Investigation of buildup dose from electron contamination of clinical photon beams

Abstract

The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements. A Monte Carlo technique was employed. The calculation was divided into two parts. First, the accelerator treatment head was simulated in detail using the EGS-PEGS electromagnetic shower code. Then, information obtained from these calculations was used to compute dose curves in a polystyrene phantom. Two cases were considered, one in which both electrons and photons were incident upon the phantom, and another in which electrons were eliminated from the incident beam. Results of these calculations agree with recent experimental findings. A decrease in buildup dose as well as a shift in dmax was observed when electrons were eliminated from the beam.

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

“…The effect has been measured and calculated earlier [1,3,9,13]. The tray perturbation factor variation with tray-surface distance indicates that the introduction of polycarbonate tray acts both as a contributor and an absorber of scattered electrons.…”

Assessment of skin dose for breast chest wall radiotherapy as a function of bolus material

“…Dose at the surface is primarily due to electron contamination from the flattening filter, beam modifiers and air. The magnitude of the surface dose depends on the field size, angle of beam incidence, air gap and the use of beam modifiers (Biggs and Ling 1979, Gerbi et al 1987, Lamb and Blake 1998, Lopez Medina et al 2005, Petti et al 1983a, 1983b, Yang et al 2004, Zhu and Palta 1998.…”

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

“…with summation of series of MLC shaped fields. [32][33][34][35][36][37] There are multiple factors that influence the Sc values: in particular, photons are scattered by structures in the accelerator head (primary collimator, flattening filter, the secondary collimator), tertiary collimators (MLCs and wedges), photons and electrons backscatter into the monitor chamber, and at very small field sizes, a portion of the X-ray source is obscured by the collimators. In recent times, linear accelerator manufacturers have made provisions to deliver radiation therapy treatments with the flattening filter removed from a traditional medical accelerator.…”

“…39 The type of phantom and depth of measurement of Sc values are topics of interest, as has been reported by several authors. 4,7,9,14,29,32 The AAPM therapy physics committee Task Group 74 (TG-74) 13 recommends the build-up caps in cylindrical shapes along with long axis parallel with beam central axis and the ion chamber placed at 10 g/cm 2 water equivalent depth for head scatter factor measurements. The present study emphasizes the need for Sc measurements at 10 cm water equivalent depth with mini phantom for 6MV-FB photon beams.…”