EXTERNAL BEAM RADIATION THERAPY
Radiation Oncology is the medical discipline that uses ionizing radiation with therapeutic intent in the management of cancer and selected benign disease. Compared to surgery, radiation is a modern medical treatment. Wilhelm Conrad Roentgen first described ionizing radiation as “a new kind of rays” or x-rays in 1895.1 The new modality was used for medical imaging within a month of its discovery and was applied to malignant disease within a few years of its discovery.
Radiation oncology encompasses a wide variety of treatment modalities and delivery methods including external beam radiation and brachytherapy, conventional fractionation and ablative therapies such as stereotactic radiosurgery (SRS), and stereotactic body radiation therapy (SBRT). Multiple types of radiation are used therapeutically including photons, electrons, protons, other particle therapy, and radioisotopes. The common mechanism of action is the deposition of energy in living tissue and subsequent damage to DNA. This chapter focuses on the more common modalities and treatments. The biology and physics of radiation therapy are well described elsewhere.2,3
Radiation oncology is an integral component of multidisciplinary cancer care. The goal of radiation therapy is to deliver the prescribed dose of radiation to a defined tumor target volume with minimal harm of the normal surrounding tissues. With advances in physics, treatment planning software, and treatment delivery systems, we have approached that goal in the last few decades.
The most common treatment delivery method is external beam radiotherapy (EBRT). EBRT is delivered with a medical linear accelerator or Cobalt machine. Modern linear accelerators treat with photons, high energy uncharged particles, and electrons, charged particles with limited depth of penetration.
Prior to the 1990s the predominant method of treatment planning and delivery was based on two-dimensional (2D) images and plans. The treatment volume was imaged with plain x-rays or fluoroscopy and a 2D treatment plan through a central plane was created (Fig. 11-1). For some sites (whole brain, pelvis, larynx), treatment fields were defined clinically by external landmarks and a port film was obtained to confirm the radiation field. This method of treatment planning and delivery is still widely used in low- and medium-resource countries. Advantages are simplicity and cost. Disadvantages are increased variation in treatment delivery (less accurate) and increased volume of normal tissue irradiated, with potential increase in long-term side effects, diminished quality of life, and future medical costs. The foundation of modern radiation oncology practice was established with 2D treatment planning and effective treatments were developed for multiple tumor sites including head and neck, cervix, lymphoma, and breast cancer.4-7
A. Lateral fluoroscopic image for breastcancer radiation planning. B. Isodose plan through central axis of breast.