1990,
The British Journal of Radiology, 63, 149-151
Correspondence (The Editors do not hold themselves responsible for opinions expressed by correspondents)
Effective dose equivalent in dual X-ray absorptiometry THE EDITOR—SIR,
The technique of dual photon absorptiometry (DPA) has been established for many years for the measurement of bone mineral density of the lumbar spine and femoral neck. Its use, however, has been mostly restricted to centres with specialized interests. A recent development of the technique has been the replacement of the dual energy isotope source by a dual energy X-ray source. This technique is referred to as dual X-ray absorptiometry (DXA), dual energy X-ray absorptiometry (DEXA) or quantitative digital radiography (QDR). Scanners based on this system are now available from several manufacturers. The use of an X-ray source has resulted in improved image resolution, shorter scan times, better precision (in vivo reproducibility of about 1%) and a reduction in radiation exposure per scan. The improved performance of the new machines has led to a wider use of the absorptiometry technique, which is now capable of fulfilling a screening role. Patients may receive DXA on several occasions in order to establish their rate of bone loss or response to treatment. It is therefore important to be able to reassure such patients about their level of X-ray exposure. In a review article on bone mineral measurement, Tothill (1989) stated that for DPA "radiation dose is low, around 0.1 mGy to a relatively small volume of tissue. The effective dose equivalent is only a few /iSv" but was not more specific. We have measured the entry dose on two commercial DXA machines (Hologic QDR-1000) at two centres, and have also measured a mean dose to the irradiated volume at one centre, with a view to making a realistic estimate of the effective dose equivalent (EDE) associated with spine and femoral neck scans. The manufacturers of the Hologic QDR-1000 quote an entry dose of 20-50 /iSv. At one centre, we obtained a mean entry dose of 35/iSv per scan, based on two thermoluminescent dosimeters each scanned 20 times in spine scan mode. At the second centre, we obtained a mean entry dose of 36.2 + 2.1 (standard deviation) ^Sv per scan from 10 measurements using a small energy-compensated GM tube dosimeter (Rad Alert, Perspective Scientific Ltd). All measurements were obtained using anthropomorphic trunk phantoms. Entry dose, however, is not particularly useful when assessing the total radiation hazard from a particular procedure. Eflect/Ye dose equ/rate/ir /s a concept borrowed from occupational exposure estimation and is now commonly applied to estimate doses from nuclear medicine investigations. It represents the total radiation dose to a number of organs
weighted according to a risk estimate for each organ concerned (ICRP, 1977). To estimate the EDE for scans of the lumbar spine, we first estimated the mean dose to the irradiated tissues. The doses measured using the GM tube dosimeter at 0%, 25%, 50%, 75% and 100% depths in a 19 cm thick elliptical hardboard phantom were respectively 36.2/iSv, 35.4/iSv, 23.2/iSv, 12.3 fxSv and 6.1 /xSv, giving a mean of 22.6/xSv per scan. Bone, marrow and female gonads are the only named risk organs (ICRP, 1977) in the measurement field. In calculating the EDE we neglected bone and marrow for special treatment
Vol. 63, No. 746
because the irradiated masses represent only a small proportion of the total mass of these organs. It is not possible, however, to exclude the female gonads from the calculation because of the variability in their position. For females, the EDE was calculated as follows: EDE = 22.6 x A x 0.30 + 23.2 x 0.25 = 6.4 /iSv where the mean measured dose has been weighted by the ratio of the mass of tissue irradiated during scanning (6 kg) to the total body mass (70 kg) and also by the weighting factor for "general" tissues (0.30); the gonad dose has been taken as the measured midline dose (23.2/iSv) with 0.25 as the appropriate weighting factor. For males, the gonads should not appear within the primary beam and so the term in the calculation of EDE relating to the gonads can be dropped, resulting in a value of approximately 0.6/xSv. For scans of the femoral neck in both males and females, the gonads should not appear in the primary beam and as a similar or slightly smaller tissue volume is scanned the EDE for the male spine is appropriate (i.e. 0.6 /xSv per scan). These values of EDE allow us to put the exposure from DXA in context. A modern chest radiograph gives an EDE of about 60/iSv, and the mean daily EDE in the UK to each person from natural sources of radiation is about 5/iSv (NRPB, 1986). Hence, an upper limit to the estimated EDE from a spine scan on a female patient on the QDR-1000 is equivalent to about the daily background if it is assumed that the gonads are in the scanning field. For femoral neck scans on females and males, and spine scans on males, the EDE is only about one tenth of this. Yours, etc., D. W. PYE W. J. HANNAN *R. HESP
Department of Medical Physics and Medical Engineering, Western General Hospital, Edinburgh and *Division of Radioisotopes, MRC Clinical Research Centre, Harrow, Middlesex (Received August 1989)
ICRP, 1977. ICRP Publication 26 (Pergamon Press, OxforcfJ. NRPB, 1986. Living with radiation (Her Majesty's Stationery Office, London). TOTHILL, P., 1989. Methods of bone mineral measurement. Physics in Medicine and Biology, 34, 543-572.
Changes in relative biological effectiveness with depth of neutron beams THE EDITOR—SIR,
In his letter on this subject in the August issue of the British Journal of Radiology (Hall, 1989), Professor E. J. Hall suggests
149
The British Journal of Radiology, 63, 149-151
Correspondence (The Editors do not hold themselves responsible for opinions expressed by correspondents)
Effective dose equivalent in dual X-ray absorptiometry THE EDITOR—SIR,
The technique of dual photon absorptiometry (DPA) has been established for many years for the measurement of bone mineral density of the lumbar spine and femoral neck. Its use, however, has been mostly restricted to centres with specialized interests. A recent development of the technique has been the replacement of the dual energy isotope source by a dual energy X-ray source. This technique is referred to as dual X-ray absorptiometry (DXA), dual energy X-ray absorptiometry (DEXA) or quantitative digital radiography (QDR). Scanners based on this system are now available from several manufacturers. The use of an X-ray source has resulted in improved image resolution, shorter scan times, better precision (in vivo reproducibility of about 1%) and a reduction in radiation exposure per scan. The improved performance of the new machines has led to a wider use of the absorptiometry technique, which is now capable of fulfilling a screening role. Patients may receive DXA on several occasions in order to establish their rate of bone loss or response to treatment. It is therefore important to be able to reassure such patients about their level of X-ray exposure. In a review article on bone mineral measurement, Tothill (1989) stated that for DPA "radiation dose is low, around 0.1 mGy to a relatively small volume of tissue. The effective dose equivalent is only a few /iSv" but was not more specific. We have measured the entry dose on two commercial DXA machines (Hologic QDR-1000) at two centres, and have also measured a mean dose to the irradiated volume at one centre, with a view to making a realistic estimate of the effective dose equivalent (EDE) associated with spine and femoral neck scans. The manufacturers of the Hologic QDR-1000 quote an entry dose of 20-50 /iSv. At one centre, we obtained a mean entry dose of 35/iSv per scan, based on two thermoluminescent dosimeters each scanned 20 times in spine scan mode. At the second centre, we obtained a mean entry dose of 36.2 + 2.1 (standard deviation) ^Sv per scan from 10 measurements using a small energy-compensated GM tube dosimeter (Rad Alert, Perspective Scientific Ltd). All measurements were obtained using anthropomorphic trunk phantoms. Entry dose, however, is not particularly useful when assessing the total radiation hazard from a particular procedure. Eflect/Ye dose equ/rate/ir /s a concept borrowed from occupational exposure estimation and is now commonly applied to estimate doses from nuclear medicine investigations. It represents the total radiation dose to a number of organs
weighted according to a risk estimate for each organ concerned (ICRP, 1977). To estimate the EDE for scans of the lumbar spine, we first estimated the mean dose to the irradiated tissues. The doses measured using the GM tube dosimeter at 0%, 25%, 50%, 75% and 100% depths in a 19 cm thick elliptical hardboard phantom were respectively 36.2/iSv, 35.4/iSv, 23.2/iSv, 12.3 fxSv and 6.1 /xSv, giving a mean of 22.6/xSv per scan. Bone, marrow and female gonads are the only named risk organs (ICRP, 1977) in the measurement field. In calculating the EDE we neglected bone and marrow for special treatment
Vol. 63, No. 746
because the irradiated masses represent only a small proportion of the total mass of these organs. It is not possible, however, to exclude the female gonads from the calculation because of the variability in their position. For females, the EDE was calculated as follows: EDE = 22.6 x A x 0.30 + 23.2 x 0.25 = 6.4 /iSv where the mean measured dose has been weighted by the ratio of the mass of tissue irradiated during scanning (6 kg) to the total body mass (70 kg) and also by the weighting factor for "general" tissues (0.30); the gonad dose has been taken as the measured midline dose (23.2/iSv) with 0.25 as the appropriate weighting factor. For males, the gonads should not appear within the primary beam and so the term in the calculation of EDE relating to the gonads can be dropped, resulting in a value of approximately 0.6/xSv. For scans of the femoral neck in both males and females, the gonads should not appear in the primary beam and as a similar or slightly smaller tissue volume is scanned the EDE for the male spine is appropriate (i.e. 0.6 /xSv per scan). These values of EDE allow us to put the exposure from DXA in context. A modern chest radiograph gives an EDE of about 60/iSv, and the mean daily EDE in the UK to each person from natural sources of radiation is about 5/iSv (NRPB, 1986). Hence, an upper limit to the estimated EDE from a spine scan on a female patient on the QDR-1000 is equivalent to about the daily background if it is assumed that the gonads are in the scanning field. For femoral neck scans on females and males, and spine scans on males, the EDE is only about one tenth of this. Yours, etc., D. W. PYE W. J. HANNAN *R. HESP
Department of Medical Physics and Medical Engineering, Western General Hospital, Edinburgh and *Division of Radioisotopes, MRC Clinical Research Centre, Harrow, Middlesex (Received August 1989)
ICRP, 1977. ICRP Publication 26 (Pergamon Press, OxforcfJ. NRPB, 1986. Living with radiation (Her Majesty's Stationery Office, London). TOTHILL, P., 1989. Methods of bone mineral measurement. Physics in Medicine and Biology, 34, 543-572.
Changes in relative biological effectiveness with depth of neutron beams THE EDITOR—SIR,
In his letter on this subject in the August issue of the British Journal of Radiology (Hall, 1989), Professor E. J. Hall suggests
149
