Relationship between source-surface distance and patient dose in fluoroscopic X-ray examinations

The International Electrotechnical Commission, IEC provided in its standard IEC 60601-1-3 (1994) to prevent the use during radioscopic irradiation of focal spot to skin distances less than 20 cm if the X-RAY EQUIPMENT is specified for RADIOSCOPY during surgery or 30 cm for other specified applications. This standard was reflected in the Japanese Industrial Standard JIS Z 4701-1997, which provided the minimum distance from focal spot to skin to be 30 cm for the use of a fluoroscopic and radiographic table (Under-table type). However, JIS had formerly provided the minimum distance to be 40 cm and so does the current Medical Treatment Law. The draft revision for the Medical Treatment Law currently discussed has consideration to adopt the value 30 cm in accordance with the current JIS. Our research intended to investigate the impact on the entrance surface dose for the change of the focal spot to skin distance from 40 cm to 30 cm. The result was 20-30% increase of the entrance surface dose for the focal spot to skin distance 30 cm. Taking patient exposure dose into account, we need further and more sufficient discussion with this result before adopting this value to the Medical Treatment Law. (author)