Radiotherapy plays an integral role in the care of many gynecologic cancers and can be used for definitive management, adjuvant therapy, or palliation. The principal basis of therapeutic radiation lies in its ability to cause ionization, or the creation of free electrons and free radicals, when absorbed by biologic matter. These highly reactive chemical species interact with critical molecules in a cell (in particular deoxyribonucleic acid [DNA]) and, if unrepaired, lead to loss of cellular reproductive capacity and eventual cell death. Ionizing radiation can be emitted from radioactive isotopes, both naturally occurring and man-made, or created using specialized high-voltage but nonradioactive equipment such as linear accelerators.
Optimal radiation for gynecologic malignancies often combines both teletherapy (external beam radiotherapy) and brachytherapy (internal radiation) with careful clinical judgment required to determine the proper weighting of each component. The challenge in radiation delivery is to deliver intended full dose(s) to selected target(s), while minimizing exposure to adjacent normal tissues. Sophisticated developments in imaging, computer-based treatment planning, and linear accelerator technology provide for ever-greater sophistication and accuracy in radiotherapy. However, such precision in dose delivery has to be accompanied by improvements in patient set-up immobilization, reproducibility, and regular tumor tracking to prevent marginal misses of the intended target volume. Advances in radiotherapy for gynecologic malignancies will be based on further integration with systemic agents (for both spatial cooperation and chemosensitization), as well as developments in targeting, tracking, and adaptive processes, featuring radiation plans that may be modified during a course of therapy to conform to changes in patient and tumor geometry.
FUNDAMENTALS OF RADIATION PHYSICS
All matter is composed of individual units called elements. Each element is defined by the physical and chemical properties of its basic component—the atom. The atom consists of a central core, the nucleus, made up of positively charged particles, called protons, and neutrons, which have no charge. The nucleus is surrounded by a "cloud" of negatively charged particles, or electrons, which move in orbits around the nucleus. In the basic "resting" state of an atom, the number of protons in the nucleus is equal to the number of orbiting electrons, making the atom electrically neutral.
The formula AZX is used to identify each atom. X is the chemical symbol for the element, A is the mass number or number of nucleons (the number of neutrons and protons in the nucleus), and Z is the atomic number (the number of protons in the nucleus). The number of protons (Z) in an atom determines its chemical properties and its elemental name. Within the periodic table of elements, as Z increases, the number of accompanying neutrons increases proportionately more (ie, A:Z ratio > 2) to maintain nuclear stability. Atoms with the same Z, but with different numbers of neutrons, share the same element name and chemical properties but are called ...