Variation of eye-lens dose with variation of the location of the beam isocenter in the head during neuro-interventional 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 x 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 x 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 x 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.