484 Obstetrics and Gynecology Board Review •••
❍ What constitutes the “clinical target volume (CTV)”?
CTV encompasses the gross tumor volume and the regions proximal to the gross tumor that are considered to
harbor potential microscopic disease.
❍ What constitutes the “planning target volume (PTV)”?
The region around the CTV that allows for variation in treatment setup and breathing motion. It does not,
however, include beam characteristics (penumbra).
❍ What are the most commonly used radiation in clinical radiation?
X-rays and g (Gamma)-rays. Electrons are also used and heavier particles have been employed in more
experimental treatments.
❍ Where do X-rays and g-rays arise from?
Their names reflect their different origins. Gamma (g)-rays arise from within the nucleus (in practice, they are
emitted from radioactive isotopes). X-rays arise from outside the nucleus, produced by bombardment of a target
with high-speed electrons.
❍ What is the Compton effect?
The Compton effect is the interaction of a photon with a loosely bound orbital electron in which part of the
incident (incoming) photon’s energy is transferred as kinetic energy to the electron and the remaining energy is
transmitted to another photon. The energy of the incoming photon determines the probability of its interaction
with a target atom’s outer electrons; as the energy increases, the probability of interaction decreases.
❍ How does ionizing radiation affect damage at the intracellular level?
Directly and indirectly. In the direct mechanism, the incoming photon displaces on electrons that directly ionize
a DNA strand causing a break in the indirect action, and the displaced electron interacts with a water molecule to
produce a hydroxyl radical (.OH), a highly reactive free radical that damages the DNA strand.
❍ What occurs following damage to the DNA following ionizing radiation?
The initial DNA damage brings about a cascade of biologic events that either interfere with mitosis or initiate
programmed cell death (apoptosis).
❍ What is the principal target of ionizing radiation?
DNA is the principal target within the nucleus of the cell.
❍ What type of DNA aberration caused by ionizing radiation is lethal to the cell?
Most biologic effects of radiation are a result of incorrect joining of breaks in two chromosomes during the
repair process. Specifically, two broken chromosomes may recombine to form dicentric (a chromosome with two
centromeres) or a centric fragment (a chromosome with no centromeres). These are lethal lesions.
❍ What is the most likely mechanism for radiation-induced carcinogenesis?
Radiation-induced carcinogenesis likely results from a translocation that moves one oncogene from a quiescent
chromosome site to an active one.