Implication of electron backscattering for electron dosimetry

The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift deltaz = -0.058 cm toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of p(wall) with depth is minimized.

Section: Iiic Contributions Of the Different Chamber Wallsmentioning

confidence: 99%

On the wall perturbation correction for a parallel‐plate NACP‐02 chamber in clinical electron beams

Zink¹,

Wulff²

2011

“…In electron dosimetry the material directly behind the detector can have a significant effect on the measured dose. Klevenhagen 26 derived results for a number of low-Z materials based on the work of Hunt et al 27 while Verhaegen 28 used Monte Carlo techniques to investigate the effect. It was found that even for materials with an atomic number close to that of water there was a significant deficiency in the backscatter dose for materials such as graphite, PMMA, and polystyrene.…”

Section: Electron Backscattermentioning

confidence: 99%

Characterization of the phantom material Virtual Water™ in high‐energy photon and electron beams

McEwen

Niven

2006

The material Virtual Water has been characterized in photon and electron beams. Range-scaling factors and fluence correction factors were obtained, the latter with an uncertainty of around 0.2%. This level of uncertainty means that it may be possible to perform dosimetry in a solid phantom with an accuracy approaching that of measurements in water. Two formulations of Virtual Water were investigated with nominally the same elemental composition but differing densities. For photon beams neither formulation showed exact water equivalence-the water/Virtual Water dose ratio varied with the depth of measurement with a difference of over 1% at 10 cm depth. However, by using a density (range) scaling factor very good agreement (<0.2%) between water and Virtual Water at all depths was obtained. In the case of electron beams a range-scaling factor was also required to match the shapes of the depth dose curves in water and Virtual Water. However, there remained a difference in the measured fluence in the two phantoms after this scaling factor had been applied. For measurements around the peak of the depth-dose curve and the reference depth this difference showed some small energy dependence but was in the range 0.1%-0.4%. Perturbation measurements have indicated that small slabs of material upstream of a detector have a small (<0.1% effect) on the chamber reading but material behind the detector can have a larger effect. This has consequences for the design of experiments and in the comparison of measurements and Monte Carlo-derived values.

“…Consistent with the conclusions drawn recently by two committees after reviewing the available data, 25,24 the P wall term is taken to be unity, with the caveat that for plane-parallel chambers, there may be a need to include a factor varying between 1.0 and 1.02, depending on the energy and material of the back wall of the chamber. [26][27][28] For cylindrical chambers, Nahum's theoretical work suggests that if there is any P wall correction, its deviation from unity is less than 1%. 29 The P cel factor for cylindrical chambers with 1 mm aluminum electrodes is 1.0 for R 50 р5.6 cm or 0.998 for higher energies as given in Ref.…”

Section: Appendix: Calculation Of K Q and K R 50mentioning

confidence: 99%

A new approach to electron‐beam reference dosimetry

Rogers

1998

A new approach is proposed for electron-beam dosimetry under reference conditions and data necessary to use this approach are presented. The approach has the following features; it uses ion chambers and starts from an absorbed-dose calibration factor for 60Co to be consistent with the present proposal for the new AAPM photon-beam protocol; it uses R50 to specify the beam quality and the reference depth, dref = 0.6R50 - 0.1 (all quantities in cm), recommended by Burns et al. [Med. Phys. 23, 383-388 (1996)]; it has a formalism which is parallel to the kQ formalism for photon-beam dosimetry; it fully accounts for the impact on stopping-power ratios of realistic electron beams; it allows an easy transition to using primary standards for absorbed dose to water in electron beams when these are available. The equation for dose to water under reference conditions is; DWQ = MPionPgrQk'R50kecalND,w60Co. The term PgrQ is not needed with plane-parallel chambers but corrects for gradient effects with cylindrical chambers and is measured in the user's beam. The parameter kecal is associated with converting the 60Co absorbed-dose calibration factor into one for an electron beam of quality Qe and contains most of the chamber to chamber variation. Calculated values of kecal are presented as well as Monte Carlo calculated Pwall values for plane-parallel chambers in a water phantom irradiated by a 60Co beam since these are needed to calculate kecal. The factor k'R50 is a function of R50 and converts the absorbed-dose calibration factor to that for the electron-beam quality of interest. Two analytical expressions are presented which are close to universal expressions for all cylindrical Farmer-like chambers and for well-guarded plane-parallel chambers respectively. Calculated values are presented graphically for electron beams with energies between 5 and 50 MeV.