Fundamental study on diagnostic reference level quantities for endoscopic retrograde cholangiopancreatography using a C-arm fluoroscopy system

<jats:title>Abstract</jats:title> <jats:p>The diagnostic reference level (DRL) is an effective tool for optimising protection in medical exposures to patients. However regarding air kerma at the patient entrance reference point (<jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub>), one of the DRL quantities for endoscopic retrograde cholangiopancreatography (ERCP), manufacturers use a variety of the International Electrotechnical Commission and their own specific definitions of the reference point. The research question for this study was whether <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> is appropriate as a DRL quantity for ERCP. The purpose of this study was to evaluate the difference between <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> and air kerma incident on the patient’s skin surface (<jats:italic>K</jats:italic> <jats:sub>a,e</jats:sub>) at the different height of the patient couch for a C-arm system. Fluoroscopy and radiography were performed using a C-arm system (Ultimax-i, Canon Medical Systems, Japan) and a over-couch tube system (CUREVISTA Open, Fujifilm Healthcare, Japan). <jats:italic>K</jats:italic> <jats:sub>a,e</jats:sub> was measured by an ion chamber placed on the entrance surface of the phantom. Kerma-area product (<jats:italic>P</jats:italic> <jats:sub>KA</jats:sub>) and <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> were measured by a built-in <jats:italic>P</jats:italic> <jats:sub>KA</jats:sub> meter and displayed on the fluoroscopy system. <jats:italic>K</jats:italic> <jats:sub>a,e</jats:sub> decreased while <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> increased as the patient couch moved away from the focal spot. The uncertainty of the <jats:italic>K</jats:italic> <jats:sub>a,e</jats:sub>/<jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> ratio due to the different height of the patient couch was estimated to be 75%–94%. <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> may not accurately represent <jats:italic>K</jats:italic> <jats:sub>a,e</jats:sub>. <jats:italic>P</jats:italic> <jats:sub>KA</jats:sub> was a robust DRL quantity that was independent of the patient couch height. We cautioned against optimising patient doses in ERCP with DRLs set in terms of <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> without considering the patient couch height of the C-arm system. Therefore, we recommend that <jats:italic>K</jats:italic> <jats:sub>a,r</jats:sub> is an inappropriate DRL quantity in ERCP using the C-arm system.</jats:p>