Investigating the effect on backscatter factor when measuring entrance surface dose using the raysafe X2 solid state radiography and fluoroscopy detector

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Abstract
Objectives: Assessment of patient dose is a critical quality assurance check towards ensuring it is kept as low as reasonably practicable. One metric used to evaluate this is entrance surface dose (ESD)—the summed magnitudes of the incident radiation and Compton scatter resulting from photon interactions within tissue. ESD can be directly measured using a dosemeter that is sensitive to backscattered radiation. However, the increasing use of solid-state detectors such as the tin backed RaySafe X2 R/F detector, means that ESD cannot be directly measured. The IAEA Technical Report No.457, recommends using backscatter factors (BSFs) as calculated by Petoussi-Henss et al in 1998 via Monte Carlo methods when measuring ESD using such dosemeters. This research seeks to practically determine a reliable method of calculating the BSF using a polymethylmethacrylate (PMMA) patient-equivalent phantom. Methods: A comparison between the solid-state detector and an ionization chamber capable of detecting backscatter was conducted. Special attention was given to BSF variation with both radiation field size and beam quality by varying additional copper and aluminium filtration to mimic the behaviour of patient-dose reducing techniques on modern fluoroscopy systems. Results: Results have shown significant correspondence with those of Petoussi-Henss et al using the simulated water and International Commission on Radiation Units and Measurements (ICRU) tissue equivalent phantoms. Conclusions: Given the results consistency, BSFs from this study have been proposed for use when measuring ESD using the RaySafe solid state detector with a PMMA patient-equivalent phantom for quality assurance purposes. Advances in knowledge: This research provides the opportunity to obtain more accurate patient dose data during quality assurance testing with modern solid-state dosemeters.

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