Application of Current Radiation Delivery Systems and Radiobiology

Photon Radiation

A photon is a particle that has no mass, travels at the speed of light, and carries the energy present in all electromagnetic radiation, including microwaves, visible light, ultraviolet light, and x-rays. Photons are the most common form of ionizing radiation used in radiotherapy. The energy of the photon influences the nature of the interaction between the photon and matter. In the kiloelectron volt (keV) energy range, which is used in most diagnostic x-ray units, photons interact with matter predominantly via the photoelectric effect . A photon interacts with a tightly bound electron, which absorbs most of the energy and is ejected from the atom; it is then free to interact with other atoms in the vicinity. This interaction is highly dependent on the atomic number of the material being irradiated. Diagnostic x-ray studies take advantage of this phenomenon. Bone, for example, is high in calcium (with an atomic number of 20) and is much more likely to interact with these photons than soft tissue, which is mostly carbon, hydrogen, and oxygen (with atomic numbers of 6, 1, and 8, respectively). This differential interaction is the basis of diagnostic x-ray imaging.

At higher energies, such as the energies of most therapeutic radiation, the Compton effect predominates. A high-energy photon in the megaelectron volt (MeV) range interacts with a loosely held orbital electron, which results in ejection of the electron from the atom and scattering of the photon. The scattered photon now has a change in its energy. Interestingly, this interaction is independent of the atomic number of the material being irradiated but is dependent on electron density. Thus images generated from therapeutic energy x-rays (ie, in the megaelectron volt range) are less useful for imaging than are diagnostic x-rays, as there is little contrast between bone and soft tissue; however, this energy range is useful in radiation therapy because it is highly penetrating and can interact equally with all tissues.

There are two main sources of high-energy photons used in radiation therapy. The first source is radioactive decay, a natural process that occurs in elements with unstable nuclei and results in the emission of energy as the nucleus cascades into a stable configuration. Cobalt-60 ( ⁶⁰ Co), the most common element used in radiation therapy units, undergoes a process known as beta decay to become nickel-60, which results in the emission of a high-energy photon known as a gamma ray (γ-ray). For ⁶⁰ Co, the average energy of the photons generated from radioactive decay is 1.25 MeV. Radiotherapy devices that take advantage of radioactive decay use a shielded radioactive source, and a small aperture or apertures result in shaping (known as collimation ) of the beam to the desired size and shape. This is the source of radiation used in devices such as the Gamma Knife (GK).

The other main source of high-energy photons is a linear accelerator, or LINAC, which uses microwaves to accelerate electrons to a high energy. These electrons are directed to collide with a high-atomic-number target , like tungsten; the resulting interactions between the target's atoms and the accelerated electrons result in the production of high-energy photons (x-rays) via a process known as bremsstrahlung , or “braking” radiation. As the electrons decelerate via their interaction with the atoms of the tungsten target, photons are produced via the bremsstrahlung phenomenon such that the law of conservation of energy is satisfied. The difference between x-rays and γ-rays is simply the site of origin: x-rays are produced by electron interactions but γ-rays are produced by nuclear decay. Unlike the γ-rays produced by ⁶⁰ Co, the energy of x-rays generated in a LINAC is based on the characteristics of the machine, and the settings of the machine can be modified by the operator; most commercial units offer multiple energy options. These x-rays are produced in the arm and head of the unit (known as the gantry ) and can be shaped by a system of controllable leaves, known as a multileaf collimator (MLC), placed between the source of the x-rays and the patients ( Fig. 50.1 ).

The physical characteristics of photons determine their biologic effects. Photons deposit dose in a characteristic pattern, in which there is an initial buildup of deposited energy after entry into the patient, followed by an exponential decrease ( Fig. 50.2 ). Photons are indirectly ionizing radiation: They liberate directly ionizing radiation, electrons, which then move on and are responsible for actual deposit of energy in some vicinity of the original interaction site. The dose buildup region is due to the initial increase in the number of liberated electrons. This dose increase in counteracted by the dwindling number of primary photons in the beam, producing the dose decrease further into the tissue. The shape of this depth-dose curve is dependent on the energy of the photon involved. Higher energy photons exhibit greater skin-sparing effect, but they have a slower rate of attenuation as they pass through tissue and have a slightly larger penumbra due to higher-energy secondary electrons. In general, photons in the 1- to 6-MeV range are used for cerebral stereotactic radiosurgery (SRS). SRS is the delivery of a high dose of ionizing radiation to a target in a single or a few (usually <5) treatments.

Proton Radiation

Charged particles have been used in radiotherapy over many decades, even though much less widespread than photons. Of the many ion species employed so far (protons, helium, and carbon, to name a few), protons are most widely adopted, with the number of centers offering proton therapy steadily increasing. A proton is the nucleus of a hydrogen atom. Protons are positively charged, subatomic particles of approximately one atomic mass unit. For radiotherapy purposes they are produced in an ion source by stripping electrons from molecular hydrogen gas, before being accelerated to therapeutic energies in a synchrotron or cyclotron. Charged particles are directly ionizing, meaning they deposit energy through collision with atomic electrons. As a particle travels through the medium it loses its energy in a myriad of these collisions and finally comes to a full stop. Because more energy is lost the slower the particle goes, a useful feature arises—a peak right at the end of the particle's travel (“Bragg peak”). The main advantages of protons over photons are the absence of dose beyond the target and the decrease in dose proximally (see Fig. 50.2 ). To cover lesions which extend in depth, multiple Bragg peaks, originating from protons with different initial energies, are superimposed to create a spread-out Bragg peak (SOBP). In opposition to the favorable dose characteristics of protons versus photons, there are two disadvantages: physical uncertainty and cost. Given the sharp dose fall-off, it is critical to be able to calculate and deliver proton dose precisely, which remains a continuing field of development with the goal of minimizing treatment margins. Second, in order to produce proton beams of therapeutic energies, relatively large accelerators are needed. To date, most facilities are the size of a football field, with multiple treatment rooms sharing one large accelerator. More compact technology is under development and has become available.