MEASUREMENT OF THE EQUIVALENT INDIVIDUAL DOSES FOR PATIENTS IN ANGIOGRAPHY PROCEDURE AND INTERVENTIONAL RADIOLOGY WITH THERMOLUMINESCENT SYSTEMS

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1 RADIOPROTECTION AND DOSIMETRY MEASUREMENT OF THE EQUIVALENT INDIVIDUAL DOSES FOR PATIENTS IN ANGIOGRAPHY PROCEDURE AND INTERVENTIONAL RADIOLOGY WITH THERMOLUMINESCENT SYSTEMS DANIELA ADAM 1, ANA STOCHIOIU 2 1 Bucharest Emergency University Hospital, Romania 2 Horia Hulubei National Institute for Physics and Nuclear Engineering, IFIN-HH, Bucharest Reactorului 30, P.O. Box MG-6, Măgurele, Romania Received July 6, 2012 The aspects of radiation injuries in IR were highlighted and the increasing concern about the high skin dose levels in cardiology and other IR procedures, as it is very important a close monitoring in order to prevent stochastic effects and, in time, deterministic effects. The present study reports on investigations that we have performed to allow the calculation of the individual penetrant radiation dose, H p (10) in interventional radiology and also the important results in determining the homogeneity of the radiation flux. In order to determine the uniformity of large area beams with a better precision, thermoluminescent matrices of 8 12cm size, containing arrays of chips, uniformly distributed in each row and column of the matrix, were used. From the measurements we obtained accurate data of the dose received by patients in different areas of the body. Measurement method using TLD's is net superior to the measurement with Dap-meter. Due to the potential for stochastic and deterministic effects existing in interventional radiology, dosimetry information is required to make the interventionalist aware of the potential for such effects. Key words: equivalent dose, TLD, DAP. INTRODUCTION International standardization in dosimetry is essential for good exploitation of the X-radiation based technology. International Atomic Energy Agency (IAEA) and other nuclear activity control bodies initiated different programs regarding ionisation radiation dosimetry, in order to help the international community, including the EU member states, to develop standards and measurement capacities of dosimetry equipments required for applying radiation in medicine. For some time there was a growing awareness that the amount of the equivalent doses to patients after performing medical procedures for diagnostic radiology contributes at growing proportion of the total dose to the population. Interventional radiology procedures (IR) and interventional cardiology procedures (IC) are registering the highest radiation doses received by patients within one year of radiological examinations. Stochastic effects that may result from these investigations are very important, especially for the young population. Rom. Journ. Phys., Vol. 58, Nos. 3 4, P , Bucharest, 2013

2 2 Measurement of the equivalent individual doses 331 To optimize patient exposure to radiation, the responsible doctor must know very well the individual penetrant dose, H p (10), to the radiosensitive organs for each type of procedure performed. A precise determination would be very difficult because of exposure to different areas of the body which may overlap in the investigation. Even when a full protocol of all exposure parameters is used, it can t provide a good estimate of the effective dose for each organ exposed to the radiation field. There are several direct and indirect methods used to determine the patient's skin dose, but one of the most accurate methods we consider that is thermoluminescent dosimetry (TLD), allowing for evaluation of the equivalent dose at various organs. The aim of the study presented in this paper is to measure the individual penetrant dose, Hp(10), from angiographic investigations with the TLD method and to determine the homogeneity of the radiation beams. MATERIALS AND METHODS For the measurement of the individual penetrant radiation dose equivalent, H p (10) values in Sievert, we used the thermoluminescent system, SD-TL type, set up at the Horia Hulubei National Institute of Physics and Nuclear Engineering, IFIN-HH, described in detail in [1], whose characteristics are in full agreement with the requirements of the standard IEC 61066:2006 [2]. It contains the dosimeters based on the use of LiF:Mg, Cu, P detectors, as chips, commercially known as GR- 200A and a TLD reader type READER ANELYSER RA 94. The calibration of the system was done according to the prescriptions of the ICRU Report 39 [3] in units of H p (10), using a standard radiation field from a 137 Cs standard source. To determine the dose in interventional radiology examinations we used a 30 cm, plexiglas phantom calibrated at PTW Freiburg, Germany, to position the detector arrays. The irradiations were done using the Integris H5000 angiography equipment produced by Philips. In order to determine the uniformity of large area beams with a better precision, thermoluminescent matrices of 8 12cm size, containing arrays of chips, uniformly distributed in each row and column of the matrix, were used. The average time for carrying out an investigation by angiography or interventional cardiology is 20 min. During this time, exposure to radiation is done in area of interest and adjacent areas of the patient to be investigated. In order to prevent deterministic effects it is important to measure the surface input dose; to have the control over stochastic effects is essentially to know deeply absorbed dose. In this respect, we measured the H p (10) values at different depths in phantom: 10 cm, 16 cm, 23 cm and 30 cm, as Fig. 1, representing the PTW-phantom, shows.

3 332 DanielaAdam, Ana Stochioiu 3 Fig. 1 Plexiglas Phantom type PTW. TLD matrices were exposed to an X radiation beam characterized by the following parameters: Tube high voltage, T = 110 kv; Intensity, I = 14.5 ma; Dose Area Product, D DAP = 12 Gy cm 2. Several measurement sets were performed: (i) the first set, 96 chips placed at 10 and 30 cm depths and an exposure time, t = 2 min; (ii) the following sets, with 15 and 11 chips, at 16, 23 and 30 cm depths with t = 1 min exposure times. After readings we obtained the individual and mean values per detectors column of dose equivalent H p (10), with the standard deviation, σ n(col) such as presented in Tables 1 5. Table 1 The values of dose equivalent [msv] for the matrix I, (10 cm depth, t = 2 min). No. of a b c d e f g h i j k l chip Mean σ n(col)

4 4 Measurement of the equivalent individual doses 333 Table 2 The values of dose equivalent [msv], for the matrix II, (30 cm depth, t = 2 min) No of a b c d e f g h i j k l chip Mean σ n (col) Table 3 The values of dose equivalent [msv] for the matrix III, (16 cm depth, t = 1 min) No a b c d e Mean σ n (col) Table 4 The values of dose equivalent [msv] for the matrix IV (23 cm depth, t = 1 min) No a b c d Mean σ n (col) Table 5 The values of dose equivalent [msv] for the matrix V (30 cm depths, t = 1 min) No a b c d Aver. value σ n (col)

5 334 DanielaAdam, Ana Stochioiu 5 Measurements are also important in determining the homogeneity of the radiation flux. By plotting the TLD dose equivalent derived from exposures of 1 min and 2 min, we can observe a variation of radiation flux that influences the radiation dose to the patient skin and collected dose at organs at risk, which are near the exposed area. In principle, the values depend of radiation flux homogeneity. Using the data from the upward tables, one represents graphically minimum and maximum dose distribution on TLD matrix surface, exposed to X radiation, such as presented in Figure 2, which provides an intuitive image of uniformity of beam. Fig. 2 - Equivalent dose distribution on TLD matrix surface, exposed to X radiation. As we can see in Fig 2. the radiation flux is not uniform on the TLD surface. Table 6 represents the summarized values of dose equivalent value [msv] of the 5 matrices of detectors, expressed as mean, minimum and maximum values, calculated from all columns, corresponding to an exposition time of 1 min. Table 6 Mean, maximum and minimum values of individual penetrant dose rate p H (10), msv/min Depths (cm) Average value (msv/min) Min value (msv/min) Max value (msv/min) ± ± ± ± ± ± ± ± ± ± ±0.006 Mean: 0.063± ± ±0.005 Mean: 0.051± ± ±0.004 Mean: 0.067±0.004

6 6 Measurement of the equivalent individual doses 335 RESULTS INTERPRETATION As first conclusion from tables, as expected, the central part of the beam has the maximum intensity; the last two columns contain lower values; regarding the matrices lines, the differences are lower. For the 30 cm, the second line values, calculated from a measurement time of 2 min are in agreement with the 1 min results, what means that for these short intervals the generator parameters remained constant. The graph below, Fig.3, represents the average value data from Table 6. Fig. 3 Individual penetrant dose rate p H (10), msv/min, as function of the depth, d. The mathematical fitting of the data, represented by the continuous line in Figure 3, results in a relation of variation of the equivalent dose rate, msv/min, as function of the depth d, in cm, of the type: H p (10)[mSv/min] = (8.089 ± 0.672) (0.589 ± 0.074) d + ( ± 0.018) This relation approximates satisfactory the variation of equivalent dose rate with the depth and allows the calculation of the value for each depth, inclusively at the entrance surface, d = 0. One can deduce that the exposure times of 20 minutes result in significant equivalent dose values. It is very important a close monitoring in order to prevent stochastic effects and, in time, deterministic effects. The aspects of radiation injuries in IR were highlighted and the increasing concern about the high skin dose levels in d 2

7 336 DanielaAdam, Ana Stochioiu 7 cardiology and other IR procedures were mentioned in Publication 85 of the ICRP [3]. The two main causes mentioned for radiation injuries were: a) the use of suboptimal equipment, and b) procedures performed by individuals inadequately trained in radiation protection. CONCLUSIONS Measurements with TLDs give a good estimation of the dose rates for various exposition times and depths. Dose Area Product (DAP), is a multiplication of the dose and the area exposed, often expressed in Gy.cm 2. Total DAP is not always a good estimator of the risk of radiation. Using the method of measurement with TLD one can have a better overview of doses at different distances and times. From the above measurements we can obtain accurate data of the dose received by patients in different areas of the body. Measurement method using TLD's is net superior to the measurement with Dap-meter. A potential for stochastic and deterministic effects exists in interventional radiology. Dosimetry information is required to make the interventionalist aware of the potential for such effects. Also, for the purposes of tracking doses to patients after IR procedures, it is necessary for details of the patient s dosimetry to be available within the patient record. Radiation protection for patients and staff is one of the main issues for IR. Acknowledgement. Thanks for assistance in achieving this article to Mrs. Prof. Dr. Maria Sahagia and Eng. Ion Tudor from Horia Hulubei National Institute for Physics and Nuclear Engineering, IFIN-HH, Bucharest. REFERENCES 1. A. Stochioiu, F. Mihai, F. Scarlat. A new passive dosimetric system with thermoluminiscent LiF:Mg, Cu, P detectors applied in individual radiation monitoring. Journal of Optoelectronics and Advanced Materials. Vol. 8, No 4, pp (2006). 2. International Standard, International Electrotechnical Commission, IEC 61066:2006. Thermoluminescent dosimetrical systems for the survey of environment and personnel. 3. International Commission for Radiation Units, ICRU Report 39. Determination of dose equivalents resulting from external radiation sources, ICRU, 1985.