EP-1579: Room scatter effects in Total Skin Electron Therapy: a Monte Carlo study

Abstract

Telephone: (972) 4 7772015; Fax: (972) 4 7773154. AbstractPurpose: Total Skin Electron Irradiation (TSEI) is a complex technique which usually involves the use of large electron fields and the dual-field approach. In this situation, many electrons scattered from the treatment room floor are produced. However, no investigations of the effect of scattered electrons in TSEI treatments have been reported. The purpose of this work was to study the contribution of floor scattered electrons to skin dose during TSEI treatment using Monte Carlo (MC) simulations.Methods: All MC simulations were performed with the EGSnrc code. Influence of beam energy, dual-field angle, and floor material on the contribution of floor scatter was investigated. Spectrum of the scattered electrons was calculated. Measurements of dose profile were performed in order to verify MC calculations.Results: Floor scatter dependency on the floor material was observed (at 20 cm from the floor, scatter contribution was about 21%, 18%, 15%, and 12% for iron, concrete, PVC, and water, respectively). Although total dose profiles exhibited slight variation as functions of beam energy and dual-field angle, no dependence of the floor scatter contribution on the beam energy or dual-field angle was found. The spectrum of the scattered electrons was almost uniform between a few hundred KeV to 4 MeV, and then decreased linearly to 6 MeV.Conclusions: For the TSEI technique, dose contribution due to the electrons scattered from the room floor may be clinically significant and should be taken into account during design and commissioning phases. MC calculations can be used for this task.

Improving treatment geometries in total skin electron therapy: Experimental investigation of linac angles and floor scatter dose contributions using Cherenkov imaging

“…The Monte Carlo simulation work of Nevelsky et al reported peak floor scatter contributions at 20 cm above the floor to be +3%, −3%, and −5% for iron, PVC, and water, compared to concrete, given a SSD = 400 cm and gantry angle 17° above the horizontal. These magnitudes are larger than those measured with the Cherenkov imaging method (+2.4%, 0.0%, −1.6%), even with a 13° increase in gantry angle (30° vs 17° above the horizontal, more directly impinging the floor) at the shorter SSD (300 cm vs 400 cm).…”

“…Nevelsky et al stated “the spectrum of the scattered electrons had a distribution which was almost uniform between a few hundred keV to 4 MeV, then decreased linearly to 6 MeV.” While most of these electrons should be above the threshold for Cherenkov light generation, the energy dependence of the number of photons emitted would deflate the contributions of lower energy‐scattered electrons. For this reason, the Cherenkov imaging method provides a novel qualitative presentation of scatter dose distribution that follow anticipated and simulated trends, however, the accuracy of the quantitative results are limited to within a few percent, depending on the material (and subsequently the energy spectrum of the scattered electrons).…”

“…Taking into consideration the already broad ranges of dose reported with in vivo dosimetry (±15%), it is not expected that the menial scatter dose magnitudes are the primary cause of heterogeneities in dose received by the patient. Rather, these dose variations could be an indication of a possible weakness in the treatment field setup, in conjunction with patient positioning, which limits the achievement of appropriate dose coverage.…”

“…In addition, interpretation of the dose distributions during TSET requires some consideration of dose from floor scatter . The second hypothesis states that the remote Cherenkov imaging method will exhibit sensitivity to dose deposited in the treatment plane as a result of the scattering interactions between the ionization radiation beam and the floor, which will be tested by introducing different scattering materials to the field and measuring differences between the collected Cherenkov images.…”

Investigation of effect of filter on the stand‐up technique for total skin irradiation by Monte Carlo simulation

“…The uncertainty for the MC simulation included room scatter effects 16 from walls, floor, ceiling, and TSI platform in the treatment room since the MC model cannot account for every detailed geometry. Furthermore, the measurements done at the treatment plane can be hard to set up accurately.…”