Reference Value Assessment for DAP Measured with a Transmission Ion Chamber in Gastrointestinal Tract Examinations

Transcription

1 Reference Value Assessment for DAP Measured with a Transmission Ion Chamber in Gastrointestinal Tract Examinations M. Río, S. Pérez, A. Herreros, A. Coll, A. Ruiz, J.Baró and J. Fernández Asesoría en Control y Protección Radiológica, ACPRO, S.L. Rafael Batlle, 24, Barcelona, Spain. josep.baro@acpro.es Abstract. In diagnostic radiological departments in the Health Service of Catalonia, Institut Català de la Salut, ICS, two different digital fluoroscopy table systems, the General Electric Prestige II and the Philips Optimus 80 Telediagnost, are being introduced. These digital devices are provided with a transmission ion chamber, a Wellhöffer Kermax Plus and a RTI Doseguard 100, respectively, in order to determine Dose Area Product, DAP. The need to establish dose reference values for complete examinations has led us, as a Radiation Protection Service, to undertake a study to evaluate partial and total Dose Area Products from typical complete medical examinations. These medical examinations are typically barium meals or barium enemas carried out in the gastrointestinal tract. The results obtained for DAP values, expressed in Gy cm 2, can be represented by a function of parameters that has an influence on the results: body mass index BMI from the patient being examined, fluoroscopic time, number of (digital and/or conventional), type of examination and working practices by doctors and technicians. Consequently, a relationship is estimated between the most significant parameters which enabled us to establish DAP reference values for barium meals and barium enemas. For barium meals the 3rd quartile observed for DAP values goes from 14 to 36 Gy cm 2 and in the case of barium enemas DAP values between 38 and 68 Gy cm 2 are found. For both types of examinations a contribution of 66% from total DAP is due to the fluoroscopic mode and the remainder, for barium meals, comes from digital. This contribution is the same for digital and conventional in the case of barium enemas. 1. Introduction The establishment and revision of reference values for dosimetric indicators has become important for radiodiagnostic and nuclear medicine departments due to the increasing streamlining of the equipment and methodology used. Nowadays, in the radiodiagnostic field in Catalonia there is a gradual replacement of conventional fluoroscopy systems for digital systems and the unavoidable need arises to determine the associated dose reference values of diagnostic examinations carried out with this equipment. Hence, ACPRO, S.L., as a Radiation Protection Service for primary medical centers in the Catalan Health Service Network, Institut Català de la Salut (ICS), has carried out a detailed dosimetric study of barium meals and barium enemas which are used for complex examinations in the gastroinstestinal tract:. The aim of this study is to determine reference values for these diagnostic examinations. This study is based on registered data (dose area product, fluoroscopic time, ) obtained from 145 barium meal and 74 barium enema examinations by integrating partial dose received at each anatomical stage of the examination. These medical examinations have been carried out at 4 different primary medical centers belonging to the Catalan Health Service in the Province of Barcelona. Two of the centers use table digital fluoroscopy systems with the PHILIPS brand, model Optimus 80 Telediagnost, provided with a transmission ion chamber by RTI Doseguard (Equipment A) and the other two centers, work with the same system made by General Electric, model Prestige II, equipped with a Wellhöffer Kermax Plus transmission ion chamber (Equipment B). The parameter dose area product (DAP) has been used as a reference value, due to the Real Decreto 1976/1999[1], which states that fluoroscopy systems must have a device that provides a DAP value throughout all of the examination. 1

2 In this study the obtained reference values for both types of tests are shown and the parameters on which they depend are analysed. Furthermore, the level of effective dose received by the patient, compared with the reference DAP values, is indicated. 2. Methodology For each of the complete examinations in the gastrointestinal tract: partial and total DAP values, partial and total time elapsed by the use of fluoroscopy mode, the number of conventional and digital, data of each patient (height, weight and age), the reason for prescription of the diagnostic examination, as well as details of the system operator, were registered. Additionally, DAP values associated with each conventional radiograph and each fluoroscopic time interval were monitored at each center. Furthermore, characteristics such as voltage (kv), intensity (ma), load (mas) and time (ms) were recorded for each partial DAP value generator. These values have been used to estimate effective dose using the EffDose 1 program. 3. Study of DAP values obtained The distribution of total DAP values obtained for the four primary medical centers considered in this study is shown in FIG. 1, for barium enema examinations, and in FIG. 2, for barium meal examinations. FIG. 1. Distribution of DAP values obtained for 74 barium enema examinations. Solid bars stand for centers with fluoroscopic equipment A and the dotted bars for equipment B. 1 A program using NRPB-SR262[2] for the Estimation of Effective Dose in Diagnostic Radiology from Entrance Surface Dose and Dose-Area Product Measurements. The Program could also be used to estimate the Dose or Dose-Area Product on the basis of Tube Potential, Total Filtration, mas, Single or Three Phase and Source Skin Distance base on ICRP 34[3] data. 2

3 FIG. 2. Distribution of DAP values obtained for 145 barium meal examinations. Solid bars stand for centers with fluoroscopic Equipment A and dotted bars for Equipment B. DAP mean values obtained for each center provide reference values for medical examinations of this type and are presented in Table I expressed in terms of the third quartile. Table I. Results for DAP values for barium enema and barium meal examinations at 4 different centers. Dose Area Product (Gy cm 2 ) Mean(N) (2) Minimum 1st 3 rd Median quartile quartile Maximum Center 1/A (1) 38 (13) Center 2/B 32 (24) Barium Center 3/A 51 (18) Enema Center 4/B 29 (19) Total 37 (74) Center 1/A 11(41) Center 2/B 29(34) Barium Center 3/A 19(27) Meal Center 4/B 13(43) Total 17(145) (1) The capital letter A and B is used for differentiating the two models of fluoroscopic equipment analysed. (2) N is the number of examinations studied. Based on these results, no correlation between DAP values and type of fluroscopic table system can be found, since for both brands similar results are observed. Analysis of the data does not reveal a significant correlation between total DAP and body mass index (BMI) values for the patients, which reflects their physical constitution. The distribution of this index, 3

4 BMI, calculated as the quotient between the weight and the squared height of the patients, reproduces the typical distribution of the population that lives in the province of Barcelona. However, as is shown in Table II, for the centers where the mean values of total DAP are greater, it is seen that the procedures or the methodology followed by the operators that perform the examinations mean greater fluoroscopic times and an increase in the number of. Table II. Methodology of the different centers during the performance of barium enema and barium meal examinations expressed in terms of mean values and standard deviations of the different kinds of radiological projections that are used. Digital Conventional time (s) Center 1/A (1) 11.7 ± ± ± 71.7 Barium Enema Center 2/B 12.9 ± ± ± Center 3/A 11.7 ± ± ± 71.4 Center 4/B 8.9 ± ± ± Center 1/A 14.3 ± ± ± 34.8 Barium Meal Center 2/B 22.1 ± ± ± Center 3/A 11.1 ± ± ± 94.6 Center 4/B 9.8 ± ± ± 58.9 (1) The capital letters A and B are used for differentiating the two models of fluoroscopic equipment analysed. To determine the influence of methodology in the DAP results during performance of the examinations, the results corresponding to one center have been analysed by grouping the data depending on the operator that has carried out the examinations. Table III. Mean values and standard deviation of different parameters that characterize the barium enema examinations for two operators at the same center, Center 3/A. Number of examinationss Total DAP (Gy cm 2 ) Number of digital time (s) DAP (Gy cm 2 ) Operator ± ± ± ± 11 Operator ± ± ± ± 13 As is shown in Table III, the methodology followed by the operator represents an important factor, because it conditions the fluoroscopic time used and hence, the greater contribution to the DAP, which constitutes almost % of the total value for both barium enemas or barium meals. 4. Estimation of effective dose The recommendations of the ICRP (1990) [4] establish that effective dose is the best for comparing the risks associated with x-ray examinations. However, its routine calculation from radiological parameters is not possible at a radiological facility, as specific information is required about dosage to each individual organ of the body. Hence, the dose area product is closely related to effective dose, since it takes into account the x-ray beam which affects the irradiated organs and also allows measurement of the total dose involved in 4

5 complete examinations, where different projections are used. This can be a useful parameter to assess patient effective dose. In the present study, the partial DAP values and the different projections applicable to the registered examinations have been used to estimate patient effective dose in radiography and fluoroscopic gap by means of the EffDose program, and hence, for the whole examination. These data have enabled conversion coefficients to be established to provide patient effective dose from DAP, which could be included in the patient s medical record. 5. Conclusions Since 75% of DAP values are located below the 3 rd quartile, we can establish from the results obtained in this work a DAP reference value of 46 Gy cm 2 for barium meal examinations and 23 Gy cm 2 for barium enemas. These proposed reference doses should be seen as a practical aid to increase awareness of the significance of observed levels of patient dose and hence to the promotion of optimisation of radiation protection in medical radiology. From the large number of parameters that can affect this dosimetric indicator, the operator that carries out the examination and its protocol is by far the most important factor, since he or she conditions the fluoroscopy time and the number of during the medical examination. However, the BMI parameter and the fluoroscopic system type do not display any significant correlation with DAP values, contrary to what was expected before this study was started. The detailed study of the projections done in each kind of examination makes their simulation possible by means of the EffDose program, as well as providing an estimation for the patient effective dose for all of the x-ray examination. A conversion factor for relating measured dose-area product to effective dose was estimated from these results. As a preliminary value, we have found a conversion factor of 0.28 msv/gy cm 2 for barium meals and 0.26 msv/gy cm 2 for barium enemas. On the other hand, it is observed from the data that in almost 90 % of the studied cases no pathologies are observed after the x-ray examination and that the mean age value for patients who undergo diagnostic examinations of this type is 60, although there are also cases of complete examinations in young patients. Therefore, we would like to point out the importance of carrying out an accurate medical diagnosis prior to prescribing radiological examinations of this type. REFERENCES [1] Real Decreto 1976/1999, de 23 de diciembre, por el que se establecen los criterios de calidad en radiodiagnóstico. [2] NRPB-262 [3] International Commission on Radiological Protection, Protection of the Patient in Diagnostic Radiology. Publication 34. Annals of the ICRP, Vol. 9/2. Pergamon Press (1983). [4] ICRP-60 5