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«CHAPTER 2 Principles of RADIATION THERAPY radiation therapy Michael J. Gazda, MS, and Lawrence R. Coia, MD This chapter provides a brief overview of ...»

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CHAPTER 2

Principles of

RADIATION THERAPY

radiation therapy

Michael J. Gazda, MS, and Lawrence R. Coia, MD

This chapter provides a brief overview of the principles of radiation therapy.

The topics to be discussed include the physical aspects of how radiation works

(ionization, radiation interactions) and how it is delivered (treatment machines,

treatment planning, and brachytherapy). Recent relevant techniques of radiation oncology, such as conformal and stereotactic radiation, also will be presented. These topics are not covered in great technical detail, and no attempt is made to discuss the radiobiological effects of radiation therapy. It is hoped that a basic understanding of radiation treatment will benefit those practicing in other disciplines of cancer management.

How radiation works

IONIZING RADIATION

Ionizing radiation is energy sufficiently strong to remove an orbital electron from an atom. This radiation can have an electromagnetic form, such as a high-energy photon, or a particulate form, such as an electron, proton, neutron, or alpha particle.

High-energy photons By far, the most common form of radiation used in practice today is the high-energy photon. Photons that are released from the nucleus of a radioactive atom are known as gamma rays. When photons are created electronically, such as in a clinical linear accelerator, they are known as x-rays. Thus, the only difference between the two terms is the origin of the photon.

Inverse square law The intensity of an x-ray beam is governed by the inverse square law. This law states that the radiation intensity from a point source is inversely proportional to the square of the distance away from the radiation source. In other words, the dose at 2 cm will be one-fourth of the dose at 1 cm.

Electron volt Photon absorption in human tissue is determined by the energy of the radiation, as well as the atomic structure of the tissue in question. The basic unit of energy used in radiation oncology is the electron volt (eV); 103 eV = 1 keV, 106 eV = 1 MeV.

PRINCIPLES OF RADIATION THERAPY 9

PHOTON-TISSUE INTERACTIONS

Three interactions describe photon absorption in tissue: the photoelectric effect, Compton effect, and pair production.

Photoelectric effect In this process, an incoming photon undergoes a collision with a tightly bound electron. The photon transfers practically all of its

RADIATION THERAPY

energy to the electron and ceases to exist. The electron departs with most of the energy from the photon and begins to ionize surrounding molecules. This interaction depends on the energy of the incoming photon, as well

–  –  –

10 CANCER MANAGEMENT: A MULTIDISCIPLINARY APPROACH

ELECTRON BEAMS

With the advent of high-energy linear accelerators, electrons have become a viable option in treating superficial tumors up to a depth of about 5 cm. Electron depth dose characteristics are unique in that they produce a high skin dose but exhibit a falloff after only a few centimeters.

Electron absorption in human tissue is greatly influenced by the presence of air cavities and bone. The dose is increased when the electron beam passes through an air space and is reduced when the beam passes through bone.

Common uses The most common clinical uses of electron beams include the treatment of skin lesions, such as basal cell carcinomas, and boosting of (giving further radiation to) areas that have previously received photon irradiation, such as the postoperative lumpectomy or mastectomy scar in breast cancer patients, as well as select nodal areas in the head and neck.

MEASURING RADIATION ABSORPTION

The dose of radiation absorbed correlates directly with the energy of the beam.

An accurate measurement of absorbed dose is critical in radiation treatment.

The deposition of energy in tissues results in damage to DNA and diminishes or eradicates the cell’s ability to replicate indefinitely.

Gray The basic unit of radiation absorbed dose is the amount of energy (joules) absorbed per unit mass (kg). This unit, known as the gray (Gy), has replaced the unit of rad used in the past (100 rads = 1 Gy; 1 rad = 1 cGy).

Exposure In order to measure dose in a patient, one must first measure the ionization produced in air by a beam of radiation. This quantity is known as exposure. One can then correct for the presence of soft tissue in the air and calculate the absorbed dose in Gy.

Percentage depth dose The dose absorbed by tissues due to these interactions can be measured and plotted to form a percentage depth dose curve. As energy increases, the penetrative ability of the beam increases and the skin dose decreases.

How radiation is delivered

TREATMENT MACHINES

Linear accelerators High-energy radiation is delivered to tumors by means of a linear accelerator.

A beam of electrons is generated and accelerated through a waveguide that increases their energy to the keV to MeV range. These electrons strike a tungsten target and produce x-rays.





X-rays generated in the 10–30-keV range are known as grenz rays, whereas the energy range for superficial units is about 30–125 keV. Orthovoltage units generate x-rays from 125–500 keV.

PRINCIPLES OF RADIATION THERAPY 11

Orthovoltage units continue to be used today to treat superficial lesions; in fact, they were practically the only machines treating skin lesions before the recent emergence of electron therapy. The maximum dose from any of these low-energy units is found on the surface of patients; thus, skin becomes the dose-limiting structure when treating patients at these energies. The depth at which the dose is 50% of the maximum is about 7 cm. Table 1 lists the physical characteristics of several relevant x-ray energies.

Megavoltage units The megavoltage linear accelerator has been the standard radiotherapy equipment for the past 20-30 years. Its production of x-rays is identical to that of lower-energy machines. However, the energy range of megavoltage units is quite broad—from 4 to 20 MeV. The depth of the maximum dose in this energy range is 1.5-3.5 cm. The dose to the skin is about 30%-40% of the maximum dose.

Most megavoltage units today also have electron-beam capabilities, usually in the energy range of about 5-20 MeV. In order to produce an electron beam, the tungsten target is moved away from the path of the beam. The original electron beam that was aimed at the tungsten target is now the electron beam used for treatment. Unlike that of photons, the electron skin dose is quite high, about 80%-95% of the maximum dose. A rule of thumb regarding the depth of penetration of electrons is that 80% of the dose is delivered at a depth (in cm) corresponding to one-third of the electron energy (in MeV). Thus, a 12-MeV beam will deliver 80% of the dose at a depth of 4 cm.

Altering beam intensity and field size When measurements are made at the point just past the target, the beam is more intense in the center than at the edges. Optimal treatment planning is obtained with a relatively constant intensity across the width of the beam. This process is accomplished by placing a flattening filter below the target.

In order for the radiation beam to conform to a certain size, high atomic number collimators are installed in the machine. They can vary the field size from 4 × 4 cm to 40 × 40 cm at a distance of 100 cm from the target, which is the distance at which most treatments are performed.

TABLE 1: Depth dose characteristics for clinical radiotherapy beams

–  –  –

kV(p) = kilovolt (peak)

12 CANCER MANAGEMENT: A MULTIDISCIPLINARY APPROACH

If it is decided that a beam should be more intense on one side than the other, high atomic number filters, known as wedges, are placed in the beam. These filters can shift the dose distribution surrounding the tumor by 15º-60º. Wedges can also be used to optimize the dose distribution if the treatment surface is curved or irregular.

Shielding normal tissue Once the collimators have been opened to the desired field size that encompasses the tumor, the physician may decide to block out some normal tissue that remains in the treatment field. This is accomplished by placing blocks (or alloy), constructed of a combination of bismuth, tin, cadmium, and lead, in the path of the beam. In this way, normal tissues are shielded, and the dose can be delivered to the tumor at a higher level than if the normal structures were in the field. These individually constructed blocks are used in both x-ray and electron treatments. A more modern technique involves multileaf collimators mounted inside the gantry. They provide computerized, customized blocking instead of having to construct a new block for each field. (See “Intensity-modulated radiation therapy.”)

PRETREATMENT PROCEDURES

Certain imaging procedures must be done before radiation therapy is begun:

Pretreatment CT Before any treatment planning can begin, a pretreatment CT scan is often performed. This scan allows the radiation oncologist to identify both tumor and surrounding normal structures.

Simulation The patient is then sent for a simulation. The patient is placed on a diagnostic x-ray unit that geometrically simulates an actual treatment machine. With use of the CT information, the patient’s treatment position is simulated by means of fluoroscopy. A series of orthogonal films are taken, and block templates that will shield any normal structures are drawn on the films.

These films are sent to the mold room, where technicians construct the blocks to be used for treatment. CT simulation is a modern alternative to “conventional” simulation and is described later in this chapter.

Guides for treatment field placement Small skin marks, or tattoos, are placed on the patient following proper positioning in simulation. These tattoos will guide the placement of treatment fields and give the physician a permanent record of past fields should the patient need additional treatment in the future.

It is imperative that the patient be treated in a reproducible manner each day.

In order to facilitate this, Styrofoam casts that conform to the patient’s contour and place the patient in the same position for each treatment are constructed.

Lasers also help line up the patient during treatment.

TREATMENT PLANNING AND DELIVERY

Determining optimal dose distribution The medical physicist or dosimetrist uses the information from CT and simulation to plan the treatment on a computer. A complete collection of machine data, including depth dose and beam profile information, is stored in the computer. The physics staff aids the radiaPRINCIPLES OF RADIATION THERAPY 13 tion oncologist in deciding the number of beams (usually two to four) and angles of entry. The goal is to maximize the dose to the tumor while minimizing the dose to surrounding normal structures.

Several treatment plans are generated, and the radiation oncologist chooses the optimal dose distribution. The beam-modifying devices discussed earlier, such as blocks and wedges, may be used to optimize the dose distribution around the tumor.

Establishing the treatment plan The planning computer will calculate the amount of time each beam should be on during treatment. All pertinent data, such as beam-on time, beam angles, blocks, and wedges, are recorded in the patient’s treatment chart and sent to the treatment machine. The radiation therapist will use this information, as well as any casts, tattoos, and lasers, to set up and treat the patient consistently and accurately each day.

Port films As part of departmental quality assurance, weekly port films are taken for each beam. They ensure that the beams and blocks are consistently and correctly placed for each treatment. Port films are images generated by the linear accelerator at energies of 6-20 MeV. Because of the predominance of the Compton effect in this energy range, these images are not as detailed as those at diagnostic film energies (as mentioned earlier), but they still add important information on treatment accuracy and ensure the quality of setup and treatment.

BRACHYTHERAPY

Brachytherapy is the term used to describe radiation treatment in which the radiation source is in contact with the tumor. This therapy contrasts with externalbeam radiotherapy, in which the radiation source is 80-100 cm away from the patient.

In brachytherapy, dose distribution is almost totally dependent on the inverse square law because the source is usually within the tumor volume. Because of this inverse square dependence, proper placement of radiation sources is crucial.

TABLE 2: Physical characteristics of commonly used radioisotopes

–  –  –

14 CANCER MANAGEMENT: A MULTIDISCIPLINARY APPROACH

Isotopes Table 2 lists commonly used isotopes and their properties. In the past, radium was the primary isotope used in brachytherapy. Recently, because of its long half-life and high energy output, radium has been replaced with cesium (Cs), gold (Au), and iridium (Ir). These isotopes have shorter half-lives than radium and can be shielded more easily because of their lower energies.

Types of implants Brachytherapy procedures can be performed with either temporary or permanent implants. Temporary implants usually have long halflives and higher energies than permanent implants. These sources can be manufactured in several forms, such as needles, seeds, and ribbons.

All temporary sources are inserted into catheters that are placed in the tumor during surgery. A few days after surgery, the patient is brought to the radiation clinic and undergoes pretreatment simulation. Wires with nonradioactive metal seeds are threaded into these catheters. Several films are taken, and the images of the seed placement can be digitized into a brachytherapy treatment planning computer.



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