Effect of electron contamination on scatter correction factors for photon beam dosimetry

Physical quantities for use in megavoltage photon beam dose calculations which are defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimator and phantom scatter correction factors, S(c) and S(p), defined at a reference depth of 10 cm. The data can be obtained from measurements at that depth in a miniphantom and in a full scatter phantom. Equations are presented that show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown that conversion of S(c) and S(p) determined at a 10 cm depth to quantities defined at the dose maximum such as (normalized) peak scatter factor, (normalized) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in S(c) at d(max) resulting from this electron contamination compared with S(c) values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, S(cel), for a number of photon beams of different accelerator types. It is shown that S(cel) may vary up to 5%. Because in the new formalisms output factors are defined at a reference depth of 10 cm, they do not require S(cel) data. The use of S(c) and S(p) values, defined at a 10 cm depth, combined with relative depth-dose data or tissue-phantom ratios is therefore recommended. For a transition period the use of the equations provided in this article and S(cel) data might be required, for instance, if treatment planning systems apply S(c) data normalized at d(max.