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

Abstract

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, Sc and Sp, 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 Sc and Sp 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 Sc at dmax resulting from this electron contamination compared with Sc values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, Scel, for a number of photon beams of different accelerator types. It is shown that Scel 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 Scel data. The use of Sc and Sp 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 Scel data might be required, for instance, if treatment planning systems apply Sc data normalized at d(max).

Measurement of Head Scatter Factor for Linear Accelerators using Indigenously Designed Columnar Mini Phantom

“…This is in agreement with that of Venselaar et al [19] and who has recommended S C measurements at 10 cm depth. The measured S C at 10 cm is compared with that of AAPM, TG-74 [21] data for both Siemens and Elekta Linear accelerators ( Table 2).…”

“…Table 2. The electron contamination scatter correction factor (S cel ) has been calculated as suggested by Venselaar et al [19] and it ranges from 0.9704 to 1.014 for the field sizes of 40×40 cm 2 to 3×3 cm 2 .…”

“…The Sc is caused by (i) source obscuring, (ii) head scattering, (iii) monitor chamber back scattering. The type of phantom and depth of measurement are the topics of interest and it has been reported by several authors [1,2,6,12,13,14,17,16,19]. The ESTRO has recommended a columnar mini phantom for the measurement of Head Scatter Factor [5].…”

“…[19] on the characteristics of S C such as, the effect of contaminating electrons, collimator exchange effect, impact of introduction of beam modifying wedges etc, and measurement techniques of head scatter correction factor for high energy X-ray beams. For many years, measurements were made with a cylindrical build-up cap of wall thickness equivalent to d max , the depth of maximum dose in a water phantom.…”

Influence of focal spot on characteristics of very small diameter radiosurgical beams

“…surements for standard square fields were also carried out at various source-chamber distances of 80, 100, 150, and 200 cm. Venselaar et al 10 performed a series of measurements to determine the scatter effect due to electron contamination from the treatment head for various field sizes of megavoltage photon beams for different accelerator types. They concluded that the scatter effect for the 6 MV x-ray beams is well below 2% for field sizes up to 40ϫ 40 cm 2 at the depth of dose maximum.…”