The effect of underlying bone on the beam angular correction in calculating the skin dose of the head in neuro-interventional imaging.

Skin dose depends on the surface shape, underlying tissue, beam energy, field size, and incident beam angle. These dependencies were determined in order to apply corrections in the skin-dose-tracking system (DTS) for accurate estimation of the risk of deterministic skin effects during fluoroscopically-guided neuro-interventional procedures. The primary-plus-scatter dose was calculated averaged over the skin thickness with underlying subcutaneous fat, and various thicknesses of skull bone on the surface of a cylindrical water phantom to simulate the head. The skin dose was calculated using EGSnrc Monte-Carlo (MC) software with 2×1010 incident photons and was normalized to the incident primary dose. Simulations were done for beam incident angles from 90 to 10 degrees with the skin surface, field sizes from 5 to 15 cm, bone thicknesses of 0, 1, 5, and 9 mm, 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 to the skin and with increasing thickness of underlying bone, while it increases with increasing field size and with increasing beam energy. The correction factor reduces the skin dose for angled rays and the reduction can be substantial for small angles of incidence, especially for angles below 50 degrees. For neuro-interventional procedures, the skin dose-area product (SDAP) with angular and bone correction is shown to be less than that without correction. The results of this study can be used to increase the accuracy of patient-skin-dose estimation for the head during fluoroscopic procedures.