Technical note: Bremsstrahlung dose in the electron beam at extended distances in total skin electron therapy

Electron beam from a linear accelerator is commonly used in total skin electron therapy (TSET) at extended distances. Since Das et al (Med Phys 21, p.1733, 1994) reported 5% bremsstrahlung dose for a 6 MeV electron beam at extended distance of 500 cm it has been accepted as common knowledge. However, measurements by Chen et al (Int J. Rad Onc Biol Phys 59 p.872, 2004) and Monte Carlo simulations by Ding et al (Phys. Med. Biol. 66, 075010, 2021) were unable to reproduce such high bremsstrahlung dose. As bremsstrahlung dose contributes to whole-body dose, which could produce bone marrow toxicity with serious complications for the outcome of the TSET, it is important to re-evaluate the magnitude of bremsstrahlung dose accurately.

The EGSnrc Monte Carlo system is used to investigate bremsstrahlung doses from 6 MeV high dose rate total skin electron (HDTSe) beams from Varian TrueBeam and Clinac Accelerators. The measurements were carried out at a depth of d_max and 5 cm in Solid Water and Acrylic phantoms at extended distances using a parallel-plate chamber and a cylindrical ion chamber.

We were able to reproduce previously reported high bremsstrahlung dose at extended distances by using a parallel plate ionization chamber. However, both the measurements made by using a cylindrical chamber and Monte Carlo simulations showed an insignificant bremsstrahlung dose (∼1%) even at SSD = 500 cm.

The bremsstrahlung doses of a 6 MeV electron beam are 0.5% to 1% for SSD from 100 to 700 cm, although it increases with the increasing extended distance. The common belief of up to 5% bremsstrahlung dose at large extended distances is incorrect. Previously reported high bremsstrahlung doses might be due to poor signal-to-noise ratio of using a parallel plate chamber for measuring very low dose or particular setup.