Wilmslow Road, Withington, Manchester M20 9BX and "University Department of Cardiology, Manchester. Royal Infirmary. (Received July 1985 and in revised ...
1986, The British Journal of Radiology, 59, 359-363
APRIL 1986
Radiation doses and somatic risk to patients during cardiac radiological procedures By K. Faulkner, B.Sc, A.R.C.S., M.Sc, Ph.D., *H. G. Love, M.B., M.R.C.P., J. K. Sweeney, B.A. and *R. A. Bardsley, D.C.R.R., S.R.R. Regional Department of Medical Physics and Bioengineering, Christie Hospital and Holt Radium Institute, Wilmslow Road, Withington, Manchester M20 9BX and "University Department of Cardiology, Manchester Royal Infirmary (Received July 1985 and in revised form September 1985)
ABSTRACT
The radiation dose to a series of adult and paediatric patients undergoing cardiac catheterisations and adults having percutaneous transluminal coronary angioplasty has been measured/determined directly using lithium fluoride thermoluminescent dosemeters and indirectly using an air ionisation chamber which indicated exposure-area product. Somatic and genetic risks are estimated from the dosimetry results. It is suggested that the magnitude of the radiation hazard is negligible compared with other clinical hazards associated with these procedures.
It is well known that high radiation doses to the patient result from cardiac catheterisations carried out under fiuoroscopic control. Scattered radiation dose to the thyroid has been studied by a number of authors (Kaude & Svahn, 1974; Rowley, 1974). Later work also included gonadal doses (Martin & Olsen, 1980). Another approach to determining patient doses involves measurements of exposure-area product (Rcm2) during cardiac radiological procedures (Kaude & Svahn, 1974; Rowley, 1974, Gustafsson & Mortensson, 1976). Modern equipment with a U-arm mount for both the X-ray tube and image intensifier has been introduced since these earlier surveys. As cardiac radiological techniques have also changed with the introduction of this new design, it was decided to carry out a new study involving both adult and paediatric patients undergoing cardiac catheterisations. In order to compare the results of this new study with previous work, it included measurements of both exposure-area product and scattered radiation dose to a number of organs. This dosimetry information will also enable an assessment of any future changes in either radiological equipment or examination technique to be made. Percutaneous transluminal coronary angioplasty (PTCA) is a recently introduced method of clearing obstructed coronary arteries. Since long screening and cinefluorography times are associated with this technique, high radiation doses to patients may be expected. Consequently, it was decided to determine the radiation dose to patients undergoing this procedure. In common with other radiological examinations there is inevitably a certain degree of risk associated with the use of ionising radiation. The patient dosimetry results have been employed to estimate the
somatic and genetic risks from the radiation dose resulting from cardiac catheterisations and angioplasties. MATERIALS AND METHODS
All cardiac radiological investigations were performed on equipment with an undercouch tube/overcouch image intensifier configuration. The image intensifier had two input field sizes (22 cm and 15 cm) and both operated under automatic exposure-rate control (exposure-rate in air at image intensifier surface 50 fiR/s). Under automatic exposure-rate control, both the tube potential and tube current were adjusted to maintain a constant exposure-rate at the input surface of the image intensifier. Typical technique factors for adults were in the range 70-110 kV and 1.4—7 mA. The corresponding technique factors for paediatric patients varied between 65 and 85 kV, and 0.5 to 1.1 mA. Cinefluorography was also performed with a pulsed X-ray output at 50 frames/second (exposure 24 /iR/frame). Automatic exposure and brightness control were employed during cinefluorography which, for adult patients, resulted in a tube potential in the range 70-120 kV and a tube current of 180-720 mA. For paediatric patients the corresponding technique factors were 65-85 kV and 130-260 mA. At the end of each examination a note was made of the total fluoroscopy and cinefluorography times. Radiation doses were measured at a number of anatomical positions using lithium fluoride thermoluminescent dosemeters (TLDs). A calibration factor for the range of diagnostic beam qualities used was experimentally determined and applied to the dosemeter readings. Dosemeters were attached to the patient using surgical tape at the following anatomical positions: (1) xiphisternum, (2) right mid-auxiliary line at level of xiphisternum, (3) left mid-auxiliary line at level of xiphisternum, (4) spinous process of ninth thoracic vertebrae, (5) thyroid, (6) iliac crest, and (7) thigh (not children). The last two dosemeters were used to monitor gonadal doses.
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TABLE I SUMMARY OF SCREENING AND CINEFLUOROGRAPHY TIMES
Type of dosemeter
Examination
Thermoluminescent Adult dosimetry (TLD) catheterisations Paediatric catheterisations PTCA Exposure-area product
Adult catheterisations Paediatric catheterisations PTCA
Number of patients
Screening time (min)
Cinefluorography time (s)
Mean
Mean
Standard deviation
Range
Standard Range deviation
8
7.2
3.0
3.0-12.0
53
22
37-79
7
6.0
3.1
2.6-10.6
22
19
5-60
15
26.9
16.7
8.7-69.5
58
31
25-158
17
6.3
3.5
1.6-12.6
53
18
25-92
6
6.4
1.7
4.2- 9.2
25
16
0-55
7
29.5
19.9
10-69.5
46
16
37-75
After irradiation the exposed dosemeters were attached to a specially designed form on which the age and sex of each patient was recorded. Initially, all dosimetry measurements on the angioplasty patients were performed using TLD, although for later patients a transparent ionisation chamber (PTW Diamentor System) attached to the X-ray tube was used in addition. This ionisation chamber measured exposure-area product and had been calibrated for these X-ray energies prior to use. The patients (both adult and paediatric), who had cardiac catheterisations, were monitored with only one type of dosimetry method, initially with TLD and later using the Diamentor. No patients having cardiac catheterisations were monitored using both methods. The Diamentor measurements of exposure-area product were converted into energy imparted (in J) using data presented by Shrimpton et al (1984). This conversion requires a knowledge of both the X-ray tube applied potential and the total filtration of the X-ray beam, which was estimated from the half-value layer measurement at a measured tube potential and was equivalent to 3.4 mm aluminium. RESULTS AND DISCUSSION
A summary of the number of patients monitored, the average screening and cinefluorography times for each type of investigation, is given in Table I. It may be seen from this table that the respective screening and cinefluorography times are similar for the same examination using both types of dosemeters. The screening times during angioplasties are much higher than adult catheterisations, which reflects the difficulties attendant in manoeuvering the guide wire and balloon catheter into position during the former procedure. Perhaps future developments in the design and construction of guide wires and balloon catheters will reduce screening and cinefluorography times, thereby reducing the radiation dose to both patients and staff.
Table II is a summary of the dosimetry results for all patients monitored. The mean, standard deviation, and range of both the Diamentor reading and the doses measured at each separate position are included in this table. The total of the four chest-level dosemeters has also been quoted, as this will partially account for changes in projection direction. During the course of some of the examinations, patient movement caused one or other of the TLDs to become detached from the patient. Consequently these dosemeters were excluded from the study, which meant for some patients it was impossible to determine the total of the four dosemeter readings. Hence there is a discrepancy between the mean of the total of the four dosemeters and the total of the four means for each position at chest level. The mean value of exposure-area product was 490 Rein2 for paediatric cardiac catheterisations, which is similar to that reported in an earlier study (Rowley, 1974), but slightly higher than that quoted in another study (Gustafsson & Mortensson, 1976) for children aged between 1 and 13 years, which is similar to the age range of the patients monitored here. The mean exposure-area product for adult cardiac catheterisations was somewhat higher than had been previously measured (Ardran et al, 1970; Rowley, 1974). Both the dose-rate in air at the image intensifier input surface and the dose per frame during cinefluorography were within an acceptable range. Given that the average screening times were shorter than in an earlier study (Ardran et al, 1970), then the increase in measured exposure-area product must arise from considerably longer cinefluorography times. As cinefluorography is a major contribution to patient exposure in cardiac catheterisations, due to the high dose-rate, any attempt at patient dose reduction should be concentrated on this side of the examination. Mean measured exposure-area product readings are similar for angioplasties and adult catheterisations, although the range of readings is far greater for
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APRIL
1986
Radiation doses and somatic risk to patients during cardiac radiological procedures TABLE II SUMMARY OF DOSIMETRY RESULTS (MGY)
Paediatric cardiac catheterisation
PTCA
Standard Range deviation
Mean
Standard Range deviation
Mean
Standard Range deviation
59.2 Total of four trunk dosemeters 5.9 Xiphisternum Right mid-auxiliary line 20.3 Left mid-auxiliary line 13.8 Spinous process T9 19.4 Thyroid 1.7 0.4 Iliac crest 0.1 Thigh
40.8
66.3
57.9
5.2-145.7
151.9
76.0
50.6-298.1
15.3 98.8 25.7 75.9 2.7 0.9 0.2
4.2 20.8 4.4 37.0 1.2 0.7 —
4.9 24.2 5.0 45.5 1.3 0.7 —
0.3- 13.7 0.1-119.7 0.2- 12.2 4.6- 73.5 0.2- 3.7 0.0- 1.8 —
10.5 75.7 4.7 55.1 2.0 1.2 0.3
7.0 85.6 4.4 64.6 0.9 1.1 0.2
2.8- 26.7 0.6-280.7 0.6- 18.1 3.3-200.7 0.8- 4.1 0.0- 3.2 0.0- 0.8
Diamentor reading (Rcm2) Energy imparted (mJ)
4800
3010
180-12500
490
360
150-570
3270
1440
2320-5780
473.0
296.8
17.7-1232.6 42.9
31.5
13.1-49.9
322.5
142.0
228.8-570.0
Dosemeters
Adult cardiac catheterisation Mean
4.6 23.8 9.3 32.8 0.8 0.3 0.1
21.1-127.1 2.81.42.83.60.80.10.1-
catheterisations. Average screening times are longer for angioplasties when compared with adult cardiac catheterisations, but the cinefluorography times are approximately equal. Thus, as the mean measured exposure-area product is lower for angioplasties, it would imply that largerfieldsizes are employed in adult cardiac catheterisations. For all three types of investigation, the total of the four trunk dosemeter readings was considerably less than that estimated from the average exposure-area product and the area of the X-ray field. In general, each procedure involves the use of many projections. There will not be a dosemeter in the primary beam, except for the PA and lateral projections. It is impractical to use extra dosemeters, as a large number would be necessary without an advance knowledge of the projection directions to be used. The overall effect of a multiplicity of projection directions is to reduce the maximum skin dose at a point, as there is a considerable difference in skin-dose distributions for these projections. The radiation doses to the other three sites arise from scattered radiation, as these dosemeters were placed on the exit side of the patient, and the effect of any tube leakage radiation on the readings will be negligible. The surface dose will therefore be a good indicator of the dose to the corresponding organ within the body. The radiation dose to the thyroid for both adult and paediatric catheterisations, given in Table II, is considerably lower than that stated in other publications (Gustafsson & Mortensson, 1976; Martin & Olson 1980). Whilst the thyroid dose during angioplasty is higher than for both adult and paediatric cardiac catheterisations, it is still lower than the thyroid dose quoted in the above publications. In the main, the radiation dose to both the ovaries and testes during adult and paediatric cardiac cathe-
terisations quoted in Table II compare favourably with previously published results (Gough et al, 1968; Kaude & Svahn, 1974; Langmead & Wall, 1976; Martin & Olson, 1980). Gonadal doses during angioplasty are approximately three times higher than the dose received by adult patients having cardiac catheterisations. Higher gonadal doses will arise from differences in the cardiologist's technique, as the movement of the guide wire and balloon catheter from the femoral artery to the heart will be observed in angioplasty using fluoroscopy. Also quoted in Table II are the calculated values for the energy imparted during each type of investigation, as the energy imparted correlates with the total somatic risk. The energy imparted has been calculated from the Diamentor reading using previously published factors (Shrimpton et al, 1984). Patients having an angioplasty will, in general, also have at least two cardiac catheterisations—one before the angioplasty and one 6 months after as a follow-up. Consequently, when assessing the total dose to a patient having an angioplasty, the contribution from two cardiac catheterisations should also be included. Somatic and genetic risk factors have been published by the International Commission on Radiological Protection (ICRP, 1977). The risk factors averaged for males and females over all ages have been employed to calculate risks. In this study a genetic risk factor of 4xlO~ 3 Sv"1 has been used for adults, which is an average over all the population and refers to the effect manifest in the first two generations, whilst for paediatric patients a factor of 1 x 10 ~2 Sv"1 was employed, since it is assumed that all such exposures were genetically significant. Genetic risks were estimated by multiplying the mean ovary and testes dose by the above genetic risk factor. For paediatric patients, testes dose was estimated from the dose to the
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59, No. 700 K. Faulkner, H. G. Love, J. K. Sweeney and R. A. Bardsley TABLE III ESTIMATED SOMATIC RISKS (FATAL MALIGNANCIES PER EXAMINATION) X 10 ~ 6 AND *GENETIC RISKS X 10 ~ 6
Risk
Adult Paediatric PTCA cardiac cardiac catheterisation catheterisation
Leukaemia Bone cancer Breast cancer Lung cancer Thyroid cancer Other cancer Total somatic Genetic: male* Genetic: female*
8 2 15 31
