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INTRODUCTION

To effectively incorporate radiation treatment into cancer care, clinicians must understand the fundamental concepts and vocabulary used in radiation oncology. Clinically, radiation therapy, often combined with chemotherapy, can be used as primary treatment for many gynecologic malignancies (Table 28-1). Additionally, radiation therapy may be recommended postoperatively if the probability of tumor recurrence is high. Radiation therapy is also used frequently in the relief of symptoms caused by metastasis of any gynecologic cancer.

TABLE 28-1Role of Radiation Therapy in the Management of Gynecologic Cancers

RADIATION PHYSICS

Radiation therapy is the focused delivery of energy in tissue to accomplish controlled biologic damage. Radiation used in this therapy can be delivered as electromagnetic waves or particles.

Electromagnetic Radiation

Photons (x-rays) and gamma rays are the two types of electromagnetic radiation used for radiation therapy. Photons, used in external beam therapy, are produced when a stream of electrons collides with a high-atomic-number target (tungsten) located in the head of a linear accelerator (Fig. 28-1). In contrast, gamma rays originate from unstable atom nuclei and are emitted during decay of radioactive materials, also termed radionuclides, which are widely used in brachytherapy.

FIGURE 28-1

Block diagram of a linear accelerator used to create external beam radiation. Either photon beams or electron beams may be produced. A. Photon beam therapy is suited for deep tumors such as the cervical cancer shown here. Beam energy is measured in millions of volts (MV). B. Electron beam therapy is indicated for superficial lesions such as inguinal lymph nodes. Beam energy is measured in millions of electron volts (MeV).

Particle Radiation

Whereas electromagnetic waves are defined by their wavelengths, particles are defined by their masses. For clinical use, particles include electrons, neutrons, protons, helium ions, heavy charged ions, and pi mesons. Except for electrons, which are available in all modern radiation oncology centers, and protons, other particles have limited clinical use. Proton facility numbers are expanding, with 30 facilities operating in the United States and 14 additional centers planned or under construction.

Particles are produced by linear accelerators or other high-energy generators and are usually delivered by external beam. Of clinically used particles, electrons are negatively charged and deposit most of their energy near the surface. In contrast, heavy charged particles, such as protons, deposit most of their energy in the absorbing tissues as their velocity decreases, that is, near the end of the ...

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