Radiation therapy, which uses ionizing radiation to kill cancer cells and shrink tumors, is part of the treatment plan for many patients with cancer. Ionizing radiation interacts with tissues via an energy transfer and a chemical reaction, in which free radicals are released and water molecules decompose into hydrogen, hydroxyl, and perhydroxyl ionic forms. These ionic forms break atomic and molecular bonds, which in turn break the double-stranded DNA structure and cause cellular death. Consequent cell death occurs in both normal tissue and malignant lesions. In radiotherapy, biochemical recovery and biologic repair occur in the normal host cells, maintaining the integrity of vital systems.
In radiation oncology, therapeutic ratio refers to a fundamental concept in which the risks and benefits to the targeted cancer cells and the surrounding tissues must be weighed. Lymphocytes are damaged by 1 gray (Gy) of radiation and central nervous system tissue by 50 Gy. Table 13-2 lists some examples of the effects of radiation on ocular and nonocular tissues.
Radiation can be delivered through external beam radiotherapy (EBRT; most common) or internal placement (brachytherapy); radiation can also be administered systemically (eg, radioactive substance bound to a monoclonal antibody). In EBRT, high-energy x-ray beams generated either by linear accelerators, which produce photons or electrons, or by cobalt machines, which use radioactive decay of an element such as cobalt 60, are aimed at the tumor site. Planning for EBRT involves not only localizing the tumor, but also determining the proper dose of radiation: one that will kill the malignant cells while minimizing damage to the surrounding noncancerous tissue. There are many other methods of EBRT, including particle therapy and stereotactic radiosurgery.
Table 13-2 Radiation Damage to Ocular and Nonocular Tissues
In brachytherapy (also called internal radiation therapy), radioactive material is implanted within or adjacent to the tumor, delivering radiation while minimizing damage to the surrounding normal tissue. The term brachytherapy refers to various types of procedures, one example of which is seed implantation, used in the treatment of prostate cancer and some uveal melanomas. See BCSC Section 4, Ophthalmic Pathology and Intraocular Tumors, for more on brachytherapy and uveal melanomas.
For some conditions, monoclonal antibodies are available as a vector to deliver radiation directly to the target tissue; these antibodies are discussed later in this chapter in the section Biologic Therapies.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.