Percentage Depth Dose

Percentage Depth Dose

One way of characterizing the central axis dose distribution is to normalize dose at depth with respect to dose at a reference depth. The quantity percentage (or simply percent) depth dose may be defined as the quotient, expressed as a percentage, of the absorbed dose at any depth d to the absorbed dose at a fixed reference depth d₀, along the central axis of the beam (Fig. 1). Percentage

Figure 1: Percentage depth dose is (D_d/D_d0), where d is any depth and d₀ is reference depth of maximum dose.

For orthovoltage (up to about 400 kVp) and lower-energy x-rays, the reference depth is usually the surface (d₀ = 0). For higher energies, the reference depth is taken at the position of the peak absorbed dose (d₀ =d_m).

In clinical practice, the peak absorbed dose on the central axis is sometimes called the maximum dose, the dose maximum, the given dose, or simply the D_max. Thus:

A number of parameters affect the central axis depth dose distribution. These include beam quality or energy, depth, field size and shape, source to surface distance, and beam collimation.

Physics of Radiation Therapy, The, 5th Edition

Faiz M. Khan PhD

Professor Emeritus

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