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

Physical quantities for use in megavoltage photon beam dose calculations wh ich are defined at the depth of maximum absorbed dose are sensitive to elec tron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimat or and phantom scatter correction factors, S-c and S-p, defined at a refere nce depth of 10 cm. The data can be obtained from measurements at that dept h in a miniphantom and in a full scatter phantom. Equations are presented t hat show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown t hat conversion of S-c and S-p determined at a 10 cm depth to quantities def ined at the dose maximum such as (normalized) peak scatter factor, (normali zed) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in S-c at d(max) re sulting from this electron contamination compared with S-c values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplicatio n factor, S-cel, for a number of photon beams of different accelerator type s. 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 requi re S-cel data. The use of S-c and S-p values, defined at a 10 cm depth, com bined with relative depth-dose data or tissue-phantom ratios is therefore r ecommended. For a transition period the use of the equations provided in th is article and S-cel data might be required, for instance, if treatment pla nning systems apply S-c data normalized at d(max). (C) 1999 American Associ ation of Physicists in Medicine. [S0094-2405(99)01310-3].