good for the cancer treatment is given below.
What is proton beam?
A proton is an atomic nucleus of hydrogen, and a particle positively charged. It is produced with hydrogen gas as the source reagent, by separating electrons from the nucleus in the high-temperature plasma state. When massive protons are accelerated in vacuum all at once, a proton beam, of which the penetrative power into a human body is strong enough to destroy cancerous cells, is produced. The special equipment used for this process is called an accelerator. The speed of protons in the accelerator comes close to 70% of the speed of light.
What are the features of proton-beam therapy?
In the above explanation, it says that “a proton beam has the strong penetrative power into a human body.” However, it is not the one and only feature of proton beam. It has “charged particle radiation,” which suits cancer treatment because of the reasons explained as follows.
When the power of proton beam reaches to maintain a certain level of energy, the protons penetrate a human body at the uniform speed and stop completely at a designated depth. Therefore, a proton beam does not go any deeper to damage normal cells. The depth is controllable and can be adjusted. Furthermore, most of the energy distribution of protons focuses just before they stop. In other words, the amount of radiation irradiated in the form of proton beam is used quite effectively at a designated depth. Please see the figure below for details.
As you see in the figure, the majority of X-ray and gamma ray radiations is absorbed near the body surface, and the amount of radiation energy becomes rather small as they go deep inside the body. On the other hand, the radiation energy of proton beam peaks deep inside the body (namely where the tumor is). Proton-beam therapy optimally exploits this feature.
Depth dose curve
Why is proton beam used for the therapy?
Merits of Proton- beam Therapy
Merits for patients