Patient entrance skin doses at Minna and Ibadan for common diagnostic radiological examinations

Abstract

Entrance surface dose from two diagnostic x-ray centers in Nigeria for three common radiological examinations is presented in this study. Entrance surface doses for 294 patients drawn from Niger state hospital Minna and Two-Tees x-ray centre, Ibadan are included in this dose survey. The air kerma for each patient was measured using thermoluminescent dosimeter chips (TLD-100). The air kerma for each patient was then multiplied by a back scatter factor of 1.35 to obtain the Entrance surface dose. Generally doses obtained in this study were found to be higher than those in published works and International Atomic Energy Agency recommended limits for chest examination. The range factor at Niger State General Hospital was as high as 12 in some cases but as low as 1 in most cases at Two-Tees X-ray center. The doses obtained for skull and abdomen examination were found to be within acceptable International Atomic Energy Agency recommended limit. INTRODUCTION In medicine ionizing radiation is used for two main purposes; diagnosis and therapy. Consequently, individuals and the populace at large receive significant exposure to radiation. Diagnostic radiology is a leading cause of man made radiation exposure to the population. It was estimated that diagnostic radiology and nuclear medicine contributed 96% to the collective effective dose from man made sources in the U.K (National Radiological Protection Board, (NRPB), 1993). Similar estimate showed that this contribution was 88% in the U.S.A (National Council on Radiation Protection and Measurement (NCRP), 1987). The health of the population would decline if ionizing radiation techniques were not available to diagnose disease and detect trauma. Nevertheless, there is no excuse for complacency and it is a basic premise of radiation protection practice that any exposure should be justified by weighing the potential harm against the perceived benefit In view of the significant benefits from properly conducted medical exposures, the principal concern in radiological protection is the reduction of examinations that are either unlikely to be helpful to patient management or involve doses that are not as low as reasonably achievable in order to meet specified clinical objectives. In order to do this, there is a need to optimize x-ray equipment and radiological techniques (NRPB 1990). Patient dose measurement is an integral part of this optimization procedure (Faulkner et al 1999). Such measurements will reveal x-ray facilities with high doses after which possible dose reduction measures may be specified. Dose measurement is also necessary so as to: establish dose constraints, determine risk to patient and to justify the examination. There are two categories of doses to patient which are important in diagnostic radiology; the effective dose E which takes into account of dose equivalent to radiosensitive organs and the entrance skin dose. Most interest in diagnostic radiology is concern with effective dose since this relates to the risk of stochastic effect such as cancer induction. Effective dose or effective dose equivalent combines a set of organ or tissue equivalent dose into one single quantity. For this, the organs equivalent dose (H T) are multiplied by organ weighing factors (w T) and then summed. E = ∑ H T . w T ……………………… 1 However evaluation of E involves calculations which are not trivial. Simple Entrance Skin Dose (ESD) is defined as the absorbed dose to air where the x-ray beam intersects the skin surface of the patient. It is a quantity that can be measured directly and can easily be compared with previous measurements and with measurement obtained at other practices and countries. It can also be used as an indicator of effective dose for particular radiographic projections. Another reason for evaluating skin doses is that the dose is greatest at the surface where radiation enters the body of the patient and the skin is therefore the main organ for which there is a possibility of deterministic effect i.e. skin burn. More over E can be calculated from ESD to various organs using tables published by the international commission on radiation units and measurement (ICRU), (ICRU, 2005) or the International Commission on Radiological Protection (ICRP), (ICRP, 1982) . The European Union (EC) introduced the use of diagnostic reference levels (DRL) as an efficient standard for radiation of patients (see EC 1997). The physical parameter recommended for monitoring the DRL in convention radiography was the ESD (EC, 1996). Also, in the national protocol for patient dose measurements in diagnostic radiology (NRPB, 1992), the measurement of the ESD was proposed for individual radiograph.