A consistent formalism for the application of phantom and collimator scatter factors

Abstract

A coherent system for the use of scatter correction factors, determined at 10 cm depth, is described for dose calculations on the central axis of arbitrarily shaped photon beams. The system is suitable for application in both the fixed source-surface distance (SSD) and in the isocentric treatment set-up. This is in contrast to some other proposals where only one of these approaches forms the basis of the calculation system or where distinct quantities and data sets are needed. In order to derive the relations in the formalism, we introduced a separation of the phenomena related to the energy fluence in air and to the phantom scatter contribution to the dose. Both are used relative to quantities defined for the reference irradiation set-up. It is shown that dose calculations can be performed with only one set of basic beam data, obtained at a reference depth of 10 cm. These data consist for each photon beam quality of measured collimator and phantom scatter correction factors, in combination with a set of (percentage/relative) depth-dose or tissue-phantom ratio values measured along the central axis of the beam. Problems related to measurements performed at the depth of maximum absorbed dose, due to the electron contamination of the beam, are avoided in this way. Collimator scatter correction factors are obtained by using a mini-phantom, while phantom scatter correction factors are derived from measurements in a full scatter phantom in combination with the results of the mini-phantom measurements. For practical reasons the fixed SSD system was chosen to determine the data. Then, dose calculations in a fixed SSD treatment set-up itself are straightforward. Application in the isocentric treatment set-up needs simple conversion steps, while the inverse approach, from isocentric to fixed SSD, is described as well. Differences between the two approaches are discussed and the equations for the conversions are given.

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

“…3,4 Recently, new formalisms for the application of scatter factors in dose calculation procedures for megavoltage pho-ton beams have been presented to avoid these ambiguities. [5][6][7] The essentials of these formalisms are ͑1͒ the choice of a reference depth of 10 cm, d ref , for the definition of all physical quantities, which is beyond the range of contaminating electrons for photon beam qualities up to at least 25 MV, ͑2͒ the separation of the output factor into a collimator scatter correction factor, S c , and a phantom scatter correction factor, S p , defined at this reference depth, and ͑3͒ the measurement of the collimator scatter correction factor S c with a narrow cylindrical beam-coaxial phantom ͑miniphantom͒. 5,6 In these formalisms all data are related to the same reference depth.…”

Dependence of the tray transmission factor on collimator setting and source-surface distance

“…This is consistent with the use of other quantities in a recommended dose calculation system, such as output factors and its constituents S c and S p , and wedge factors. 2,3,5 The influence of electron contamination on depth-dose characteristics due to the presence of a tray must be investigated separately and can be taken into account by using depth-dose data measured specifically for this purpose. As pointed out by Schaeken and Van Loon, a shift of d max can be found and the loss of skin sparing can be considerable.…”

NCS Report 15: Quality assurance of 3-D treatment planning systems for external photon and electron beams

“…Develop a MU calculation program, either for manual calculation or using a computer program, based on the formalisms given in ESTRO Booklets 3 [36] and 6 [37] or NCS Report 12 [38]. See also Venselaar et al [39].…”