Megavoltage Equipment These are high energy machines with energies greater than 1,000 kilovolts. Megavoltage machines let the radiation oncologist treat internal tumors without giving an excessive dosage to the skin. Commonly used megavoltage machines include Cobalt 60 and Linear Accelerators that produce the maximum dose at an internal depth determined by the energy of the machine. A 25-megavolt Betatron, for example, delivers its maximum dose almost 2 in. (4.5 cm) beneath the skin.
• Cobalt 60 became available in the 1950s and is still used. This machine contains a radioactive source housed in a
specially designed lead container. As the cobalt isotope decays over time, it produces a beam in the megavoltage
range.
• The Linear Accelerator, one of the most popular machines now being used, is available in energies ranging from
4 to 35 megavolts. Many of these machines can treat patients with x-rays or electrons or a combination of both.
The combination is called a mixed beam and allows the radiation oncologist to tailor the treatment to a specific
tumor and reduce the dose to surrounding tissues.
The machine works by producing electrons that are accelerated along a wave guide to a high velocity. These
electrons can be directly used for treatment in the electron mode or they can be directed at a target within the
machine to produce the x-rays that are then used for treatment. The high energy electrons used in the electron
mode have a specific range of penetration into tissue, determined by the energy of the beam selected. The higher
the megavoltage, the greater the penetration. Because an electron therapy beam produces a high skin dose, the
whole course of treatment will seldom be given with only the electron beam unless you are being treated for
