Biomedical Engineering Reference
In-Depth Information
beams of X-rays are directed toward the patient. The beams pass
through the patient, undergoing near-exponential attenuation, and
deposit dose
3
along the way; the interactions of secondary electrons
(see Chapter 4) eventually lead to cell death.
The principal aspects of a single photon beam which can be mani-
pulated are:
the beam quality (i.e., the maximum photon energy);
the beam direction (its angle relative to a point within the patient);
the beam intensity;
4
the shape of the field;
5
the intensity profile.
6
How and why these manipulations may be made will be discussed
in later chapters. In conventional radiation therapy, as opposed to
intensity-modulated radiation therapy which will be discussed below,
the intensity profile of the beam is chosen to be as uniform as possible
throughout the body of the field, and to fall off as sharply as possible
at the field edges. As a consequence, such beams will tend to
3
The dose of radiation is characterized by the energy imparted per unit mass
of the irradiated medium. The unit of dose is the Gray (written Gy);
1 Gy = 1 J
-1
⋅
kg (see Chapter 4).
4
The term “intensity” is used widely in radiation therapy, but not always
in a consistent manner. Indeed, its meaning is context sensitive and often
ambiguous. A dictionary definition of “intensity”, as used in physics, is
“the measurable amount of a property, such as force, brightness, or a
magnetic field.” (OED, 2001). This leaves open the question of what the
property is. And, “intensity” may either refer to the
flux
of the property
(e.g., number of photons crossing unit area per unit time), or its
fluence
(e.g., number of photons crossing unit area) which is the integral of flux
over time (Webb S and Lomax A, 2001). In talking about dose, the beam
intensity may either be understood to refer to the dose rate (dose per unit
time), or the total dose. One has to rely on context (and, one hopes, the
explicit use of units) to decide which meaning is intended. In the context
of the graphic representation of images, “image intensity” usually means
the relative fluence of transmitted light through a semi-transparent medium
such as film, or of the emitted light from a video display of the image.
5
The term
field
refers to the area within the lateral margins of the radiation
within a plane normal to the beam direction and upstream of the patient.
6
The term
intensity profile
refers to the lateral distribution of dose within a
plane normal to the beam direction.
