Letter to the Editor

Abstract

Sir, Comments on ‘Increasing film-focus distance (FFD) reduces radiation dose for X-ray examinations’ by P.C. Brennan, S. McDonnell and D. O'Leary. In a recent publication, Brennan et al.(1) claim that increasing the film-focus distance (FFD) from 100 to 130 cm will reduce effective doses (E) by 33% for a pelvic X-ray examination. This conclusion is based on a reported 33% reduction in the entrance surface dose (ESD), by the use of the same effective dose per unit ESD conversion coefficient (E/ESD) at both FFD values. However, E/ESD values are only valid for the specific geometry used in the Monte Carlo simulations, and will generally vary with FFD. To obtain values of E at different FFDs, one should calculate the kerma area product (KAP) values at each FFD distance, and obtain effective doses using E/KAP conversion factors provided by the National Radiological Protection Board (NRPB)(2). For a given X-ray beam quality and anatomic projection, the KAP is directly proportional to energy imparted to the patient, which in turn will be (approximately) proportional to effective dose.