Biologic Aspects of Radiation Oncology

[center]Biologic Aspects of Radiation Oncology

Radiation Deposits Energy in Tissue
and Produces Ionization Events

Radiation can be
considered as packets of energy in the form of photons (e.g., x-rays,
ultraviolet light) or particles (e.g., protons, neutrons, -particles, and
electrons). As these packets of energy penetrate into tissue, they produce
ionizations either directly or indirectly in biologically important molecules.
The subatomic collisions caused by the particle types of radiation can induce
direct biologic damage within cells, which is termed direct ionization. X-rays,
on the other hand, transfer their energy to chemical intermediates within
tissue, and it is these intermediates that produce the actual biologic damage.
This is called indirect ionization

Ionization events occur as these
packets of energy travel through tissue and deposit their energy. Radiation dose
is the term that describes the quantity of energy deposited per mass of tissue.
Radiation oncologists frequently express radiation using the International
System unit gray (Gy), which equals 1 J/kg. An older unit of dose is the rad,
which is equal to 1/100 Gy or 1 cGy

In the case of electromagnetic
radiation, high-energy x-ray photons collide with orbiting electrons of biologic
molecules, which leads to the ejection of fast electrons (Fig.below). These fast
electrons can then directly damage biologic target (e.g., DNA) in a process
called direct action. More commonly, however, the fast electrons collide with
the plentiful water molecules in tissue. This results in the production of
hydroxyl radicals (OH–), which in turn can damage biologic targets (termed
indirect action). This indirect action is thought to account for approximately
two-thirds of the biologic damage that is produced by x-rays

Radiation
can also be delivered directly in the form of electrons (or -particles), and
this produces events that are similar in principle to those of x-rays. Electron
radiation can be thought of as the secondary fast electrons that are created by
x-rays. However, these electrons are directly produced from a machine or a
radioactive isotope (e.g., phosphorus , unlike the secondary fast electrons
that are produced in tissue by x-rays
[/center]

_________________