A pilot programme on patient dosimetry on paediatric interventional cardiology in Chile

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1 A pilot programme on patient dosimetry on paediatric interventional cardiology in Chile Patricia Miranda *1, Fernando Leyton 2, Eliseo Vano 3, Alfonso Espinoza 2, Raul Ramirez 4, Otto Delgado 2, Pedro Ortiz-Lopez 4, Leopoldo Romero 1 and Luis Cardenas 1 1 Hospital Luis Calvo Mackenna, Av. Antonio Varas 360, Providencia, Santiago, Chile 2 Instituto de Salud Pública, Marathon 1000, Ñuñoa, Santiago, Chile 3 Universidad Complutense y Hospital San Carlos, Madrid, España 4 International Atomic Energy Agency, Vienna, Austria Abstract. This paper presents the design and some preliminary results of a pilot programme on patient dosimetry in paediatric interventional cardiology (PIC) in Chile. The collected parameters during the survey have been: procedure identification, patient age, gender, weight, height, kerma area product (P ka ), cumulative dose at the interventional reference point, number of cine series, total number of frames and fluoroscopy time. These data will also permit the estimation of typical radiological risks of the procedures for different age groups. Preliminary results are being calculated for the following age groups: <1; 1- <5; 5- <10 and 10- <16 years. Kerma area product values and parameters related to imaging protocols of the procedures have been initially obtained from one of the most representative paediatrics centres of Chile. The x-ray system used in this centre was a biplane Siemens Axiom Artis with image intensifiers. The sample contains patients aged between 6 days and < 16 years. The patient distribution for the different age groups was 68 for <1 year; 95 for 1- <5 years; 37 for 5-< 10 years and 59 for 10-<16 years. Median values of P ka for diagnostic procedures were: 1.0; 1.6; 2.0 and 4.9 Gy.cm 2 respectively, and for therapeutic ones: 1.1; 1.3; 1.4 and 5.8 Gy.cm 2 respectively. No significant statistical differences were found between diagnostic and therapeutic groups. Results are similar (slightly lower) to other recent published values in Europe, suggesting that an approach to propose patient dose reference levels could also be advisable in paediatric cardiology. Dependence of air kerma area product on patient weight is, as expected, stronger for P KA per minute fluoroscopy and per acquisition frame on patient weight than the dependence for the whole procedure, due to the influence of procedure complexity. Nevertheless, influence of patient weight on air kerma area product becomes plausible, even for the whole procedure, by appropriate data representation. Suggestions to optimize the practice of paediatric cardiology are concluded. The next steps of the programme will concentrate on image quality evaluation, diagnostic information obtained from the different cine series and fluoroscopy runs and analysis of the complexity for the different procedures to be correlated with patient doses. The goal will be to establish criteria for the best management of patient doses in paediatric fluoroscopy guided procedures. KEYWORDS: paediatric interventional cardiology, kerma area product, patient dose, reference levels. Introduction Paediatric cardiology is absolutely different from the adult cardiology not only in the age of the patients (newborns until fifteen years of age) but also because of the diversity of structural anomalies in congenital heart diseases. This is important to understand because paediatric procedures are longer than procedures in adults [1]. Fluoroscopy guided procedures are used in paediatric patients more often than a few years ago, because more complex congenital heart diseases have the possibility of cardiac surgery, complex diagnostic studies are necessary including several biplane angiographic recordings, measurement of local pressure curves and oxygen saturation (with and without oxygen), before and after cardiac surgery. * Presenting author, patriciamiragon@gmail.com 1

2 On the other hand the development of an increasing number of devices for interventional procedures helps to avoid in some cases cardiac surgery (atrial septal defect ASD- closure with device, patent ductus arteriosus PDA- closure with coil or device, angioplasty with stent, etc). Finally the hybrid approach, involving the collaboration between interventionists and surgeons in the same procedure is another area of activity contributing to the increase of paediatric interventional cardiology. Patient doses during paediatric interventional procedures are a critical issue because these patients may receive high doses of ionizing radiation in the years when they have more sensible tissues and have greater potential to develop malignancies associated with x-rays. According to the International Commission on Radiological Protection (ICRP) the probability of inducing fatal cancer in the paediatric patients is 2 to 3 times greater than adults [2]. The staff involved in these studies should know the entrance dose rates for the different operation modes and protocols and the benefit on image quality when patient thickness is bigger or when electronic magnification is applied [3]. This paper presents the design and initial results of a pilot programme on patient dosimetry in paediatric interventional cardiology (PIC) in Chile, developed with the support of the International Atomic Energy Agency (IAEA) and the Institute of Public Health of Chile. Detailed results will be published in a next extended paper. Methods and Materials The methodology on national surveys recommended by the European Commission Consortium SENTINEL [4] has been followed. Contacts with the National Society of Cardiology in Chile have been established and all centres with activity in PIC have been contacted. A basic quality control of the x ray systems, including calibration of dosimetry tools, has been scheduled. In one of the centres (Calvo Mackenna Hospital) with approximately 50% of all PIC activity in Chile, a full characterization of the x ray system has been carried out to help in optimization of procedures. The collected parameters during the survey have been: procedure identification, patient age, gender, weight, height, kerma area product value (P ka ), cumulative dose at the interventional reference point (entrance surface air kerma, ESAK (K e )) [5-6], number of cine series, total number of frames and fluoroscopy time. These data will also permit the estimation of typical radiological risks of the procedures for different age groups. The procedures where divided into two groups, therapeutic and diagnostic. The total P ka for every patient corresponds to the sum of the P ka for each C-arm corrected by the transmission factor ft (0.81) of the table and the mattress for the arc A. In the case of the arc B, the adopted correction factor was 1 [7]. (P ka ) = ft * (P ka ) arc A + (P ka ) arc B (1) The entrance surface air kerma (ESAK) was calculated according to the proposed methodology described by Martinez et al. [7]. All the patient dose data presented in this study correspond to the Calvo Mackenna Hospital (Hemodynamic Laboratory) and procedures have been carried out on a biplane Siemens Axiom Artis with image intensifiers [8]. The sample includes 259 patients aged between 6 days and <16 years. Results 2

3 Results have been grouped for the following age bands: <1; 1- <5; 5- <10 and 10- <16 years. From the total sample of 259 patients, 126 were females and 133 males. The patient distribution for the different age groups was 68 for <1 year; 95 for 1- <5 years; 37 for 5-< 10 years and 59 for 10-<16 years. The frequency of the different procedures registered according to its type is shown on Table 1. The diagnostic procedures were divided into diagnostic and complex diagnostic. Conditions for this last group were the following: When the main cardiologist is working with a resident (cardiologist in training), a large number of cine series have been obtained, samples of blood have been taken with and then without oxygen, or the pathology of the patient requires image acquisition at a high number of frames per second (60 f/s). Table 1: Type of procedures performed and their frequency Type of procedure Name of procedure Frequency Diagnostic Diagnostic normal (d) 76 Diagnostic complex (D) 21 Therapeutic Aortic angioplasty (AoA) 14 Pulmonary angioplasty (PA) 44 Pulmonary angioplasty with stent (PAS) 12 Atrial septal defect (ASD) closure 8 Aortic valvuloplasty (AV) 9 Pulmonary valvuloplasty (PV) 27 Patent ductus arteriosus (PDA) closure 41 Other (OT) 7 Table 2 shows the summary of the kerma area product (P KA ) values for all the procedures. Table 2: Results for P KA (Q xx are quartiles; SD are the standard deviations) for all the procedures. Age bands (years) Q 25 P ka Q 50 P ka Q 75 P ka Mean P ka SD P ka < < < < Median values of P KA for diagnostic procedures were: 1.0; 1.6; 2.0 and 4.9 Gy.cm 2 respectively, and for therapeutic ones: 1.1; 1.3; 1.4 and 5.8 Gy.cm 2 respectively. No significant statistical differences were found between diagnostic and therapeutic groups (according to the Mann-Whitney (Wilcoxon) statistical test). In interventional procedures, there are a number of factors influencing the entrance surface air kerma, K e, and the air kerma area product, P KA, Some of these factors are even more dominant than the patient weight, thus masking the weight dependence of K e and P KA. Procedural complexity and variability between operators are dominant factors that cause large variations in K e and P KA for the same patient weight, and generally lead to a relatively poor correlation 3

4 between P KA and patient weight. The masking effect can be reduced by filtering these factors, in order to better visualize the dependence of P KA on patient weight. To achieve it, patient P KA data can be clustered into weight groups and averaged for each group into a single value. In this study, P KA values were clustered into five weight groups (< 10 kg, kg, kg, kg, and >40 kg). The results are shown in Figures 1 to 3, from which the weight dependence becomes evident and the R 2 for fits to exponential functions are: for P KA per minute fluoroscopy, for P KA per frame in cine acquisition and (R=0.9473) for total P KA versus body weight. The latter still retains a higher influence of the procedure complexity and variability in spite of the filtering, but the overall correlation is still relatively good. Figure1. Kerma area product per minute fluoroscopy versus body weight kerm a area product / tim e fluoroscopy (cgycm ^2m in y = e x R 2 = FLUOROSCOPY 0 < > 40 Weight (kg) Figure 2. Kerma area product per cine frame versus body weight ke rm a are a product / fram e (cgycm ^ 2 ) CINE ACQUISITION y = e x R 2 = < > 40 Weight (kg) 4

5 Figure 3. Kerma area product for the whole procedure versus body weight kkerma area product KAP (cgycm^ kerma area product v/s body weight y = 78,531e 0,4495x R² = 0,8974, R=0,9473 < > 40 Weight (kg) Discussion Dose values reported in this study for paediatric cardiology are lower than most of the published ones (Table 3). Table 3: Comparison between median KAP (Gy.cm 2 ) for paediatric cardiology procedures reported by different authors (figures adapted by the authors of this paper). Age bands (years) Boothroyd et al. [10] (1997) Rassow et al. [11] (2000) () Bacher et al. [12] (2005) () Martinez et al. [7] (2007) () This paper (2008) () < < < < Complex procedures may require longer fluoroscopy times and number of frames. In addition, some of them may require use of higher dose rate modes among other factors, Thus, complexity of procedures is a dominant factor on the total value kerma area product. For this reasons, there is generally large variation of kerma area product for the same patient weight, which blurs the correlation factor between the kerma area product and the patient weight and may give the appearance of a relative poor weight dependence. However, air kerma area product retains a strong dependence on patient weight and, for the same complexity, patient exposure is larger in heavier patients, as revealed by filtering the data and representing them appropriately. It is important to keep this dependence in mind especially when complex procedures are needed for heavier patients. With larger number of patient data, it should be possible to do a deeper and more detailed analysis of these correlations. In particular, the influence of the complexity of the procedures will be analyzed in a future study as it will have a relevant influence on the results, especially for the higher age band (10-5

6 <16 y). Values of kerma area product are given in table 4, sorted out from the highest to the lowest value of kerma area product Table 4: Mean values of P KA for different procedures Type of examination P ka mean Pulmonary angioplasty with stent 8.10 Diagnostic complex (D) 6.47 Closure atrial septal defect 5.14 Other 4.54 Aortic valvuloplasty 3.63 Pulmonary angioplasty 3.56 Diagnostic normal (d) 3.46 Therapeutic (global) 3.44 Aortic angioplasty 2.98 Diagnostic (global) 2.44 Pulmonary valvuloplasty 2.00 Patent ductus arteriosus closure Conclusion Our results on patient doses are in the same range (but lower) than other values published recently in Europe, suggesting that an approach to propose patient dose reference levels could also be advisable in paediatric cardiology. Dependence of air kerma area product on patient weight is, as expected, stronger for P KA per minute fluoroscopy and per acquisition frame on patient weight than the dependence for the whole procedure. Nevertheless, the dependence of patient exposure on patient weight becomes plausible by a appropriate data representation. Suggestions to optimise the practice of paediatric interventional cardiology could be proposed using these preliminary patient dose values. The next steps of the programme will include image quality evaluation and diagnostic information obtained from the different series of cine and fluoroscopy runs. Analysis of the complexity for the different procedures and its correlation with patient doses will be one of the challenges for paediatric cardiology. The goal should be to establish criteria for the best management of patient doses in paediatric fluoroscopy guided procedures. Acknowledgements This work has been partly supported by the International Atomic Energy Agency (IAEA) Technical Cooperation Project RLA/9/057 and the Institute of Public Health of Chile. References 1. Lock J.E Diagnostic and Interventional Catheterization in Congenital Heart Disease, Second Edition (Norwell, MA: Kluwer Academic) 2. ICRP. The 2007 Recommendations of the International Commission on Radiological Protection. Publication 103. Ann ICRP. 2007; 37(2-4). 3. Vano E, Ubeda C, Leyton F, Miranda P. Radiation dose and image quality for paediatric interventional cardiology.phys Med Biol. 2008;53(15): SENTINEL. Safety and Efficacy for New Techniques and Imaging using New Equipment to Support European Legislation. European Coordination Action ( ). Available 6

7 on (last access 25 July 2008). 5. ICRU 2005 Patient Dosimetry for X Rays Used in Medical Imaging ICRU Report 74 (Bethesda, MD: International Commission on Radiological Units and Measurements). 6. IEC 2000 Medical electrical equipment: Part Particular requirements for the safety of x-ray equipment for interventional procedures International Electro technical Commission (IEC) , 1st edn (Geneva, Switzerland: International Electrotechnical Commission) 7. Martinez LC, Vano E, Gutierrez F, Rodríguez C, Gilarranz R, Manzanas MJ. Patient doses from fluoroscopically guided cardiac procedures in pediatrics. Phys Med Biol. 2007; 52(16): Siemens (last accessed 13 June 2008). 9. Onnasch DG, Schröder FK, Fischer G, Kramer HH. Diagnostic reference levels and effective dose in paediatric cardiac catheterization. Br J Radiol. 2007; 80(951): Boothroyd A, McDonald E, Moores BM, Sluming V, Carty H. Radiation exposure to children during cardiac catheterization. Br J Radiol. 1997; 70: Rassow J, Schmaltz AA, Hentrich F, Streffer C. Effective doses to patients from paediatric cardiac catheterization. Br J Radiol. 2000; 73(866): Bacher K, Bogaert E, Lapere R, De Wolf D, Thierens H. Patient-specific dose and radiation risk estimation in pediatric cardiac catheterization. Circulation. 2005; 111(1): Bernardi G, Padovani R, Morocutti G, Vano E, Malisan MR, Rinuncini M, Spedicato L, Fioretti PM. Clinical and technical determinants of the complexity of percutaneous transluminal coronary angioplasty procedures: analysis in relation to radiation exposure parameters. Catheter Cardiovascular Interv. 2000; 51(1):1-9. 7