Scattered photons as the cause for the observed d_max shift with field size in high‐energy photon beams

Abstract

Measurements on the Sagittaire linear accelerator and Allis-Chalmers betatron at M. D. Anderson Hospital indicate that the observed dmax shift with field size is due to the presence of Compton-scattered photons in the therapy beam, and not electrons as proposed by others. Separating the primary from other radiation components indicates that the secondary fraction represents a percentage contribution to the overall beam that increases as the collimators are opened. This is consistent with what would be expected from Compton scattering and explains the effective softening of the beam as field size increases.

Measurement of head scatter factors of linear accelerators with columnar miniphantoms

“…The outer edge of the flattening filter gives rise to x-ray energy-fluence spectra that are lower in energy. 33,34,53 The change of the energy-fluence spectrum with field size will change the transmission, scatter, and buildup in copper and lead miniphantoms. These changes in scatter, transmission, and buildup with beam energy will be different in miniphantoms fabricated with different materials.…”

“…Different field sizes and source-tosurface distance will cause the value of d max to shift. [30][31][32][33][34] ͑3͒ As the photon energy of a beam increases the value of d max increases. This causes the cylindrical buildup cap to become very large in diameter.…”

Measurement and comparison of head scatter factor for 7 MV unflattened (FFF) and 6 MV flattened photon beam using indigenously designed columnar mini phantom

“…This method suffers from a number of problems, like detector response difference for electrons and photons, 8,9 absence of unique value of d max for different field sizes and source-to-surface (SSD) distance. [10][11][12] To solve the above problem, AAPM therapy physics committee Task Group 74 (TG74) 13 recommends the build-up caps in cylindrical shapes along with long axis parallel with the beam central axis and the ion chamber placed at 10 g/cm 2 water equivalent depth for Sc measurements. These build-up caps are generally called columnar mini phantoms.…”

Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom

“…Historically, Sc is measured at depth of maximum dose (d max ) with a water equivalent build-up cap and wall thickness equivalent to depth of maximum dose in water phantom. [15161718] This method suffers from a number of problems like detector response difference for electrons and photons,[192021] absence of unique value of d max for different field sizes and source-to-surface distance (SSD),[222324] and an increase in d max values with respect to photon energy. To solve the above problem, AAPM therapy physics committee Task Group 74 (TG74)[25] recommends the build-up caps in cylindrical shapes along with long axis parallel with beam central axis and the ion chamber placed at 10 gm/cm 2 water equivalent depth for Sc measurements.…”