in 1957. Gordon's right eye was removed January 11, 1957, because his cancer had spread. His left eye, however, had only a localized tumor that prompted
Henry Kaplan to try to treat it with the electron beam. Conventionally, the energy of diagnostic and therapeutic
gamma- and
X-rays is on the order of
kiloelectronvolts (keV) or
megaelectronvolts (MeV), and the energy of therapeutic electrons is on the order of megaelectronvolts. The beam is made up of a spectrum of energies: the
maximum energy is approximately equal to the beam's maximum
electric potential within a
linear accelerator times the
electron charge. For instance, a 1 megavolt beam will produce photons with a maximum energy around 1 MeV. In practice, the mean X-ray energy is about one-third of the maximum energy. Beam quality and hardness may be improved by
X-ray filters, which improves the homogeneity of the X-ray spectrum. Medically useful X-rays are produced when electrons are accelerated to energies at which either the
photoelectric effect predominates (for diagnostic use, since the photoelectric effect offers comparatively excellent contrast with effective atomic number
Z) or
Compton scattering and
pair production predominate (at energies above approximately 200 keV for the former and 1 MeV for the latter), for therapeutic X-ray beams. Some examples of X-ray energies used in medicine are: •
Very low-energy superficial X-rays – 35 to 60 keV (mammography, which prioritizes soft-tissue contrast, uses very low-energy kV X-rays) • Superficial radiotherapy X-rays – 60 to 150 keV •
Diagnostic X-rays – 20 to 150 keV (mammography to CT); this is the range of photon energies at which the photoelectric effect, which gives maximal soft-tissue contrast, predominates. •
Orthovoltage X-rays – 200 to 500 keV •
Supervoltage X-rays – 500 to 1000 keV •
Megavoltage X-rays – 1 to 25 MeV (in practice, nominal energies above 15 MeV are unusual in clinical practice). Megavoltage X-rays are by far most common in radiotherapy for the treatment of a wide range of cancers. Superficial and orthovoltage X-rays have application for the treatment of cancers at or close to the skin surface. Typically, higher-energy megavoltage X-rays are chosen when it is desirable to maximize "skin-sparing" (since the relative dose to the skin is lower for such high-energy beams). Medically useful photon beams can also be derived from a radioactive source such as
iridium-192,
caesium-137, or
cobalt-60. (
Radium-226 has also been used as such a source in the past, though has been replaced in this capacity by less harmful radioisotopes.) Such photon beams, derived from
radioactive decay, are approximately
monochromatic, in contrast to the continuous
bremsstrahlung spectrum from a linac. These decays include the emission of
gamma rays, whose energy is
isotope-specific and ranges between 300 keV and 1.5 MeV.
Superficial radiation therapy machines produce low energy x-rays in the same energy range as diagnostic x-ray machines, 20–150 keV, to treat skin conditions.
Orthovoltage X-ray machines produce higher energy x-rays in the range 200–500 keV. Radiation from orthovoltage x-ray machines has been called "deep" due to its greater penetrating ability, allowing it to treat tumors at depths unreachable by lower-energy "superficial" radiation. Orthovoltage units have essentially the same design as
diagnostic X-ray machines and are generally limited to photon energies less than 600 keV. X-rays with energies on the order of 1 MeV are generated in
Linear accelerators ("linacs"). The first use of a linac for medical radiotherapy was in 1953. Commercially available medical linacs produce X-rays and electrons with an energy range from 4 MeV up to around 25 MeV. The X-rays themselves are produced by the rapid deceleration of electrons in a target material, typically a
tungsten alloy, which produces an X-ray spectrum via bremsstrahlung radiation. The shape and intensity of the beam produced by a linac may be modified or collimated by a variety of means. Thus, conventional, conformal, intensity-modulated,
tomographic, and stereotactic radiotherapy are all provided using specially modified linear accelerators.
Cobalt units use radiation from cobalt-60, which emits two gamma rays at energies of 1.17 and 1.33 MeV, a dichromatic beam with an average energy of 1.25 MeV. The role of the cobalt unit has largely been replaced by the linear accelerator, which can generate higher energy radiation. Nonetheless, cobalt treatment still retains some applications, such as the
Gamma Knife, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator.
Sources and properties of X-rays Bremsstrahlung X-rays are produced by bombarding energetic
cathode rays (
electrons) onto a target made of a material with high atomic number, such as
tungsten. The target acts as a sort of
transducer, converting part of the electrons' kinetic energy into energetic
photons. Kilovoltage X-rays are typically produced using an
X-ray tube, in which electrons travel through a
vacuum from a hot
cathode to a cold
anode, which also acts as the target. However, it is impractical to produce megavoltage X-rays using this method; instead, a
linear accelerator is most commonly used to produce X-rays of such energy. X-ray emission is more forward-directed at megavoltage energies and more laterally directed at kilovoltage energies. Consequently, kilovoltage X-rays tend to be produced using a reflection-type target, in which the radiation is emitted back from the target's surface, while megavoltage X-rays tend to be produced with a transmission target in which the X-rays are emitted on the side opposite that of electron incidence. Reflection type targets exhibit the
heel effect and can use a rotating anode to aid in heat dissipation.
Compton scattering is the dominant interaction between a megavoltage beam and the patient, while the
photoelectric effect dominates at keV energies. Additionally, Compton scattering is much less dependent on
atomic number than the photoelectric effect; while kilovoltage beams enhance the distinction between muscle and bone in
medical imaging, megavoltage beams suppress that distinction to the advantage of teletherapy.
Pair production and
photoneutron production increase at higher energies, only becoming significant at energies on the order of 1 MeV. X-ray energy in the keV range is described by the electrical voltage used to produce it. For instance, a 100 kVp beam is produced by a 100 kV voltage applied to an X-ray tube and will have a maximum photon energy of 100 keV. However, the beam's spectrum can be affected by other factors as well, such as the voltage
waveform and external
X-ray filtration. These factors are reflected in the beam's
half-value layer (HVL), measured in-air under conditions of "good geometry". A typical superficial X-ray energy might be 100 kVp per 3 mmAl – "100 kilovolts applied to the X-ray tube with a measured half-value layer of 3 millimeters of
aluminum". The half-value layer for orthovoltage beams is more typically measured using copper; a typical orthovoltage energy is 250 kVp per 2 mmCu. For X-rays in the MeV range, an actual voltage of the same magnitude is not used in production of the beam. A 6 MV beam contains photons of no more than 1 MeV, rather than 6 MeV; the energy of such a beam is instead generally characterized by measuring the ratio of the beam's intensity at varying depths in a medium. Kilovoltage beams do not exhibit a build-up effect and thus deposit their maximum dose at the surface, i.e. d = 0 or D = 100%. Conversely, megavoltage beams do exhibit the buildup effect deposit; they deposit their maximum dose at some depth below the surface, i.e. d > 0. The depth of dose maximum is governed by the range of the electrons liberated upstream during Compton scattering. At depths beyond d, the dose profile of all X-ray beams decreases roughly exponentially with depth. Though actual values of d are influenced by various factors, the following are representative benchmark values. ==Electrons==