Determination of the primary dose component and primary linear attenuation coefficient in a 6 MV photon beam using a small attenuator

It is a common technique in radiotherapy treatment planning systems to simplify the calculations by splitting the radiation beam into two components: namely the primary and scattered components. The contributions of the two components are evaluated separately and then summed to give the dose at the point of interest.Usually the primary dose is obtained experimentally by extrapolating the ionization measured within the medium to zero-field size (Godden, 1983). This approach offers the opportunity to obtain the primary component of dose without the need for a non-linear extrapolation. It is based on a paper by Nizin and Kase from 1988.The primary dose can be obtained from four measurements of ionization in narrow beam geometry and two measurements of ionization in a large beam in a phantom. If these measurements are performed over a range of different depths, the primary linear attenuation coefficient can also be obtained.The value for the primary dose at dmax in a 10 cm × 10 cm field obtained in a 6 MV beam using this method is Dp(dmax, 10 cm × 10 cm) = 0.925 Gy/100 MU for a 1 cm thick lead attenuator and is Dp(dmax, 10 cm × 10 cm) = 0.941 Gy/100 MU for a 2 cm thick lead attenuator. The primary linear attenuation coefficient is μ0 = 0.0445 ± 0.0007 cm−1.The obtained values of the primary dose component compare well with the extrapolation of the phantom scatter correction factor to zero-field size from measurements done in the same beam and also to literature (Rice and Chin, 1990). One can thus conclude that this method has the potential to provide an independent measurable verification of calculations of primary dose.