Why KV or MV for X-rays While KeV or MeV for Gamma-Rays and Electrons?

If you are in radiation oncology, you may have come across terms like kV (kilovolt) or MV (megavolt) for X-rays and keV (kiloelectronvolt) or MeV (megaelectronvolt) for gamma-rays and electrons. Understanding why these specific units are used for different types of radiation is crucial for professionals in the field. The question is also asked frequently in exam viva.

The answer lies in historical and practical reasons. Let's find out how.

Understanding the Basics

X-rays:

They are generated by accelerating electrons by applying potential difference (in X-ray tube) or by more than just potential difference like using RF fields (in Linear accelerator) and then suddenly decelerating them upon hitting a target, usually made of tungsten.

Gamma-Rays:

Gamma-rays are also a form of electromagnetic radiation but are emitted from the nucleus of a radioactive atom.

Electrons:

Electrons used in radiation therapy (known as beta particles if emitted from the nucleus) are high-energy particles.

Why different units?

The energy of gamma-rays and electrons is usually expressed in kiloelectronvolts (keV) or megaelectronvolts (MeV). This unit is more appropriate as it directly represents the energy carried by the photons or particles. For instance, a 1 MeV gamma-ray has an energy equivalent to one million electron volts.

The energy of X-rays is often expressed in kilovolts (kV) or megavolts (MV), referring to the potential difference used to accelerate the electrons. For instance, a 120 kV X-ray tube means that the electrons are accelerated by a potential difference of 120,000 volts before striking the target.

The difference in units arises from the history and the nature of the radiation.

X-rays initially when discovered were generated by X-ray tubes only that operated based on an applied potential difference. The kilovolt (kV) or megavolt (MV) rating of an X-ray machine represents the peak voltage applied to the X-ray tube, which correlates with the maximum energy of the X-rays produced. This is why X-ray energy is typically expressed in kV or MV.

But main reason behind different units is not just history and convention but practical due to different nature of these rays.

The energy of monoenergetic beams like an electron beam, can be appropriately described by the kinetic energy of the electrons within  the beam as it's same for all the electrons of that beam. Therefore, it's appropriate to use a unit of kinetic energy, such as electron volts (eV), kiloelectronvolts (keV) and million electron volts (MeV). Same is the case of gamma-rays which is also monoenergetic.

But X-ray beam is a spectrum of photons with different energies, therefore polyenergetic and hence a unit of kinetic energy will NOT be appropriate because it carries photons with wide range of different kinetic energies. Therefore, a unit of potential difference is used i.e. voltage (V), kilovoltage (KV) and megavoltage (MV).

Notably, some radioactive sources also produce photons with more than one energies like Co-60 which produces photons with two different energies, but practically "polyenergetic" typically refers to a spectrum rather than few discrete energies and therefore it is still practical to measure it's energy in eV.

Take home message:

The use of kV and MV for X-rays and keV and MeV for gamma-rays and electrons is more than just a convention; it reflects the physical principles behind their nature. X-rays are described in terms of the voltage, while gamma-rays and electrons are described by their intrinsic energy levels. In case of x-ray tubes, the energy of x-rays produced can be simply described by the potential difference applied but in case of linear accelerator as the electrons are not accelerated just by applying potential difference, it represents the equivalent energy.

Have you heard of the interesting concept of Radiation Hormesis? Click here to read an in-depth article on Radiation Hormesis.