Keywords: Radiation protection, Broad beam transmission, Diagnostic exposure. The growth of interventional techniques in radiology and the consequent close ...
1991, The British Journal of Radiology, 64, 69-71
Short communication Broad-beam transmission data in lead for scattered radiation produced at diagnostic energies By D. J . Rawlings, BSc, M I P S M , K. Faulkner, BSc, MSc, PhD and R. M. Harrison, PhD, FinstP FIPSM Regional Medical Physics Department, Newcastle General Hospital, Westgate Road, Newcastle Upon Tyne, NE4 6BE (Received March 1990 and in revised form June 1990) Keywords: Radiation protection, Broad beam transmission, Diagnostic exposure
The growth of interventional techniques in radiology and the consequent close proximity of staff to patients during screening emphasizes the need for provision of proper protective clothing and for appropriate personnel monitoring for radiological staff. In order to comply fully with the Ionising Radiation Regulations (HMSO, 1985a) and the associated Approved Code of Practice and Guidance Notes (HMSO, 1985b; HMSO, 1988), an employer must demonstrate not only that staff doses are below any relevant limits but also that they are as low as is reasonably practicable. Since heavy protective aprons may be unduly inconvenient to wear and since routine personnel monitoring makes use of just one or two dosemeters, it is important for both the optimization of protective clothing and the estimation of whole body effective dose equivalent (IPSM, 1977) that the broad beam shielding properties of lead under relevant working conditions are known (Faulkner & Harrison, 1988). Although there are several sources which tabulate the broad beam shielding characteristics of lead and other materials at diagnostic X-ray energies (BSI, 1971; HMSO, 1971; O'Riordan & Brotherton, 1968; Binks, 1943; NCRP, 1976), a number of limitations exist when these data are applied in practice to lead apron shielding:
commonly found in coronary care units. Tube voltages were checked using a Keithley non-invasive divider which had a calibration traceable to primary standards. A 1.8 litre ion chamber, connected to a digital electrometer (MDH) was placed close to the phantom but outside the primary beam so that it detected radiation scattered through approximately 90° (see Fig. 1). Specifications for the chamber and electrometer indicated a measurement resolution of 0.01 /iGy repeatable to within 1%. The energy response of the chamber, over the range of qualities encountered, was uniform to within 1%. The chamber, protected from X-ray tube leakage by 6 mm lead shielding, was positioned with its centre 13 cm from the nearest face of the phantom. The radiation striking the chamber was taken to represent that incident on a member or staff attending the patient. A series of large lead sheets 500 x 500 x 0.15 mm
X-Ray Focus
(1) the data represent primary beam attenuation whereas personnel need to be protected against scatter; (2) data for only a limited number of tube voltages are represented possibly leading to difficult interpolation; (3) a very large range of thicknesses is covered by the data and accuracy may be compromised at the low lead equivalent thicknesses of aprons. In this paper, measurements of broad beam attenuation of scatter in lead rubber aprons under realistic conditions are described, and the results compared with existing primary beam data.
Experimental arrangement A wax phantom (60 x 30 x 23 cm) was placed on an X-ray table and irradiated with a vertical X-ray beam of field size 27 x 17 cm2 and a source to surface distance of 50 cm. This was taken to represent a patient undergoing a routine interventional procedure, for example in a coronary care ward or in a specialist radiographic facility. The X-ray apparatus consisted of a Medio DLX X-ray set with a Machlett Dynamax tube and a 16° tungsten target. A fully rectified unsmoothed voltage waveform was used with generating voltages in the range 50-120 kVp. The voltage waveform was similar to those provided by generators of mobile image intensifier TV systems Vol. 64, No. 757
Figure 1. Apparatus for measurement of broad beam shielding characteristics of scatter from a wax phantom irradiated by diagnostic X rays.
69
Short communication
100
Table I. Broad beam transmission of 90° scatter by lead aprons as a percentage of unshielded dose 120 kV 110 kV 100 kV
Generating voltage (kVp)
0.25 mm Pb apron(%)
0.35 mm Pb apron(%)
50 60 70 80 90 100 110 120
0.92 1.7 3.3 4.4 5.8 8.6 12 15
0.23 0.62 1.5 2.1 2.8 4.6 6.6 8.6
90 kV 80 kV
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Thickness of Lead (mm) Figure 2. Scatter transmission characteristics.
was placed in the 6 cm gap between scatterer and detector, and the transmitted radiation dose was measured at different thicknesses of lead. A small correction was made for the residual scatter from the walls and floor of the room. This was assessed separately at each generating voltage by placing a thick shield (3.4 mm lead) between scatterer and detector. A further ion chamber, connected to an analogue electrometer and stable to within 2%, was placed in the primary beam to act as a monitor.
Results The results shown in Fig. 2 may be compared with primary beam data (HMSO, 1971) over a limited number of generating voltages. Although there was generally good agreement, it was found that at the highest generating voltages, the scatter data indicated significantly greater penetration than the recommended primary beam data. According to the measured data, an apron of 0.25 mm lead equivalent thickness would transmit 15% of the broad beam scatter at a generating potential of 120 kVp, whereas at the same thickness only approximately 10% of the primary beam would be transmitted under broad beam conditions. Transmission data for common lead apron thicknesses are shown in Table I.
Discussion Although the maximum energy of the scattered radiation is less than that of the primary beam, beam hardening acts to increase the quality of the radiation beam within the patient (Birch et al, 1979). Depth dose characteristics over the diagnostic range of energies (for example, Wall et al, 1988) indicate that significant beam hardening generally occurs within about 5 cm of the surface but that beyond this depth there is little change in beam quality. It would appear therefore that the observed results may be consistent with the effects of beam
70
hardening of both primary and secondary radiation within the patient. Keane and Spiegler (1951) measured first half value layer data from radiation scattered from deep within a phantom and, consistent with the current results, found values to be higher than those of the primary beam. However, Trout and Kelley ('972) measured attenuation data over a similar quality range and generally found primary beam quality to be greatest, in contrast to the present result. However a field size of 900 cm2 at the phantom surface is specified. This is much larger than that typically found in practice. In addition, the volume of the ionization chamber and the size of the lead attenuators are not specified and so a direct comparison with the current work may not be appropriate. The data shown in Fig. 2 are appropriate for use in optimizing protection or estimating whole body effective dose equivalent to a member of staff carrying out duties close to a patient during screening procedures (Faulkner & Harrison, 1988). The data are tabulated for a range of generating voltages and lead equivalences which are relevant for these purposes. Since beam hardening takes place whenever radiation is scattered from deep within a patient it is apparent that any member of staff required to wear a lead apron may be subject to beam hardened scatter, except where the scattering volume is very small or there is otherwise a large proportion of superficial scatter. Since only 5 cm of tissue would normally be required for complete beam hardening, it is also apparent that under most circumstances the spectrum of scattered radiation could not be expected to be strongly dependent upon field size. The attenuation data presented here are therefore generally applicable whenever radiation, scattered laterally from a patient undergoing a diagnostic procedure, is considered. Overall uncertainty in the data is indicated by the error bars in Fig. 2, and may be attributed to small variations in the X-ray generating potential and uncertainties in the residual scatter from sources other than the phantom. This may be contrasted with the uncertainty associated with the interpretation of data from existing primary sources, limited by the large range of lead equivalences, and small number of beam qualities represented. The data in Table I may be used as a guide to the scatter transmission characteristics of commonly used lead aprons. This data emphasizes the need for the specification of lead apron thickness dependent on the tube potential range used clinically, and is in accordance with the advice given in paragraph 3.70 of the Guidance Notes (HMSO, 1988). If the advice in the guidance notes is followed, then it may be deduced from the data presented in Table I that the scatter transmission through body lead aprons will be less than 10%. The British Journal of Radiology, January 1991
Short communication Acknowledgments The authors would like to thank Professor K. Boddy for his continued support and encouragement, and Messrs K. Robson and N. Marshall for assistance with the graphics. References BINKS, W., 1943. Protection in industrial radiology. British Journal of Radiology, 16, 49-53. BIRCH, R., MARSHALL, M. & ARDRAN, G. M., 1979. Catalogue
of Spectral Data for Diagnostic X-Rays (Hospital Physicists' Association, London). BSI, 1971. Recommendations for Data on Shielding from Ionising Radiation, Part 2. Shielding from X Radiation, BSI Standard No. 4094 (British Standards Institution, London). FAULKNER, K. & HARRISON,
R. M.,
1988. Estimation
of
effective dose equivalent to staff in diagnostic radiology. Physics in Medicine and Biology, 33, 83-92. HMSO, 1971. Handbook of Radiological Protection, Part 1 Data (Her Maesty's Stationery Office, London). 1985a. The Ionising Radiations Regulations 1985 (Her Majesty's Stationery Office, London). 1985b. Approved Code of Practice, The Protection of Persons against Ionising Radiation arising from any Work Activity (Her Majesty's Stationery Office, London).
Vol. 64, No. 757
1988. Guidance Notes for the Protection of Persons against Ionising Radiations arising from Medical or Dental Use (Her Majesty's Stationery Office, London). ICRP, 1977. Recommendations of the ICRP, ICRP Publication 26 (Pergamon Press, Oxford). KEANE, B. E. & SPIEGLER, G., 1951. Stray radiation from diagnostic X-ray beams. British Journal of Radiology, 24, 198-203. NCRP, 1976. Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies up to WMeV, NCRP Report No. 49 (National Council on Radiation Protection and Measurements, Bethesda). O'RibRDAN, M. C. & BROTHERTON, L., 1968. Transmission of 50 to 200 kV Pulsating Potential X rays through Lead, Radiological Protection Service Report R.P.S./1/40 (Radiological Protection Service, Oxford). TROUT, E. D. & KELLEY, J. P., 1972. Scattered radiation from a tissue equivalent phantom for X-rays from 50 to 300 kVp. Radiology, 104, 161-169. WALL, B. F., HARRISON, R. M. & SPIERS, F. W., 1988. Patient
Dosimetry Techniques in Diagnostic Radiology, Institute of Physical Sciences in Medicine Report 53 (Institute of Physical Sciences in Medicine, York).
71
