••• Chapter 49^ Radiation Therapy, Chemotherapy, Immunotherapy, and Tumor Markers^487
❍ Which phase in the cell cycle has been shown to be resistant to irradiation?
The S phase.
❍ What constitutes a rad. and a Gray (Gy)?
The rad. is a unit that is defined as the absorption of 0.01 joule per kilogram of the medium (1 rad. = 0.01 J/kg).
One Gray (Gy) equals 100 rad.
❍ Define “maximum dose.”
The point of maximum dose for high-energy X-rays and g-rays is several millimeters below the skin. The dose to
the point for any given field is referred to as the maximum dose.
❍ The minimal tumor dose is the lowest delivered to the tumor/target volume. The maximum tumor dose
should be what percentage above the minimal tumor dose?
The maximum tumor dose should be no more than 10% to 15% over the minimum dose.
❍ What is the integral dose?
It is the total dose delivered over the entire volume or to the body of the patient. It is defined in terms of rad-gram
or megarad-gram.
❍ What is an isodose curve?
An isodose curve represents points of equal distance and is used to provide a visual representation of the dose
distribution within a single plane. A series of curves are drawn at 10% increments normalized to the dose at the
reference depth. The shape of the isodose curve will be influenced by the type of radiation, the size of its source, its
field, the filters employed, and integral dose.
❍ What is a dose profile?
It is the representation of the dose in an irradiated volume as a function of spatial positions along a single line.
❍ What are three distinct types of radiation?
(1) An alpha (a) or helium nucleus that has a positive charge.
(2) A beta (b) particle or electron that has a negative charge.
(3) Gamma (g) rays that originate within the nucleus of the atom and have no charge.
❍ How does “intracavitary” differ from “surface-dose” and “interstitial” brachytherapy?
“Intracavitary” brachytherapy consists of placing applicators with radioactive compounds within a body cavity so
as to gain proximity to the target tissue. In contrast, “interstitial” brachytherapy consists of surgically implanting
radioactive sources directly into the target tissue. “Surface-dose” brachytherapy consists of an applicator or mold
containing radioactive sources designed to deliver a constant dose to a skin or mucosal surface.
❍ The vaginal cylinder, tandem, and colpostat applicators are used in what type of brachytherapy?
They are used for intracavitary brachytherapy.