The Physics of Radiation Therapy Quiz

Answer: Indirectly Ionizing Radiation

X-rays are indirectly ionizing radiation, as they firstly interact with matter by transferring its energy to electrons. The electrons are then free to roam and ionize tissue, causing damage to cells.

Answer: They are man made.

Gamma rays and X-rays are both ionizing radiation consisting of highly energetic photons. The distinction between them is that gamma rays are produced as a result of nuclear decay of a radioactive element, and X-rays are produced by accelerating a beam of electrons at a metal target using high voltages. Both have no charge, do the same damage to tissue (assuming they have the same energy of course), and pass through water quite easily.

Answer: Gamma Decay

Although beta decay also occurs within the cobalt-60 source, the beta particles do not penetrate more than a few milimetres in the head of the machine. Its the gamma decay that occurs after the initial beta decay, where the usable gamma ray is released with enough energy to ionize the tissue in its way.

Answer: Compton Scattering

Compton Scattering is the main process by which photons transfer their energy to a tissue in the energy around 500kV to up to 10MV. Photoelectric Absorption occurs at kilovoltage energies like those found in diagnostic X-ray facilities, CT scanners and some superficial skin treatment machines.

Answer: Electron and positron.

This interaction is actually a testimony to a famous Einstein equation. The science world already knew that energy could not be destroyed or created, only transferred between different forms. He however was the first to prove that energy could be converted to mass, and mass into energy, by the formula E=m*c^2.

A photon has zero mass and zero charge. When a photon of high energy interacts in a pair production process, it disappears. Therefore the energy must have been converted to something, right? It turns out, that the energy changes into two particles, both with opposite charge and opposite mass! But how can we have opposite mass? Well the positron is something called "anti-matter", and is known in the science world as the exact opposite of the electron.

When these two particles come back together and interact with each other, they disappear! Guess what they turn into? Yep, energy.

Answer: They were produced by the slowing down of electrons as they bend around the strong charge of the target nuclei.

Bremsstrahlung is a German word meaning "to slow down" or "to brake". As a beam of electrons are aimed at a heavy metal target such as tungsten, they are repelled and deflected by the strong positive charge in the nuclei (lots of protons), and as a result some of their energy is released in the form of photons. Those photons are termed bremsstrahlung X-rays and is the basis upon which 90% of modern radiation treatment is built upon.

Answer: Radiation damages all cells, but healthy cells can repair themselves

Radiation is non-discriminatory in that it damages healthy cells as well as cancerous ones. Healthy cells can repair themselves though, while cancer cells cannot, meaning that the malignant cells die off while the healthy ones regenerate. This is why radiotherapy is usually spread over several sessions to allow healing to take place.

Shrinking tumors with radiation is actually common practice only when a tumor is operable. It however is not the main use of curative radiation.

Answer: Brachytherapy

Brachytherapy is derived from the Greek, meaning short distances. Teletherapy obviously derived from the same Greek root as words such as telecommunications and telescope, meaning from a distance.

Answer: Isocenter

The isocenter is the point at which the teletherapy machine should rotate, and is located 100cm from the source of the radiation at all angles.

Answer: False

Contrary to all the mis-information by the media, the radiation therapy and nuclear science industry is safer than most industries perceived as "safe". You are more likely to receive a dangerous dose of radiation on a 4 hour flight than a lifetime as a radiation worker.