Purpose: During fluoroscopic-interventional procedures, the x-ray beam is not always at normal incidence to the skin surface. Because of its sensitivity for deterministic effects, an accurate estimate of skin dose is important and we thus investigated the effect on skin dose of the angle of x-ray beam incidence.
Methods: EGSnrc Monte-Carlo (MC) software was used to calculate the primary and scattered radiation dose averaged over various depths into the entrance surface of a water phantom as a function of incident x-ray beam angle. The incident primary dose was calculated from the dose in air at the field center and the primary dose averaged over the depth was calculated using the attenuation coefficients published by NIST and by integrating over the beam-energy spectrum. The total, the primary and the scatter dose components were determined for incident angles from 90 to 10 degrees, for â€œskinâ€? thicknesses from 0.5 to 5 mm, and for beam energies from 60 to 120 kVp. The phantom had an entrance surface of 40x40 cm and was 20 cm thick. All MC simulations used 5x10Â¹â?° photons incident on the phantom.
Results: The total primary-plus-scatter dose averaged over skin thickness decreases considerably with decreasing angle of incidence for angles below 50 degrees due principally to the increased path length and thus attenuation of the primary at oblique angles for a given skin thickness. The amount of decrease is less with increasing beam energy due to reduced attenuation. The total dose is rather invariant with skin thickness as the primary is lower for greater thicknesses while the scatter component increases with thickness.
Conclusion: The depth-averaged dose to the patientâ€™s skin changes substantially with beam incident angle and corrections to the absorbed skin dose should be applied for those parts of the skin over which the beam is not incident normally.
Funding Support, Disclosures, and Conflict of Interest: This research is supported by Canon Medical Systems.