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Correspondence |
A comparison of male and female ESD and DAP values measured during a national survey of doses to patients in the UK [1] and the corresponding DRLs presented in the National Protocol for Patient Dose Measurements [2] indicates that National and European adult patient DRLs are governed by the levels of exposure to male patients.
The reason for this lies, to a large extent, in the recommendation that the mean weight of an adult patient sample, in any ESD/DAP audit, should lie within 5 kg of 70 kg [2]. In the national patient dose survey [1], the average weight of female patients was roughly 60 kg whilst that of the male patients was 70 kg. Similar weight differences for female and male patients have also been noted in studies involving fluoroscopy [3] and radiography [4]. Also, the weights of female and male adult voxel phantoms, developed for computational radiation protection studies, have been designed around similar weight differences [5–7]. The decision to employ a single-weight dependent DRL value has never been scientifically justified as an effective basis for optimization of practices in respect of female patients.
As I indicated in my letter, results of the 1985 national survey [1] demonstrate that the mean ESD for female patients undergoing a PA chest X-ray examination was 30% lower than that of the male patients. Such differences in ESD were also noted, for example, for the AP abdomen (27%), lat thoracic spine (33%), lat lumbar spine (25%) examinations. Other studies have also shown similar differences in ESD [4], for PA chest (70%), knee (100%) and AP skull (70%), examinations.
The mean DAP value for female patients was significantly lower than that of male patients [1] for the PA chest (86%), thoracic spine (22%), barium meal (39%), IVU (43%), cholangiography and cholecystography (43%) examinations. Other studies have shown similar differences in DAP values for female patients [3]; barium meal (36%), barium swallow (45%) etc. In the case of the DAP values, such differences can arise from unknown variations in the ESD and field sizes employed.
Under present circumstances, local DRLs derived for patients in a women's hospital would not be relevant to females in a mixed-sex patient group of 70 kg average weight. Different audit standards, therefore, can exist for different female populations. Similarly, female DAP values for a PA X-ray examination of the chest may exceed their own population norm by up to 86% [1], due to unknown variations in either ESD or field size, before the need for an investigation is indicated. Such an indication may be dependent upon the patient mix, including weights, in the audit sample, as well as specifically radiation protection considerations. Why should easily demonstrable, optimized practices for female patients be hidden under patient sampling/weight considerations involving male patients?
Besides exposure differences for female patients, other factors merit consideration in respect of optimization strategies for female patients:
Given such extensive differences, why are female patients not considered to be a separate exposed group with its own DRL values, so that their own unique safety and imaging needs can demonstrably be given full and direct consideration in respect of radiation safety?
The main reason for the widespread installation of DAP meters on X-ray imaging equipment throughout the NHS [paragraph 2 of B Wall's comments] is due to the fact that funds were made available from central government (presumably on the advice of the NRPB), in the early 1990s, for the purchase and distribution of 200 DAP meters. Prior to this, extensive medical physics effort had been expended on developing the use of calibrated X-ray source data for patient dose assessments. This effort included the background science [12] and calibration methods [13]. Such an approach harmonises patient dose assessments in both radiotherapy and diagnostic radiology and is based upon over 50 years of scientific effort. Unfortunately this was ignored in the National Protocol [2].
The reduction in patient doses in diagnostic radiology over the past 20 years [paragraph 7 of B Wall's comments] is due to the introduction of rare earth intensifying screens in radiography and sodium iodide phosphors in fluoroscopic units. These initiatives have been supported by the introduction of low attenuation materials in table-tops and Bucky stands. The UK national dose audits have faithfully recorded their well-documented effects. Our manufacturer colleagues are to be commended upon their contribution to patient dose reduction.
Yours etc.,
1 Co-ordinator, Radiological Unification Strategies [RADIUS] Group, EC Radiation Protection Research Programme
Received for publication November 18, 2005. Revision received January 4, 2006. Accepted for publication January 11, 2006.
References
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