Megavoltage X-rays
Encyclopedia
Megavoltage X-rays are produced by linear accelerators ("linacs") operating at voltage
s in excess of 1000 kV
(1 MV) range, and therefore have an energy
in the MeV
range (see external beam radiotherapy
for an explanation of the maximum and mean energies as a function of voltage). Beams with the voltage range of 4-25 MV are used to treat patients because radiation oncologist
s find that they penetrate well to deep sites within the body.
For instance, in one particular technique of prostate
radiotherapy, a patient will be treated with 5 radiation beams of 15 MV X-rays. While the beams will point into the patient from different angles, all the beams will be pointed towards one point or centre in the prostate (so called an isocentric technique
). In this way the linac can rotate around the patient who does not need to move. In fact, patient motion is a source of positioning error which a radiation therapist
tries to eliminate.
By crossing multiple beams, the radiation dose delivered internally in the prostate is much higher (~70-78 Gy
) than the radiation dose delivered on the entry and exit tracks of each individual beam (~40 Gy). This lower dose outside the prostate rarely leads to side effects or any detectable change.
By convention, the voltage is used to characterize X- and gamma-ray beams (in volts), whilst electron
beams are characterized by their energies (in electronvolts).
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s in excess of 1000 kV
KV
-In physics and chemistry:* Voltage-gated potassium channel,a large family of potassium channels* Kilovolts , 1000 volts* The rating of RPMs per volt as used in reference to the rotational speed of brushless motors...
(1 MV) range, and therefore have an energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
in the MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
range (see external beam radiotherapy
External beam radiotherapy
External beam radiotherapy or teletherapy is the most common form of radiotherapy. The patient sits or lies on a couch and an external source of radiation is pointed at a particular part of the body...
for an explanation of the maximum and mean energies as a function of voltage). Beams with the voltage range of 4-25 MV are used to treat patients because radiation oncologist
Radiation oncologist
A radiation oncologist is a doctor who specializes in the treatment of cancer patients, using radiation therapy as the main modality of treatment. Radiation can be given as a curative modality, either alone or in combination with surgery and/or chemotherapy. It may also be used palliatively, to...
s find that they penetrate well to deep sites within the body.
For instance, in one particular technique of prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....
radiotherapy, a patient will be treated with 5 radiation beams of 15 MV X-rays. While the beams will point into the patient from different angles, all the beams will be pointed towards one point or centre in the prostate (so called an isocentric technique
Isocentric technique
An isocentric technique is where all beams used in a radiation treatment have a common focus point, a.k.a the isocentre.-Working:The movements of the linear particle accelerator are threefold:...
). In this way the linac can rotate around the patient who does not need to move. In fact, patient motion is a source of positioning error which a radiation therapist
Radiation therapist
The Radiation Therapist is an allied health professional who works in the field of radiation oncology...
tries to eliminate.
By crossing multiple beams, the radiation dose delivered internally in the prostate is much higher (~70-78 Gy
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....
) than the radiation dose delivered on the entry and exit tracks of each individual beam (~40 Gy). This lower dose outside the prostate rarely leads to side effects or any detectable change.
By convention, the voltage is used to characterize X- and gamma-ray beams (in volts), whilst electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
beams are characterized by their energies (in electronvolts).