proton
Meaning of proton: A particle of basic physics with an unusual clinical afterlife, because a beam of protons can be made to stop exactly where it is wanted.
Definition of proton
A proton is a subatomic particle carrying a single positive electrical charge, found in the nucleus of every atom. The number of protons in a nucleus defines which chemical element that atom is. In medicine, beams of accelerated protons are used to deliver radiation treatment, because they release most of their energy at a set depth and then stop.
What does a proton do in an atom?
The nucleus of an atom is built from protons and uncharged neutrons, with one exception: the common form of hydrogen is a lone proton with no neutron at all. Proton count is identity. An atom with six protons is carbon and nothing else; change the number and the element changes, which is what happens during radioactive decay. Change only the neutron count and the result is an isotope of the same element, chemically alike but different in mass and sometimes unstable. Negatively charged electrons, far lighter, balance the protons in a neutral atom and are what actually take part in chemical bonding.
Why is a proton beam used for treatment?
| Photon beam, ordinary radiotherapy | Proton beam | |
|---|---|---|
| Dose on entry | Highest near the surface | Modest |
| Dose at depth | Falls off gradually | Rises to a sharp peak at a depth set by the beam energy |
| Dose beyond the target | Continues through and out the far side | Essentially none |
That sharp peak, and the abrupt stop after it, is the whole argument for proton beam radiation therapy. It matters most where a structure that must be spared sits directly behind the target, and in children, where the total volume of developing tissue receiving any dose has lifelong consequences. The costs are real too: the equipment is large and expensive, the depth of the peak depends on exactly what the beam passes through so positioning must be precise, and for many tumor sites the advantage over a well-planned photon beam radiation therapy has not been established.
Where else do protons matter in the body?
A hydrogen atom stripped of its electron is simply a proton, and the concentration of these in a fluid is what pH measures, so every discussion of acid and base is a discussion of protons. Cells build their energy supply by pumping protons across a membrane inside mitochondria and letting them flow back through a turbine-like enzyme. Stomach lining cells pump protons to make gastric acid, which is why the drugs that suppress it are called proton pump inhibitors. Magnetic resonance imaging works by tipping the protons of hydrogen in water and fat with a magnetic field and listening as they realign.
Used in a sentence
Proton beam therapy was considered as one option for delivering radiation to the tumor.
Good to know
Proton beam therapy differs from conventional radiation therapy in how the dose is distributed with depth, a physical property that is central to why one approach is chosen over the other for a given treatment area.
Word origin
Where “proton” comes from
From Greek protos, meaning first, chosen because the proton was the first particle to be identified within the atomic nucleus.