Dosimetric effect by shallow air cavities in high energy electron beams

Abstract

a b s t r a c tThis study evaluates the dosimetric impact caused by an air cavity located at 2 mm depth from the top surface in a PMMA phantom irradiated by electron beams produced by a Siemens Primus linear accelerator. A systematic evaluation of the effect related to the cavity area and thickness as well as to the electron beam energy was performed by using Monte Carlo simulations (EGSnrc code), Pencil Beam algorithm and Gafchromic EBT2 films. A home-PMMA phantom with the same geometry as the simulated one was specifically constructed for the measurements. Our results indicate that the presence of the cavity causes an increase (up to 70%) of the dose maximum value as well as a shift forward of the position of the depthedose curve, compared to the homogeneous one. Pronounced dose discontinuities in the regions close to the lateral cavity edges are observed. The shape and magnitude of these discontinuities change with the dimension of the cavity. It is also found that the cavity effect is more pronounced (6%) for the 12 MeV electron beam and the presence of cavities with large thickness and small area introduces more significant variations (up to 70%) on the depthedose curves.Overall, the Gafchromic EBT2 film measurements were found in agreement within 3% with Monte Carlo calculations and predict well the fine details of the dosimetric change near the cavity interface. The Pencil Beam calculations underestimate the dose up to 40% compared to Monte Carlo simulations; in particular for the largest cavity thickness (2.8 cm).

Investigating the Effect of Air Cavities of Sinuses on the Radiotherapy Dose Distribution Using Monte Carlo Method

“…In fact, the Monte Carlo simulation is one of the most J Biomed Phys Eng 2019; 9 (1) www.jbpe.org Seif F. et al accurate calculation methods for radiotherapy dose [2,3]. In the studies carried out in this field, the occurrence of disorders due to the air inhomogeneities close to the intersection of air-tissue, and consequently, overdose and underdose in the surrounding areas have been reported [4][5][6][7][8][9][10]. For instance, when treating a tumor close to an air cavity in the head and neck region, underdose could occur in the intersection of air-tissue [11,12].…”