General shape of depth dose curve

From MedPhysWiki

The general shape of the central axis depth dose curve for electron beams differs from that of photon beams (see Fig. 8.1). Figure 8.1(a) shows depth doses for various electron beam energies and Fig. 8.1(b) shows depth doses for 6 and 15 MV X ray beams.

Typically, the electron beam central axis depth dose curve exhibits a high surface dose (compared with megavoltage photon beams), and the dose then builds up to a maximum at a certain depth referred to as the electron beam depth of dose maximum z_max. Beyond z_max the dose drops off rapidly and levels off at a small low level dose component referred to as the bremsstrahlung tail. These features offer a distinct clinical advantage over the conventional X ray modalities in the treatment of superficial tumours.

A typical high energy linac may produce electron beams with discrete energies in the range from 4 to 25 MeV.

Electron beams can be considered almost monoenergetic as they leave the accelerator; however, as the electron beam passes through the accelerator exit window, scattering foils, monitor chambers, collimators and air, the electrons interact with these structures, resulting in:

• A broadening of the beam’s electron energy spectrum;
• Bremsstrahlung production contributing to the bremsstrahlung tail in the electron beam percentage depth dose (PDD) distribution.

On initial contact with the patient, the clinical electron beam has an incident mean energy Failed to parse (Missing texvc executable; please see math/README to configure.): \bar E_0

that is lower than the electron energy inside the accelerator.

The ratio of the dose at a given point on the central axis of an electron beam to the maximum dose on the central axis multiplied by 100 is the PDD, which is normally measured for the nominal treatment distance (i.e. the distance between the accelerator exit window and the patient’s skin) and depends on field size and electron beam energy.