Skin dose is dependent on the incident beam angle and corrections are needed for accurate estimation of the risk of deterministic effects of the skin. Angular-correction factors (ACF) were calculated and incorporated into our skin-dosetracking system (DTS) and the results compared to Monte-Carlo simulations for a neuro-interventional procedure. To obtain the ACF’s, EGSnrc Monte-Carlo (MC) software was used to calculate the dose averaged over 0.5, 1, 2, 3, 4 and 5 mm depth into the entrance surface of a water phantom at the center of the field as a function of incident beam to skin angle from 90-10 degrees for beam field sizes from 5-15 cm and for beam energies from 60-120 kVp. These values were normalized to the incident primary dose to obtain the ACF. The angle of incidence at each mesh vertex in the beam on the surface of the DTS patient graphic was calculated as the complement of the angle between the normal vector and the vector of the intersecting ray from the tube focal spot; skin dose at that vertex was calculated using the corresponding ACF. The skin-dose values with angular correction were compared to those calculated using MC with a matching voxelized phantom. The results show the ACF decreases with decreasing incident angle and skin thickness, and increases with increasing field size and kVp. Good agreement was obtained between the skin dose calculated by the angular-corrected DTS and MC, while use of the ACF allows the real-time performance of the DTS to be maintained.
Monte-Carlo software was used to calculate the “patient’s skin-dose” averaged over 1 mm skin thickness as a function of incident beam-to-skin angle from 90 to 10 degrees for entrance-beam sizes from 5 to 15 cm, energies from 60 to 120 kVp, and thicknesses of Cu beam filters from 0.2 to 0.5 mm in a water phantom to obtain an angular-correction-factor (ACF). The Matlab tool, ‘cftool’, was used to fit these ACF’s to formulas as a function of incident beam angle and kVp, allowing the ACF to be quickly determined for accurate skin-dose calculation during fluoroscopically-guided procedures.
The patient’s eye-lens dose changes for each projection view during fluoroscopically-guided neuro-interventional procedures. Monte-Carlo (MC) simulation can be done to estimate lens dose but MC cannot be done in real-time to give feedback to the interventionalist. Deep learning (DL) models were investigated to estimate patient-lens dose for given exposure conditions to give real-time updates. MC simulations were done using a Zubal computational phantom to create a dataset of eye-lens dose values for training the DL models. Six geometric parameters (entrance-field size, LAO gantry angulation, patient x, y, z head position relative to the beam isocenter, and whether patient’s right or left eye) were varied for the simulations. The dose for each combination of parameters was expressed as lens dose per entrance air kerma (mGy/Gy). Geometric parameter combinations associated with high-dose values were sampled more finely to generate more high-dose values for training purposes. Additionally, dose at intermediate parameter values was calculated by MC in order to validate the interpolation capabilities of DL. Data was split into training, validation and testing sets. Stacked models and median algorithms were implemented to create more robust models. Model performance was evaluated using mean absolute percentage error (MAPE). The goal for this DL model is that it be implemented into the Dose Tracking System (DTS) developed by our group. This would allow the DTS to infer the patient’s eye-lens dose for real-time feedback and eliminate the need for a large database of pre-calculated values with interpolation capabilities.
EGSnrc Monte-Carlo software was used to calculate the “patient’s skin dose” as a function of incident beam angle for cylindrical water phantoms with underlying subcutaneous fat and various thicknesses of bone. Simulations were done for incident angles from 90 to 10 degrees, entrance beam sizes from 5 to 15 cm, and energies from 60 to 120 kVp. The depth-averaged scatter-plus-primary to incident-primary dose ratio decreases with decreasing skin incident angle and increasing underlying bone thickness, and increases with increasing field size and energy. Corrections for these factors improve the accuracy of skin-dose estimation for neuro-interventional procedures with our Dose-Tracking-System.
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.
The lens of the eye can receive a substantial amount of radiation during neuro-interventional fluoroscopic procedures, increasing the risk of cataractogenesis for the patient. The purpose of this study is to investigate the variation of eye lens dose with a variation of the location of the beam isocenter in the head. The primary x-ray beam of a Toshiba (Canon) Infinix fluoroscopy machine was modeled using EGSnrc Monte Carlo code and the lens dose was calculated using 2 × 1010 photons incident on the anthropomorphic Zubal computational head phantom for each simulation. The Zubal phantom is derived from a CT scan of an average adult male and has internal organs, including the lenses, segmented for dose calculation. Computations were performed with the head shifted vertically +/- 4 cm and in the cranial-caudal and lateral directions incrementally up to 6 cm in either direction. At each position, the gantry was rotated to various LAO/RAO and CAU/CRA angles, both 5 cm × 5 cm and 10 cm x 10 cm entrance field sizes were used and the kVp was varied. The results show that substantial changes in lens dose occur when the head is shifted and can result in a dose difference between eyes of over 6 times at certain beam angles for the 5 cm × 5 cm field size. The results of this study should provide increased accuracy in lens dose estimation during neuro interventional procedures and, when incorporated into our real-time dose-tracking system, help interventionalists manage patient lens dose during the procedure to minimize risk.
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