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Patient Dose Surveys and the Use of Local and National Diagnostic Reference Levels M.L

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Patient Dose Surveys and the Use of Local and National Diagnostic Reference Levels M.L IAEA-CN-85-248 XA0101683 PATIENT DOSE SURVEYS AND THE USE OF LOCAL AND NATIONAL DIAGNOSTIC REFERENCE LEVELS M.L. Ramsdale, D. Peet, P. Hollaway, A. Rust Regional Radiation Protection Service, Royal Surrey County Hospital, Guildford, Surrey, United Kingdom Abstract Patient doses have been assessed routinely as part of quality assurance programmes in a group of National Health Service and private hospitals in a southern English health region. Surveys of patient dose have been conducted in plain film radiography, fluoroscopy, computed tomography, mammography and dental radiography. In keeping with national guidance, dose parameters used were entrance skin dose for radiography, dose area product for fluoroscopy, effective dose and dose length product for computed tomography (CT), mean glandular dose for mammography and air kerma and dose width product for dental intra-oral and orthopantomography (OPG) respectively. Comparison of results against recommended standards showed that in plain film radiography and fluoroscopy, doses were well below national reference levels and corresponding local reference levels were adopted at about 75% of the national figures. Elsewhere in CT, mammography and dental radiography, doses were generally in line with national trends. Overall, as an integral part of the QA programme, the dose surveys have contributed greatly to the users understanding of patient dose and in several instances has led to real optimisation. 1. Introduction As part of the provision of a radiation protection and diagnostic radiology physics service to around 50 hospitals, 30 mammography units and 130 dental practices in the South East of England, sampling of the doses received by patients during common X-ray examinations is undertaken. Results are compared to relevant Diagnostic Reference Levels (DRLs) as an aid to optimisation and in compliance with the EC Directive [1] and UK legislation [2] on the protection of the patient. 2. Methods Patient dose surveys for plain film radiography and fluoroscopy have been undertaken periodically in accordance with the national dose protocol which recommends that such surveys are carried out at least once every 3 years [3]. Surveys of patient dose have also taken place in computed tomography, mammography and dental X-ray. For plain film radiography, the patient dosimetry programme started in 1993 and two cycles were completed covering 64 and 77 X-ray departments respectively. In accordance with the recommended protocol, thermoluminescent dosemeters (TLDs) were used to measure entrance skin doses for 5 common examinations; chest, skull, lumbar spine AP, lateral lumbar spine and pelvis. Only standard size patients (60 to 80 kg) were used in the study with generally 10 patients per examination. Dose area product (DAP) is the recommended dose parameter for complex examinations including fluoroscopy. Since 1995 DAP data have been collected for barium studies, angiography and interventional procedures covering 37 screening rooms in 23 hospitals involving about 10,000 patients. In CT the effective dose for common examinations has been assessed from knowledge of exposure protocols and measured computed tomography dose index (CTDI) values utilising published CT scanner data [4]. An initial patient dose survey of 8 CT scanners was 434 IAEA-CN-85-248 undertaken in 1996 and subsequently repeated in 1999/2000 for 7 of the original units plus 3 new scanners. In keeping with the European protocol [5], doses are now also compared to the parameter dose length product (DLP). The Service undertakes routine performance measurements on 30 mammography units locally including 12 in the national breast screening programme and 18 located in the symptomatic mammography sector. In accordance with the recommended protocol [6] assessment of mean glandular dose is undertaken routinely every 6 months for the 'standard breast' model. Calculation of mean glandular dose using exposure and breast thickness data from samples of patients undergoing mammography is also carried out periodically. Radiation protection and performance measurements are undertaken on all dental X-ray on a 3-yearly cycle. Since 1997 doses have been assessed on 357 intra-oral and 70 OPG X-ray units using measurements of radiation output. In keeping with national guidance [7] dose parameters used were skin entrance dose for intra-oral and dose width product for OPG. 3. Results The results of two rounds of patient dose surveys for plain film radiographic examinations carried out over 1992-1995 and 1996-1999 respectively is summarised in table I. The mean values presented across all hospitals for the common examinations studied are compared to the national reference doses published in 1992. The local doses were well below national levels and local reference doses, also shown, were derived from the first round of local dose measurements. Table 1. Skin entrance dose for plain film radiography No of Mean dose % National Local ref pts (mGy) Reference Dose dose (mGy) 92-95 96-99 92-95 96-99 Abdo AP 682 5.1 4.8 51 48 7 Chest PA 1601 0.14 0.12 50 40 0.2 L spine AP 866 5.9 5.2 59 52 7 L spine Lat 876 15.3 13.2 51 44 20 Pelvis 638 5.2 4.0 52 40 7 The results of DAP measurements for more than 7500 patients are summarised in table II, highlighting the mean DAP for barium enemas and meals across 30 screening rooms. In the 1992 dose protocol these were the only examinations commonly undertaken in local screening rooms where reference doses were provided. Again, local doses were generally well within national figures and consequently local reference doses based on these values were adopted. The relative impact of local and national reference doses for barium enemas is illustrated in the distribution of doses in Figure 1. 435 IAEA-CN-85-248 Table 2. Dose area product values for fluoroscopy examinations Examination Nos of Mean DAP % National Ref Local Ref Dose patients Gy cm2 Level Gy cm2 Ba Enema 3242 31.8 53 40 Ba Meal 1032 11.7 47 15 Ba Swallow 540 7.5 - 10 BaFT 184 13.7 - 20 Femoral Arteriogram 676 54.6 - 80 Coronary Angiogram 372 35.9 - 50 Coronary Angioplasty 88 34.8 - 35 Venogram 558 4.4 - 5 ERCP 399 11.1 - 12 HSG 216 5.9 - 8 Nephrostomy 92 11.9 - 16 Fibroid embolisation 255 84.0 - 105 70 - J160 1992 National Reference level "8 n £ 40 - 30-- - : 10 - nifllill Hospital Figure 1. Dose area product for barium enemas Table 3. Effective and dose length product for computed tomography examinations Examination Dose Dose-length Product (mGy (mSv) cm) Mean NRPB Minimu Maximu Mean EC Ref Mean m m Brain 2.08 1.78 316 2196 870 600 Neck 2.35 - 143 1107 552 - Pelvis 6.93 7.12 216 655 423 600 Abdomen 7.01 7.58 469 989 414 800 Chest (normal) 9.04 7.80 162 790 409 650 Liver 6.00 7.17 151 723 364 - IAM 0.63 0.35 100 641 298 - L-spine 4.64 3.33 56 450 292 - Chest (high 1.34 - 30 104 92 - resolution) 436 IAEA-CN-85-248 The mean effective dose for common CT examinations, based on a standard patient model, is shown in table III. These values are also compared to the means from a national survey conducted by the National Radiological Protection Board (NRPB) [8]. The corresponding range and mean of DLPs is also presented for the same nine examinations. 3.00 Number Figure 2. Mammography Dose to the "standard breast The results for mammography in Figure 2 indicate variation in dose due to equipment factors only as a 'standard breast' is defined. Variations in X-ray equipment design, speed of the film and screen combination and variation in film optical density are the main factors affecting the distribution of patient dose. The reference dose indicated on the graph is used as a maximum recommended level, above which further optimisation of the system is indicated or justification for continued use is required. Surveys of patient dose based upon exposure factor data for real examinations additionally include variations due to the size and composition of the breast and also operation of advanced automatic exposure systems with the capability to modify X-ray beam quality. Patient doses in dental radiology are illustrated in figs 2 and 3 for mandibular molar intra-oral and OPG examinations respectively. Number Figure 3. Dental Intra-oral doses (Mandibular Molar) mGy 437 IAEA-CN-85-248 Number Figure 4. Dental Panoramic doses (mGy mm) 4. Discussion and conclusions In the UK reference doses for radiography and fluoroscopy, based on dose measurements in the late 1980's and very early 1990's, were published in 1992 as part of the national dose protocol [3] and have remained unchanged. The first round of local dose measurements for radiography confirmed, not unexpectedly regarding the increase in film speed and other dose reduction features introduced since the 1980's, that examinations at all hospitals were generally well within national reference doses. Typically the mean doses for each examination were about 50% of the reference dose. Since the national reference dose no longer impacted on the optimisation process, local reference doses, based on the 75th percentile of the dose distribution from the first round of measurements, were adopted. As shown in table I, the second round of measurements showed slight reductions in mean doses for most examinations. A similar picture emerges for the fluoroscopy dose surveys where the measured dose area products for barium enemas and meals were again at about 50% of the national reference dose. Local reference doses were adopted in the same manner for these and ten other examinations. In CT no formal reference doses have been published in the UK and instead the doses for examinations have been compared to mean dose values published by the NRPB [8] following a survey of CT practice.
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