Sensitivity of coefficients for converting entrance surface dose and kerma-area product to effective dose and energy imparted to the patient

We investigate the sensitivity of the conversions from entrance surface dos e (ESD) or kerma-area product (KAP) to effective dose (E) or to energy impa rted to the patient (epsilon) to the likely variations in tube potential, f ield size, patient size and sex which occur in clinical work. As part of a factorial design study for chest and lumbar spine examinations, the tube po tentials were varied to be +/-10% of the typical values for the examination s while field sizes and the positions of the field centres were varied to b e representative of values drawn from measurements on patient images. Varia tion over sex and patient size was based on anthropomorphic phantoms repres enting males and females of ages 15 years (small adult) and 21 years (refer ence adult). All the conversion coefficients were estimated using a mathema tical phantom programmed with the Monte Carlo code EGS4 for all factor comb inations and analysed statistically to derive factor effects. In general, t he factors studied behaved independently in the sense that interaction of t he physical factors generally gave no more than a 5% variation in a convers ion coefficient. Taken together, variation of patient size, sex, field size and field position can lead to significant variation of E/KAP by up to a f actor of 2, of E/ESD by up to a factor of 3, of epsilon/KAP by a factor of 1.3 and of epsilon/ESD by up to a factor of 2. While KAP is preferred to de termine epsilon, the results show no strong preference of KAP over ESD in d etermining E. The mean absorbed dose (D) over bar in the patient obtained b y dividing epsilon (determined using KAP) by the patient's mass was found t o be the most robust measure of E.