Room: Track 1
Purpose: The skin dose varies with the angle of x-ray incidence and that angle changes with gantry orientation and surface curvature during fluoroscopic-interventional procedures. The skin dose point spread function (SDPSF) was calculated as a function of incident angle and convolved over the beam area for curved surfaces to determine the skin dose distribution. This dose map was compared with the results of Monte Carlo calculations.
Methods: EGSnrc Monte-Carlo (MC) software was used to calculate the SDPSF as a function of incident angle to the surface using a 1 mm² beam averaged over 1 mm depth on the entrance surface of a 40x40 cm by 20 cm thick water phantom. This distribution was normalized to the incident primary dose, which was calculated using NIST mass-energy absorption coefficients and by integrating over the beam-energy spectrum. The skin dose was determined by convolving the SDPSF with a 10x10 cm entrance beam field over flat and cylindrical surfaces and compared with the dose calculated by MC for the full field with the same exposure geometry. The SDPSF were determined for incident angles from 90 to 10 degrees and for beam energies from 60 to 120 kVp. All MC simulations used 2x10¹° photons incident on the phantom.
Results: The SDPSF magnitude decreases with decreasing angle of incidence primarily due to increasing x-ray attenuation with increasing path length through the skin for the same thickness. The SDPSF convolution over a 20 cm diameter cylinder surface agreed with the full-field MC results, with an average dose difference between MC calculation and SDPSF convolution of 2.4 % over the curved surface.
Conclusion: Accurate skin dose mapping over a curved surface can be achieved through convolution of an angular-dependent dose point spread function. This method can be used to quickly estimate the dose distribution for fluoroscopically-guided procedures.
Funding Support, Disclosures, and Conflict of Interest: Research support from Canon Medical Systems