Comparison of the patient’s skin dose for flat and curved surfaces as a function of x-ray beam angle of incidence

It is important to determine the patient’s skin dose accurately for fluoroscopic interventional procedures in order to estimate the risk of deterministic injury. The purpose of this study is to investigate how the patient’s skin dose changes as a function of x-ray beam incident angle for flat and curved surfaces. The primary and scatter dose was calculated averaged over a 2.0 mm depth at the surfaces of both cubic and cylindrical water phantoms to simulate different patient curvature. The total skin dose was calculated using EGSnrc Monte-Carlo (MC) software with 10¹⁰ photons incident and the primary dose was calculated at the central axis using the mass energy absorption coefficients published by NIST and integrated over the beam-energy spectrum. Simulations were done for incident angles from 90 to 10 degrees, beam field sizes from 5 to 15 cm, cylinder diameters from 20 to 30 cm, and beam energies from 60 to 120 kVp. The results show the scatter-plus-primary to incident-primary dose ratio decreases with decreasing incident angle due to increased primary attenuation and decreases from cubic to cylindrical phantom and with decreasing cylinder diameter at all angles due to reduced backscatter. These results can be used to determine angular correction factors needed to accurately estimate patient skin dose when the beam is not normal to the entrance surface during fluoroscopic procedures.