Effect of Air Space on Depth Dose in Electron Beam Therapy

Abstract

In electron beam therapy, alterations in dosimetry occur as a result of air space between the end-of-treatment cone and the skin surface. A large air gap may be introduced in order to obtain a field size larger than that available at the cone end. Needed dosimetry corrections related to these air space problems are discussed, along with a proposed method of measuring effective source-to-cone end distance. Data presented show the modifications of a dosimetric field which occur with an increase in the air space below the treatment cone.

Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

“…] When electrons scattered from the collimator are sometimes added to the beam for field uniformity, extended treatment distances can affect field flatness and penumbra width. As discussed by Hogstrom et al (1981), and Khan et al (1978), the penumbra width (80%-20%) at shallow depths increases with distance from the collimator, particularly at the lower electron energies (< 10 MeV). This effect is illustrated in Fig.…”

Validation of Varian TrueBeam electron phase–spaces for Monte Carlo simulation of MLC‐shaped fields

“…The Markus chamber was aligned along the central beam axis using the light field crosshairs and patient positioning lasers. Dose uncertainty resulting from up to a 2 mm error in vertical positioning of the solid water surface was estimated using the technique outlined by Khan et al 32 to be less than 0.65%, although monthly tracking of the optical distance indicator's precision suggests it to be better than 2 mm, even at 70 cm SSD. Dose error associated with lateral positioning of the detector relative to the beam axis was less than 0.3% while the conversion from charge to dose carried an uncertainty of 1.1%.…”

An enhanced sector integration model for output and dose distribution calculation of irregular concave shaped electron beams

“…To account for the effect of the SSD, all measurements were repeated for SSD= 110 cm, in addition to standard SSD= 100 cm. In cases when any other SSDs than 100 cm or 110 cm are required, effective source to surface distance 26 ͑ESD͒ is calculated from the measured DD͑z͒ using the following formula:…”