Radiation oncology is the discipline of medicine involving the use of ionizing radiation to treat malignancy. 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
Therapeutic ionizing radiation can be divided into two categories: high-frequency electromagnetic radiation (x-rays and γ-rays) and particulate radiation (neutrons, protons, and heavy ions). 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 to 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 EBRT 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, three-dimensional 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) has permitted the intensities and shapes of each beam to be individually adjusted in complex ways across treatment fields. This further improved the ability of the radiation oncologist to treat 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 (Figure 4–1). Recently, image-guided radiotherapy (IGRT) has emerged as a way to ensure accurate daily tumor localization during the delivery of IMRT treatment plans. For head and neck cancers, radiation is typically directed to the areas of gross tumor as well as regions suspected to be at risk of microscopic tumor involvement including bilateral cervical and retropharyngeal lymph nodes in most cases. These “elective” treatment volumes are determined based on known patterns of lymphatic spread of head and neck cancers and are selected based on the extent of disease and primary tumor site.
Intensity-modulated radiotherapy plan demonstrating radiation dose distribution ...