Chapter 93

As a discipline, radiation oncology primarily deals with the use of ionizing radiation in the treatment of malignant diseases. Occasionally therapeutic doses of radiation may be used to treat benign processes, such as heterotopic bone formation, meningiomas, and other benign proliferative diseases. Radiation has a very prominent role in the management of breast cancer after lumpectomy, as adjuvant therapy after mastectomy, in the management of locoregional relapse of disease, and in the palliation of metastatic disease. Management of breast cancers can typically cover up to 20% to 25% of a general radiation oncology practice. The next few chapters will cover in more detail the role of radiation therapy after breast conserving therapy, the rapidly evolving role of partial breast irradiation, postmastectomy radiation, and the role of radiation in palliation and metastatic disease.

A basic review of the underlying principles of the physics, radiation biology, and radiation planning is essential to understanding the role of radiation therapy in the management of breast cancer. It is beyond the scope of this text to comprehensively review the underlying principles of radiation physics and biology, and the reader is referred to more comprehensive textbooks for more detailed information.1-5 However, in this chapter we will highlight some of the basic principles of physics and radiation biology and outline some of the basics of radiation planning to give the reader an appreciation of the underlying principles, rationale, and process of radiation in the management of breast cancer.

Radiation therapy uses a form of electromagnetic radiation, commonly referred to as ionizing radiation, generated from x-rays, gamma rays, electrons, and other forms of particles. Ionizing radiation consists of highly energetic particles that can eject at least 1 electron from an atom. Ionizing ability depends on the energy of individual particles or waves. Although x-rays or gamma rays are the principle form of ionizing radiation used in conventional radiation therapy, other forms of ionizing radiation include protons, beta particles, neutrons, alpha particles, and heavy ions. The ability of photons to ionize an atom or molecule varies across the electromagnetic spectrum. Although x-rays and gamma rays have high enough energy to ionize almost any molecule or atom, near ultraviolet and visible light are ionizing to very few molecules, and microwaves and radiowaves are nonionizing radiation within the electromagnetic spectrum.3,4

Ionizing radiation ejects electrons from atoms. These fast electrons continue on to produce additional ionizations, amplifying the effect of the initial photon interactions. One of the more critical and essential interactions of x-rays is the creation of ion pairs from interactions with the abundant water in cells and tissues that lead to the formation of free radicals. This process represents the primary mechanism through which damage is created in irradiated cells. Certain substances, such as antioxidants or high doses of certain vitamins, theoretically can absorb some of these free radicals, thereby counteracting the effects of radiation.6,7

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