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Radiation dose to the heart in paediatric interventional cardiology
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Radiol. Prot. 35 257 (http://iopscience.iop.org/0952-4746/35/2/257) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.11.21.2 This content was downloaded on 17/05/2015 at 20:59
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Society for Radiological Protection J. Radiol. Prot. 35 (2015) 257–264
Journal of Radiological Protection doi:10.1088/0952-4746/35/2/257
Radiation dose to the heart in paediatric interventional cardiology D A Keiller1 and C J Martin2 1
Radiotherapy Physics, Beatson Oncology Centre, 1053 Great Western Road, Glasgow G12 0YN, UK 2 Health Physics, West House, Gartnavel Royal Hospital, 1055 Great Western Road, Glasgow G12 0XH, UK E-mail: [email protected] Received 4 June 2014, revised 17 January 2015 Accepted for publication 21 January 2015 Published 18 March 2015 Abstract
Recent ICRP publications have reviewed evidence for induction of heart disease. Studies suggest the threshold dose to the heart may be as low as 500 mGy. Doses to the heart from paediatric interventional procedures performed in Glasgow between April 2012 and July 2013 to correct congenital heart defects were investigated to assess the level of potential risk of cardiovascular disease. For common procedures, doses were found to be typically less than 50 mGy, with the highest dose in the period for which data are available estimated to be 330 mGy. These results suggest that any increased risk due to paediatric interventional cardiology is likely to be small, but cumulative doses over a number of years could reach the threshold for effects. Keywords: interventional cardiology, radiation dosimetry, deterministic effects S Online supplementary data available from stacks.iop.org/JRP/35/020257/mmedia (Some figures may appear in colour only in the online journal) 1. Introduction Recent ICRP publications [1, 2] have reviewed the evidence for effects of ionising radiation on the cardiovascular system. Whilst it is difficult to control for confounding effects when studying occupational and medical exposures [3, 4], the life span study of survivors of the Hiroshima and Nagasaki atomic bombs suggests that there may be a statistically significant increased risk of cardiovascular disease associated with radiation doses to the heart in excess of 500 mGy [5]. The ICRP therefore recommend that the radiation dose to the heart is considered in the process of optimisation of medical exposures [2]. 0952-4746/15/020257+8$33.00 © 2015 IOP Publishing Ltd Printed in the UK
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J. Radiol. Prot. 35 (2015) 257
Reports in the literature of results relating directly to the dose to the heart from medical exposures are limited. However, there have been numerous studies of the cumulative air kerma (CAK) at the reference point which approximates the position of the skin surface and kermaarea product (KAP) for cardiac procedures. A summary of statistics from an International Atomic Energy Agency study reports that the CAK for percutaneous coronary interventions exceeded 4.9 Gy in 5% of the 556 cases assessed [6], while another study reported a CAK for one patient from two consecutive procedures of 13 Gy [7]. Cardiac interventions in children are of particular concern as this patient group has a low underlying risk of cardiovascular disease and long life expectancy, which is important given the estimated lag period of at least 10 years after radiation exposure for induction of cardiovascular effects. These procedures also have the potential to give high radiation doses to the heart, which may be in the primary beam throughout much of the procedure. The doses received by paediatric patients undergoing interventional cardiology procedures in Glasgow were investigated in order to assess the likely risks of cardiovascular effects arising from such procedures. 2. Materials and methods The paper record of interventional cardiology procedures performed between April 2012 and July 2013 at the Royal Hospital for Sick Children in Glasgow was obtained. All procedures were performed using a Philips Allura Xper FD10/10 biplane cardiovascular x-ray system which was installed in April 2007. This record contained: patient age, height and weight; type of procedure; total incident kerma area product (KAP); total incident air kerma per tube (a biplane system with two x-ray tubes is in use); the projection angles used and the number of acquisition runs from each projection. Observations had shown that the position of the reference point used for assessment of cumulative air kerma was a reasonable approximation to the position of the skin surface for an average paediatric patient. Data were transferred to a spreadsheet and separated by procedure type, with only those procedures which were performed an average of at least once per month being included in the analysis. The procedures considered were: cardiac catheter and angiography (CC and A), 104 cases; closure of patent ductus arteriosus (PDA), 52 cases; closure of atrial septal defect (ASD), 19 cases; radiofrequency ablation (RFA), 42 cases; and balloon angioplasty 33 cases. Data were analysed to determine the parameters for a typical procedure, defined as being a hypothetical average procedure performed on a hypothetical average patient of average age, height and weight. These data were then used to perform Monte Carlo simulations using the PCXMC software package (version 2.0, STUK, Helsinki, Finland) to calculate organ doses. Simulations were performed both for typical procedures as defined above and for the procedure of each type with the highest recorded KAP. In addition to the information available from the patient records, the following parameters had to be estimated: tube potential, focus to surface distance, field size, tube filtration and field position. Tube potential was estimated from quality assurance reports to be 75 kV as this is the setting selected by the AEC for a 15 cm thick water phantom, and filtration was equivalent to 3.5 mm aluminium to which 0.4 mm copper is added for the majority of proedures. Field size was calculated from incident air kerma and KAP. For the purposes of Monte-Carlo simulation, it was assumed that all fields are square and that all fields for a given procedure are of the same size. It was assumed that fields were positioned such that the heart is at the centre of the primary beam throughout all procedures, as this represents a worst-case scenario which will tend to overestimate the radiation dose to the heart. 258
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J. Radiol. Prot. 35 (2015) 257
Table 1. Mean patient demographics for typical procedures.
Type of Procedure
Number of procedures
Patient Mean Age (years)
Mean Weight (kg)
CC and A PDA RFA Balloon angio. ASD
104 52 42 33 19
1 2 11 5 6
8.2 11.8 46 18.4 21
Table 2. Patient demographics procedures of different types giving the highest KAP.
Procedure
Age (years)
Height (cm)
Weight (kg)
CC and A PDA RFA Balloon angio. ASD
16 5 months 10 16 4
Unknown 63 Unknown 165 106
69 5.3 47.2 48.2 17
Table 3. Summary of heart doses calculated by PCXMC for common procedures giving both high dose and typical cases.
Procedure
Heart dose (typical procedure) (mGy)
Heart dose (high dose procedure) (mGy)
CC and A PDA RFA Balloon angio. ASD
43 31 13 43 23
330 130 75 175 240
The PCXMC package uses geometric phantoms to represent patients and has a series of phantoms representing patients with the following ages: newborn, 1 year, 5 years, 10 years, 15 years and adult. For high dose cases, the nearest age appropriate phantom was selected and then scaled for height and weight. For typical procedures, the phantoms were scaled for age and weight only, as patient height is not consistently recorded, especially for very young patients for whom height is hard to measure. 3. Results 3.1. Patient heart doses
Parameters for the average patient and for the patient receiving the highest dose are given in tables 1 and 2 respectively. The average and maximum heart doses from PCXMC simulations are given in table 3. The tube projection is determined by two angles: the cranio-caudal tilt and the left or right anterior oblique projection as shown in figure 1. The cranio-caudal tilt is defined as being the angle between the direction of travel of the beam and the direction normal to the head-feet axis of the patient as shown in figure 1(a). The local convention is that cranial projections described with positive numbers represent the tube being moved towards the head and pointed towards the feet (figure 1(b)). The anterior oblique projection is defined such that 259
D A Keiller and C J Martin
J. Radiol. Prot. 35 (2015) 257
Figure 1. Diagram illustrating the definition of cranio-caudal and left/right anterior oblique projections. (a) Cranio-caudal tilt. (b) PA and oblique projections. Table 4. Tube parameters used for Monte Carlo simulations.
Procedure CC and A
Mean KAP (range) (Gycm2) 5.7 (0.80–241.47)
PDA
3.19 (1.87–10.52)
RFA
4.3 (1.05–28.49)
Balloon angio 10.64 (1.33–108.98) ASD
4.95 (2.08–33.23)
Tube
Mean Cumulative Representative Skin Dose (range) Projection (mGy) Angle
Frontal Lateral Frontal
56 (5.4–1048) 44 (10.1–1199) 36 (15.8–138.5)
Lateral Frontal Lateral Frontal Lateral Frontal
37 (0.9–168.3) 32 (4.9–89.2) 23 (3.2–221.2) 83 (17.8–639.9) 65 (11.1–366) 52 (23.5–165.7)
Lateral
23 (17.7–349.1)
Cranial 35 Cranial 15 RAO 30 PA LAO90 RAO30 LAO45 LAO9,Caudal33 LAO 56 PA Cranial 20 LAO90
Representative Number of Fluoro runs — — 7 3 — — — — — 26 2 —
an angle of 0° represents a postero-anterior (PA) projection, and 90° represents a horizontal beam from the patients right or left as appropriate. The x-ray tube was underneath the table in order to reduce staff radiation doses [2]. Exposure parameters used for Monte Carlo simulations are given in table 4. 3.2. Dose coefficients
The simulations in PCXMC were extended to produce dose coefficients which give the heart dose per milligray incident air KERMA for common projections as a function of patient age, using the age standardised phantoms in the software as shown in figure 2. These coefficients were calculated using a cranio-caudal angle of 0° for typical field sizes: 9 cm × 9 cm for LAO 90° and PA; 9.5 cm × 9.5 cm for RAO 30° and 11 cm × 11 cm for LAO 45°. The relationship between heart dose and field size is shown in figure 3. 3.3. Repeat procedures
Some patients require multiple procedures, either because a therapeutic procedure fails or because the first procedure is diagnostic and indicates the requirement for further intervention. 260
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J. Radiol. Prot. 35 (2015) 257
Figure 2. Dose coefficients as a function of age.
Figure 3. Dose coefficients as a function of field size for square fields.
From the first nine months of study data, five patients who underwent multiple procedures were investigated. One patient received three RF ablations. This was not investigated further, as even in the worst case scenario, the maximum radiation dose received by the heart would be less than 250 mGy. 261
D A Keiller and C J Martin
J. Radiol. Prot. 35 (2015) 257
One patient underwent three CC and A procedures, giving an estimated heart dose of 165 mGy, and three patients underwent a CC and A procedure followed by a balloon dilatation of the arteries giving heart doses of 74, 78 and 140 mGy. 3.4. Errors and uncertainties
Sources of uncertainty in the results presented above include: • The computational model used by PCXMC which approximates organs using defined geometrical shapes scaled for age, height and weight. This fails to take into account variations in anatomy between patients • The uncertainty in tube potential given that this is varied by the automatic exposure control and is not recorded by the radiographers. Repeating simulations at 70 kV and 80 kV gives an estimated uncertainty of ± 5% • The statistical uncertainty of the Monte Carlo simulation given that only a finite number of photon histories is calculated. PCXMC estimates this as ± 2% • Uncertainty in focus to surface distance. Repeating simulations at 55 cm and 65 cm FSD gives an estimated uncertainty of ± 4% • The accuracy of the Monte Carlo code itself. A published comparison of PCXMC with the MCNP code gives a difference between the two codes of as much as 35% when calculating organ doses. The reasons for these differences were not investigated but it is suggested that differences between voxel-based phantoms and geometrical phantoms are a large contributor to this difference [8] • Only the total incident air kerma per x-ray tube and the total KAP per procedure were available, so it was assumed that the incident air kerma was divided according the acquisition runs. This gives an error if the same tube was used for several projections with acquisition runs of different lengths • Field size and position. PCXMC can only model square fields, which gives an error as fields are actually hexagonal, the size of this error is estimated geometrically as 5% . It is also assumed that the heart is always in the centre of the field, which will overestimate heart dose in some cases. Taking the quantified errors and adding in quadrature suggests the error on organ doses is of order ± 36%, being dominated by the anatomical differences between computational phantoms and the anatomy of real patients. 4. Discussion From the results presented above, it seems unlikely that a patient in Glasgow would receive a radiation dose to the heart exceeding 500 mGy in a single procedure. However, some patients undergo multiple procedures and it is possible that a patient undergoing two or more complex procedures could receive a radiation dose to the heart in excess of 500 mGy. Those patients for whom data are available who underwent multiple procedures received estimated heart radiation doses of between 70 and 170 mGy, suggesting that it would be unusual for a patient to receive a dose greater than 500 mGy even in the case of multiple procedures. However, it should be borne in mind that only patient data over a period of 15 months were studied, and that only the most commonly performed procedures were included in the analysis. It is expected that doses are lower in common procedures as a result of staff familiarity and because unusual procedures are likely to be more complex than common procedures. It is also 262
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Table 5. Heart doses derived from 90th percentile KAP data reported for a cardiac catheterisation laboratory in NCRP 2010 [6]. These assume a 9 cm × 9 cm field with a frontal tube in a PA projection and a lateral tube in left lateral projection.
Patient weight range (kg)
Patient age (years)
Frontal plane 90% KAP (Gycm2)
Lateral plane 90% KAP (Gycm2)
Heart dose (mGy)
2–4 4–10 30–40 70–85
0 1 10 16
16.4 36 93 229
15.5 30.3 107 321
205 385 727 1266
likely that the number of patients undergoing multiple procedures has been underestimated for two reasons: the short follow-up period, and the change during the study period from hospital issued patient numbers to the Scotland-wide system of CHI numbers as patient identifier. The authors are not aware of data in the literature on doses to the heart in paediatrics with which comparisons can be made, but KAP data are available. Since mean KAP results at different hospitals for similar fluoroscopic examinations vary by factors of 5 to >10 between different hospitals in the UK [9] and sources for similar procedures in different countries can be even more variable [3], it is likely that some patients will receive doses to the heart above 500 mGy. Mean KAP values from other studies for diagnostic paediatric interventional cardiac procedures of 8.6 Gycm2 [10] and 17 Gycm2 [11]; and for therapeutic procedures up to 29.5 Gycm2 [11] are in a similar range, if not higher than results in this study (table 4). There are also some data on cumulative skin doses [12] which are again in a similar range if not higher than the doses presented in this study. Dose conversion coefficients have been reported for adult phantoms [13, 14], and are of similar magnitude to those reported in this study for 15 and 18 year old patients. In NCRP report 168 [6], a summary is given of radiation data for all procedures performed in a children’s hospital over the period 2005–2009. This gives values for the KAPs with the frontal (F) and lateral (L) tubes for different weight ranges as shown in table 5. Heart doses were estimated from 90 th percentile of the KAP distribution reported in the study using the dose conversion coefficients presented above. It was assumed that all exposures used a 9 cm × 9 cm field with the frontal tube in a PA projection and the lateral tube in a left lateral projection, as this was the approximate average field size used in Glasgow and these projections are used most frequently. The results suggested that some of the more exposed patients aged older than 10 years are likely to have received doses to the heart above the threshold for cardiac effects (table 5). These procedures are likely to have been performed in order to address life threatening problems, but results indicate the need to optimise techniques in order to maintain radiation doses as low as reasonably practicable. The majority of the procedures in the present study recorded heart doses substantially below the values showin in table 5. The only procedure that approached a similar dose level was the 330 mGy received by a 16 year old patient undergoing cardiac catheterisation and angiography. 5. Conclusions and further work The highest radiation dose to the heart recorded in this study of paediatric interventional cardiology procedures performed in Glasgow was 330 mGy. This is two thirds of the threshold for cardiovascular disease suggested by the ICRP [1], and indicates that patients are only likely to receive doses exceeding this threshold in very rare circumstances. This means that it is unlikely that there will be a statistically significant increase in risk of cardiovascular disease 263
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J. Radiol. Prot. 35 (2015) 257
resulting from these procedures. Nevertheless, there is likely to be a risk for a small number of patients and optimisation of the imaging performance of interventional cardiology units is important in order to minimise this. KAP data from the NCRP study [6] indicates that heart dose for some patients over 10 years old at some hospitals can exceed 500 mGy. Further studies of heart doses from paediatric interventional cardiology in other hospitals would enable the size of any potential problem to be gauged. In addition, further work to quantify the radiation doses and risks from patients undergoing multiple procedures, especially when the time between procedures is greater then a few months, is required in order to fully evaluate risks from paediatric interventional cardiology. References [1] ICRP 2013 Statement on tissue reactions/early and late effects of radiation in normal tissues and organs: threshold doses for tissue reactions in a radiation protection context ICRP Publication 118 Annals of the ICRP vol 41 (Oxford: Elsevier) [2] ICRP 2013 Radiation protection in cardiology ICRP Publication 120 Annals of the ICRP vol 42 (Oxford: Elsevier) [3] UNSCEAR 2006 Report volume 1 effects of ionising radiation Technical Report United Nations Scientific Committee on the Effects of Atomic Radiation [4] IAGIR 2010 Rce-16: circulatory disease risk: report of the independent advisory group on ionising radiation Technical Report Health Protection Agency [5] Shimizu Y et al 2010 Radiation exposure and circulatory disease risk: Hiroshima and nagasaki atomic bomb survivor data 1950–2003 Br. Med. J. 340 b5349 [6] National Council on Radiation Protection and Measurement 2010 Radiation Dose Management and for Fluoroscopically Guided Interventional Procedures (NCRP Report vol 168) (Bethesda, MD: National Council on Radiation Protection and Measurements) [7] Vano E, Escaned J, Vano-Galvan S, Fernandez J M and Galvan C 2013 Importance of a patient dosimetry and clinical follow-up program in the detection of radiodermatitis after long percutaneous coronary interventions Cardiacvasc. Interv. Radiol. 36 330–7 [8] Schultz F W, Geleijns J, Spoelstra F M and Zoetelief J 2003 Monte-Carlo calculations for assessment of radiation dose to patients with congenital heart defects and to staff during cardiac catheterisations Br. J. Radiol. 76 638–47 [9] Hart D, Hillier M C and Shrimpton P C 2012 HPA report CRCE-034: doses to patients from radiographic and fluoroscopic x-ray imagin procedures in the UK: 2010 review Technical Report Health Protection Agency [10] Martinez L C et al 2007 Patient doses from fluoroscopically guided cardiac procedures in paediatrics Phys. Med. Biol. 52 4749–59 [11] Chida K, Ohno T, Kakizaki S, Takegawa M, Yuuki H, Nakada M, Takahashi S and Zuguchi M 2010 Radiation dose to the paediatric cardiac catheterization and intervention patient J. Roentgenol. 195 1175–9 [12] El Sayed M H et al 2012 Radiation exposure in children during the current era of paediatric cardiac intervention Pediatr. Cardiol. 33 27–35 [13] Park S H et al 2008 Dose conversion coefficients calculated using tomographic phantom, KTMAN-2, for x-ray examination of cardiac catheterisation Radiat. Prot. Dosim. 128 351–8 [14] Johnson P et al 2009 The influence of patient size on dose conversion coefficients: a hybrid phantom study for adult cardiac catheterization Physics 54 3613–29
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Radiation dose to the heart in paediatric interventional cardiology
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Radiol. Prot. 35 257 (http://iopscience.iop.org/0952-4746/35/2/257) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.11.21.2 This content was downloaded on 17/05/2015 at 20:59
Please note that terms and conditions apply.
Society for Radiological Protection J. Radiol. Prot. 35 (2015) 257–264
Journal of Radiological Protection doi:10.1088/0952-4746/35/2/257
Radiation dose to the heart in paediatric interventional cardiology D A Keiller1 and C J Martin2 1
Radiotherapy Physics, Beatson Oncology Centre, 1053 Great Western Road, Glasgow G12 0YN, UK 2 Health Physics, West House, Gartnavel Royal Hospital, 1055 Great Western Road, Glasgow G12 0XH, UK E-mail: [email protected] Received 4 June 2014, revised 17 January 2015 Accepted for publication 21 January 2015 Published 18 March 2015 Abstract
Recent ICRP publications have reviewed evidence for induction of heart disease. Studies suggest the threshold dose to the heart may be as low as 500 mGy. Doses to the heart from paediatric interventional procedures performed in Glasgow between April 2012 and July 2013 to correct congenital heart defects were investigated to assess the level of potential risk of cardiovascular disease. For common procedures, doses were found to be typically less than 50 mGy, with the highest dose in the period for which data are available estimated to be 330 mGy. These results suggest that any increased risk due to paediatric interventional cardiology is likely to be small, but cumulative doses over a number of years could reach the threshold for effects. Keywords: interventional cardiology, radiation dosimetry, deterministic effects S Online supplementary data available from stacks.iop.org/JRP/35/020257/mmedia (Some figures may appear in colour only in the online journal) 1. Introduction Recent ICRP publications [1, 2] have reviewed the evidence for effects of ionising radiation on the cardiovascular system. Whilst it is difficult to control for confounding effects when studying occupational and medical exposures [3, 4], the life span study of survivors of the Hiroshima and Nagasaki atomic bombs suggests that there may be a statistically significant increased risk of cardiovascular disease associated with radiation doses to the heart in excess of 500 mGy [5]. The ICRP therefore recommend that the radiation dose to the heart is considered in the process of optimisation of medical exposures [2]. 0952-4746/15/020257+8$33.00 © 2015 IOP Publishing Ltd Printed in the UK
257
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J. Radiol. Prot. 35 (2015) 257
Reports in the literature of results relating directly to the dose to the heart from medical exposures are limited. However, there have been numerous studies of the cumulative air kerma (CAK) at the reference point which approximates the position of the skin surface and kermaarea product (KAP) for cardiac procedures. A summary of statistics from an International Atomic Energy Agency study reports that the CAK for percutaneous coronary interventions exceeded 4.9 Gy in 5% of the 556 cases assessed [6], while another study reported a CAK for one patient from two consecutive procedures of 13 Gy [7]. Cardiac interventions in children are of particular concern as this patient group has a low underlying risk of cardiovascular disease and long life expectancy, which is important given the estimated lag period of at least 10 years after radiation exposure for induction of cardiovascular effects. These procedures also have the potential to give high radiation doses to the heart, which may be in the primary beam throughout much of the procedure. The doses received by paediatric patients undergoing interventional cardiology procedures in Glasgow were investigated in order to assess the likely risks of cardiovascular effects arising from such procedures. 2. Materials and methods The paper record of interventional cardiology procedures performed between April 2012 and July 2013 at the Royal Hospital for Sick Children in Glasgow was obtained. All procedures were performed using a Philips Allura Xper FD10/10 biplane cardiovascular x-ray system which was installed in April 2007. This record contained: patient age, height and weight; type of procedure; total incident kerma area product (KAP); total incident air kerma per tube (a biplane system with two x-ray tubes is in use); the projection angles used and the number of acquisition runs from each projection. Observations had shown that the position of the reference point used for assessment of cumulative air kerma was a reasonable approximation to the position of the skin surface for an average paediatric patient. Data were transferred to a spreadsheet and separated by procedure type, with only those procedures which were performed an average of at least once per month being included in the analysis. The procedures considered were: cardiac catheter and angiography (CC and A), 104 cases; closure of patent ductus arteriosus (PDA), 52 cases; closure of atrial septal defect (ASD), 19 cases; radiofrequency ablation (RFA), 42 cases; and balloon angioplasty 33 cases. Data were analysed to determine the parameters for a typical procedure, defined as being a hypothetical average procedure performed on a hypothetical average patient of average age, height and weight. These data were then used to perform Monte Carlo simulations using the PCXMC software package (version 2.0, STUK, Helsinki, Finland) to calculate organ doses. Simulations were performed both for typical procedures as defined above and for the procedure of each type with the highest recorded KAP. In addition to the information available from the patient records, the following parameters had to be estimated: tube potential, focus to surface distance, field size, tube filtration and field position. Tube potential was estimated from quality assurance reports to be 75 kV as this is the setting selected by the AEC for a 15 cm thick water phantom, and filtration was equivalent to 3.5 mm aluminium to which 0.4 mm copper is added for the majority of proedures. Field size was calculated from incident air kerma and KAP. For the purposes of Monte-Carlo simulation, it was assumed that all fields are square and that all fields for a given procedure are of the same size. It was assumed that fields were positioned such that the heart is at the centre of the primary beam throughout all procedures, as this represents a worst-case scenario which will tend to overestimate the radiation dose to the heart. 258
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J. Radiol. Prot. 35 (2015) 257
Table 1. Mean patient demographics for typical procedures.
Type of Procedure
Number of procedures
Patient Mean Age (years)
Mean Weight (kg)
CC and A PDA RFA Balloon angio. ASD
104 52 42 33 19
1 2 11 5 6
8.2 11.8 46 18.4 21
Table 2. Patient demographics procedures of different types giving the highest KAP.
Procedure
Age (years)
Height (cm)
Weight (kg)
CC and A PDA RFA Balloon angio. ASD
16 5 months 10 16 4
Unknown 63 Unknown 165 106
69 5.3 47.2 48.2 17
Table 3. Summary of heart doses calculated by PCXMC for common procedures giving both high dose and typical cases.
Procedure
Heart dose (typical procedure) (mGy)
Heart dose (high dose procedure) (mGy)
CC and A PDA RFA Balloon angio. ASD
43 31 13 43 23
330 130 75 175 240
The PCXMC package uses geometric phantoms to represent patients and has a series of phantoms representing patients with the following ages: newborn, 1 year, 5 years, 10 years, 15 years and adult. For high dose cases, the nearest age appropriate phantom was selected and then scaled for height and weight. For typical procedures, the phantoms were scaled for age and weight only, as patient height is not consistently recorded, especially for very young patients for whom height is hard to measure. 3. Results 3.1. Patient heart doses
Parameters for the average patient and for the patient receiving the highest dose are given in tables 1 and 2 respectively. The average and maximum heart doses from PCXMC simulations are given in table 3. The tube projection is determined by two angles: the cranio-caudal tilt and the left or right anterior oblique projection as shown in figure 1. The cranio-caudal tilt is defined as being the angle between the direction of travel of the beam and the direction normal to the head-feet axis of the patient as shown in figure 1(a). The local convention is that cranial projections described with positive numbers represent the tube being moved towards the head and pointed towards the feet (figure 1(b)). The anterior oblique projection is defined such that 259
D A Keiller and C J Martin
J. Radiol. Prot. 35 (2015) 257
Figure 1. Diagram illustrating the definition of cranio-caudal and left/right anterior oblique projections. (a) Cranio-caudal tilt. (b) PA and oblique projections. Table 4. Tube parameters used for Monte Carlo simulations.
Procedure CC and A
Mean KAP (range) (Gycm2) 5.7 (0.80–241.47)
PDA
3.19 (1.87–10.52)
RFA
4.3 (1.05–28.49)
Balloon angio 10.64 (1.33–108.98) ASD
4.95 (2.08–33.23)
Tube
Mean Cumulative Representative Skin Dose (range) Projection (mGy) Angle
Frontal Lateral Frontal
56 (5.4–1048) 44 (10.1–1199) 36 (15.8–138.5)
Lateral Frontal Lateral Frontal Lateral Frontal
37 (0.9–168.3) 32 (4.9–89.2) 23 (3.2–221.2) 83 (17.8–639.9) 65 (11.1–366) 52 (23.5–165.7)
Lateral
23 (17.7–349.1)
Cranial 35 Cranial 15 RAO 30 PA LAO90 RAO30 LAO45 LAO9,Caudal33 LAO 56 PA Cranial 20 LAO90
Representative Number of Fluoro runs — — 7 3 — — — — — 26 2 —
an angle of 0° represents a postero-anterior (PA) projection, and 90° represents a horizontal beam from the patients right or left as appropriate. The x-ray tube was underneath the table in order to reduce staff radiation doses [2]. Exposure parameters used for Monte Carlo simulations are given in table 4. 3.2. Dose coefficients
The simulations in PCXMC were extended to produce dose coefficients which give the heart dose per milligray incident air KERMA for common projections as a function of patient age, using the age standardised phantoms in the software as shown in figure 2. These coefficients were calculated using a cranio-caudal angle of 0° for typical field sizes: 9 cm × 9 cm for LAO 90° and PA; 9.5 cm × 9.5 cm for RAO 30° and 11 cm × 11 cm for LAO 45°. The relationship between heart dose and field size is shown in figure 3. 3.3. Repeat procedures
Some patients require multiple procedures, either because a therapeutic procedure fails or because the first procedure is diagnostic and indicates the requirement for further intervention. 260
D A Keiller and C J Martin
J. Radiol. Prot. 35 (2015) 257
Figure 2. Dose coefficients as a function of age.
Figure 3. Dose coefficients as a function of field size for square fields.
From the first nine months of study data, five patients who underwent multiple procedures were investigated. One patient received three RF ablations. This was not investigated further, as even in the worst case scenario, the maximum radiation dose received by the heart would be less than 250 mGy. 261
D A Keiller and C J Martin
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One patient underwent three CC and A procedures, giving an estimated heart dose of 165 mGy, and three patients underwent a CC and A procedure followed by a balloon dilatation of the arteries giving heart doses of 74, 78 and 140 mGy. 3.4. Errors and uncertainties
Sources of uncertainty in the results presented above include: • The computational model used by PCXMC which approximates organs using defined geometrical shapes scaled for age, height and weight. This fails to take into account variations in anatomy between patients • The uncertainty in tube potential given that this is varied by the automatic exposure control and is not recorded by the radiographers. Repeating simulations at 70 kV and 80 kV gives an estimated uncertainty of ± 5% • The statistical uncertainty of the Monte Carlo simulation given that only a finite number of photon histories is calculated. PCXMC estimates this as ± 2% • Uncertainty in focus to surface distance. Repeating simulations at 55 cm and 65 cm FSD gives an estimated uncertainty of ± 4% • The accuracy of the Monte Carlo code itself. A published comparison of PCXMC with the MCNP code gives a difference between the two codes of as much as 35% when calculating organ doses. The reasons for these differences were not investigated but it is suggested that differences between voxel-based phantoms and geometrical phantoms are a large contributor to this difference [8] • Only the total incident air kerma per x-ray tube and the total KAP per procedure were available, so it was assumed that the incident air kerma was divided according the acquisition runs. This gives an error if the same tube was used for several projections with acquisition runs of different lengths • Field size and position. PCXMC can only model square fields, which gives an error as fields are actually hexagonal, the size of this error is estimated geometrically as 5% . It is also assumed that the heart is always in the centre of the field, which will overestimate heart dose in some cases. Taking the quantified errors and adding in quadrature suggests the error on organ doses is of order ± 36%, being dominated by the anatomical differences between computational phantoms and the anatomy of real patients. 4. Discussion From the results presented above, it seems unlikely that a patient in Glasgow would receive a radiation dose to the heart exceeding 500 mGy in a single procedure. However, some patients undergo multiple procedures and it is possible that a patient undergoing two or more complex procedures could receive a radiation dose to the heart in excess of 500 mGy. Those patients for whom data are available who underwent multiple procedures received estimated heart radiation doses of between 70 and 170 mGy, suggesting that it would be unusual for a patient to receive a dose greater than 500 mGy even in the case of multiple procedures. However, it should be borne in mind that only patient data over a period of 15 months were studied, and that only the most commonly performed procedures were included in the analysis. It is expected that doses are lower in common procedures as a result of staff familiarity and because unusual procedures are likely to be more complex than common procedures. It is also 262
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Table 5. Heart doses derived from 90th percentile KAP data reported for a cardiac catheterisation laboratory in NCRP 2010 [6]. These assume a 9 cm × 9 cm field with a frontal tube in a PA projection and a lateral tube in left lateral projection.
Patient weight range (kg)
Patient age (years)
Frontal plane 90% KAP (Gycm2)
Lateral plane 90% KAP (Gycm2)
Heart dose (mGy)
2–4 4–10 30–40 70–85
0 1 10 16
16.4 36 93 229
15.5 30.3 107 321
205 385 727 1266
likely that the number of patients undergoing multiple procedures has been underestimated for two reasons: the short follow-up period, and the change during the study period from hospital issued patient numbers to the Scotland-wide system of CHI numbers as patient identifier. The authors are not aware of data in the literature on doses to the heart in paediatrics with which comparisons can be made, but KAP data are available. Since mean KAP results at different hospitals for similar fluoroscopic examinations vary by factors of 5 to >10 between different hospitals in the UK [9] and sources for similar procedures in different countries can be even more variable [3], it is likely that some patients will receive doses to the heart above 500 mGy. Mean KAP values from other studies for diagnostic paediatric interventional cardiac procedures of 8.6 Gycm2 [10] and 17 Gycm2 [11]; and for therapeutic procedures up to 29.5 Gycm2 [11] are in a similar range, if not higher than results in this study (table 4). There are also some data on cumulative skin doses [12] which are again in a similar range if not higher than the doses presented in this study. Dose conversion coefficients have been reported for adult phantoms [13, 14], and are of similar magnitude to those reported in this study for 15 and 18 year old patients. In NCRP report 168 [6], a summary is given of radiation data for all procedures performed in a children’s hospital over the period 2005–2009. This gives values for the KAPs with the frontal (F) and lateral (L) tubes for different weight ranges as shown in table 5. Heart doses were estimated from 90 th percentile of the KAP distribution reported in the study using the dose conversion coefficients presented above. It was assumed that all exposures used a 9 cm × 9 cm field with the frontal tube in a PA projection and the lateral tube in a left lateral projection, as this was the approximate average field size used in Glasgow and these projections are used most frequently. The results suggested that some of the more exposed patients aged older than 10 years are likely to have received doses to the heart above the threshold for cardiac effects (table 5). These procedures are likely to have been performed in order to address life threatening problems, but results indicate the need to optimise techniques in order to maintain radiation doses as low as reasonably practicable. The majority of the procedures in the present study recorded heart doses substantially below the values showin in table 5. The only procedure that approached a similar dose level was the 330 mGy received by a 16 year old patient undergoing cardiac catheterisation and angiography. 5. Conclusions and further work The highest radiation dose to the heart recorded in this study of paediatric interventional cardiology procedures performed in Glasgow was 330 mGy. This is two thirds of the threshold for cardiovascular disease suggested by the ICRP [1], and indicates that patients are only likely to receive doses exceeding this threshold in very rare circumstances. This means that it is unlikely that there will be a statistically significant increase in risk of cardiovascular disease 263
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resulting from these procedures. Nevertheless, there is likely to be a risk for a small number of patients and optimisation of the imaging performance of interventional cardiology units is important in order to minimise this. KAP data from the NCRP study [6] indicates that heart dose for some patients over 10 years old at some hospitals can exceed 500 mGy. Further studies of heart doses from paediatric interventional cardiology in other hospitals would enable the size of any potential problem to be gauged. In addition, further work to quantify the radiation doses and risks from patients undergoing multiple procedures, especially when the time between procedures is greater then a few months, is required in order to fully evaluate risks from paediatric interventional cardiology. References [1] ICRP 2013 Statement on tissue reactions/early and late effects of radiation in normal tissues and organs: threshold doses for tissue reactions in a radiation protection context ICRP Publication 118 Annals of the ICRP vol 41 (Oxford: Elsevier) [2] ICRP 2013 Radiation protection in cardiology ICRP Publication 120 Annals of the ICRP vol 42 (Oxford: Elsevier) [3] UNSCEAR 2006 Report volume 1 effects of ionising radiation Technical Report United Nations Scientific Committee on the Effects of Atomic Radiation [4] IAGIR 2010 Rce-16: circulatory disease risk: report of the independent advisory group on ionising radiation Technical Report Health Protection Agency [5] Shimizu Y et al 2010 Radiation exposure and circulatory disease risk: Hiroshima and nagasaki atomic bomb survivor data 1950–2003 Br. Med. J. 340 b5349 [6] National Council on Radiation Protection and Measurement 2010 Radiation Dose Management and for Fluoroscopically Guided Interventional Procedures (NCRP Report vol 168) (Bethesda, MD: National Council on Radiation Protection and Measurements) [7] Vano E, Escaned J, Vano-Galvan S, Fernandez J M and Galvan C 2013 Importance of a patient dosimetry and clinical follow-up program in the detection of radiodermatitis after long percutaneous coronary interventions Cardiacvasc. Interv. Radiol. 36 330–7 [8] Schultz F W, Geleijns J, Spoelstra F M and Zoetelief J 2003 Monte-Carlo calculations for assessment of radiation dose to patients with congenital heart defects and to staff during cardiac catheterisations Br. J. Radiol. 76 638–47 [9] Hart D, Hillier M C and Shrimpton P C 2012 HPA report CRCE-034: doses to patients from radiographic and fluoroscopic x-ray imagin procedures in the UK: 2010 review Technical Report Health Protection Agency [10] Martinez L C et al 2007 Patient doses from fluoroscopically guided cardiac procedures in paediatrics Phys. Med. Biol. 52 4749–59 [11] Chida K, Ohno T, Kakizaki S, Takegawa M, Yuuki H, Nakada M, Takahashi S and Zuguchi M 2010 Radiation dose to the paediatric cardiac catheterization and intervention patient J. Roentgenol. 195 1175–9 [12] El Sayed M H et al 2012 Radiation exposure in children during the current era of paediatric cardiac intervention Pediatr. Cardiol. 33 27–35 [13] Park S H et al 2008 Dose conversion coefficients calculated using tomographic phantom, KTMAN-2, for x-ray examination of cardiac catheterisation Radiat. Prot. Dosim. 128 351–8 [14] Johnson P et al 2009 The influence of patient size on dose conversion coefficients: a hybrid phantom study for adult cardiac catheterization Physics 54 3613–29
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