Radiation oncology is the discipline of medicine involving the use of ionizing radiation to treat malignant neoplasias. The radiation oncologist aims to deliver a precise dose of ionizing radiation to a defined tumor volume while minimizing damage to the surrounding normal structures. Due to the increasingly multidisciplinary nature of oncology, an understanding of radiation therapy is crucial to the surgeon involved in the combined-modality treatment of the patient with head and neck cancer.
External Beam Radiation Therapy (Teletherapy)
Therapeutic ionizing radiation can be divided into two categories: high-frequency electromagnetic radiation (X-rays and γ-rays) and particulate radiation (electrons, neutrons, protons). The amount of radiation absorbed per unit mass of tissue is known as the absorbed dose. The most commonly used unit for absorbed dose is the gray (Gy), which is equivalent to one joule of energy absorbed per kilogram of tissue. One Gy is also equal to 100 cGy, or 100 rads (the previously used unit of absorbed dose).
In the head and neck, primary radiotherapy is most frequently delivered via a linear accelerator with 6-megavolt (MV) photons. Anatomic location and desired depth of penetration are the main criteria used in choosing which type and energy of external beam to employ. Less commonly employed forms of external beam radiotherapy (EBRT) include 6–20 MeV electron beams, 60Co γ-rays, and superficial (40–100 kV) or orthovoltage (250 kV) X-rays.
The past several decades have seen great advances in external beam treatment delivery schemes. Traditionally, radiotherapy was often delivered with a single field using superficially penetrating X-rays. The development of machines capable of deeper delivery of radiation (ie, linear accelerators) allowed centrally located tumors to be treated with parallel opposed radiation portals. During the early 1990s, with advances in computer and imaging technology, 3D conformal radiotherapy was introduced. This allowed for noncoplanar beam arrangements that conform to the target in three dimensions. The more recent development of intensity-modulated radiotherapy (IMRT) permitted the intensity of each beam to fluctuate in complex ways across the field. This further improved the ability of the radiation oncologist to cover irregularly shaped tumor volumes, facilitating a higher degree of dose conformality and minimization of damage to surrounding normal tissues, most importantly the parotid glands and spinal cord. Recently, image-guided radiotherapy (IGRT) has emerged as a way to ensure accurate daily tumor localization during the delivery of IMRT treatment plans.
Brachytherapy is a form of radiotherapy in which a radioactive source is placed inside or adjacent to the area requiring treatment. Selected radioisotopes contained within specialized instruments deliver radiation to the tumor or tumor bed at a short distance. The treatment may involve permanent implantation of the radiation source, or a temporary placement after which the source is withdrawn. Brachytherapy treatment can be delivered via interstitial implants (eg, base of tongue, neck, or tumor bed), intracavitary applicators (eg, recurrent nasopharyngeal cancer), or molds (eg, skin, ...