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
Correspondence that "the change of RBE [relative biological effectiveness] with depth is generally not seen for high energy neutron b e a m s . . . where the neutron spectrum produced has a single peak". I should like to remind your readers of a study using monoenergetic neutrons from a D - T generator (Nias et al, 1971). We found a 1\% reduction in the RBE value when HeLa cells were irradiated at 10 cm depth compared with those irradiated at the surface. Yours, etc.,
References HALL, E. J., 1989. Changes in relative biological effectiveness with depth of neutron beams. British Journal of Radiology, 62, 765.
A. H. W. NIAS
biological parameters in a 14 MeV neutron field. International Journal of Radiation Biology, 20, 145-151.
Richard Dimbleby Department of Cancer Research, United Medical and Dental Schools, St Thomas' Hospital, London SE1 7EH {Received August 1989)
(Author's reply) THE EDITOR—SIR,
(1) The point at issue in my letter (Hall, 1989), and the paper on which it was a comment (Hornsey et al, 1988), was the biological consequences of the difference in neutron spectra characteristic of the reactions involving protons on beryllium versus deuterons on beryllium; 14 MeV D-T neutrons are quite different. They can hardly be regarded as high-energy neutrons, and are nominally monoenergetic except for scattered neutrons from the collimator, and a large y-ray component. (2) In reading Professor Nias's reminder that he and his colleagues found a reduction in neutron RBE when HeLa cells were irradiated with 14 MeV neutrons at 10 cm depth compared with those irradiated in air, I was intrigued to note that their paper describing this work is entitled "Constancy of biological parameters in a 14 MeV neutron field". This paper concludes that " . . . the neutron therapist can be assured that however much the proportion of primary neutrons, scattered neutrons and y-radiation may vary at different positions in a 14 MeV monoenergetic neutron beam, variations in the biological parameters are small". Between writing this paper in 1971 and his present letter in 1989, Professor Nias appears to have changed his mind and now thinks the data show a change of RBE with depth. On a quick check on the constancy of the neutrons RBE with depth, we have fitted his survival measurements for oxic cells to a linear quadratic form where i refers to either in air or at 10 cm depth of water. Using the standard method of iterative reweighting, our maximum likelihood estimates for a//? are 5.5 + 3.0 Gy in air and 4.0 + 2.1 Gy at depth in water. From these numbers, the RBE in air relative to that in water can be estimated, as well as its statistical uncertainty. For example, at 2 Gy (air does), the RBE is 1.2 + 0.9. These results would seem to confirm Professor Nias's original statement that "the difference is of doubtful significance". Yours, etc., ERIC J. HALL
Center for Radiological Research, College of Physicians and Surgeons of Columbia University, 630 West 168th Street, New York, NY 10032, USA (Received August 1989)
150
HORNSEY, S., MYERS, R., PARNELL, C. J., BONNETT, D.
E.,
BLAKE, S. W. & BEWLEY, D. K., 1988. Changes in relative
biological effectiveness with depth of the Clatterbridge neutron beam. British Journal of Radiology, 61, 1058-1062. NIAS, A. H. W., GREENE, D. & MAJOR, D., 1971. Constancy of
Screening for breast cancer THE EDITOR—SIR,
It is a pity that in the otherwise excellent review of the UK Breast Screening situation by Professor Forrest he perpetuates two errors of his working party. The first is that a single view is adequate for the initial breast screen. The evidence from Sweden and elsewhere at the time of the publication of the Working Group Report (1979) was quite clearly that the two views were required for the initial screen. The second and more serious error is the recommendation of a 3 year screening interval for which there was, and is, no justification apart from a financial one. To plan propsective studies to prove what is manifestly obvious to every specialist in the field is not only a waste of money and time but is dubious ethically. Yours etc., A. F. MACDONALD
Neuroradiology, In-patient X-ray Department, Foresterhill, Aberdeen AB9 2ZB (Received August 1989) (Author's reply) THE EDITOR—SIR,
Dr MacDonald's concern is well known, but it was not one which was shared by his colleagues at the time of the working group's report. Our recommendations for single-view mammography for the basic screen and an interval for screening of 3 years were based on the facts then available. The most recent report of the two-counties study from Sweden is relevant, as it indicates that the 30% reduction in mortality in women invited to be screened by single-view mammography at intervals of 33 months is still maintained at 8 years (Tabar et al, 1989). Dr MacDonald fails to acknowledge that our recommendations for single-view mammography were "initial" and that the frequency of 3 years was a starting point which should be kept under review. We must know the truth on these matters but this can come only from properly conducted trials, such as those now being set up under the aegis of the UK Coordinating Committee on Cancer Research, and not from the subjective opinions of individual radiologists. To say that prospective well-controlled trials are unethical is but to deny the scientific basis of modern medical practice. Yours etc., P. FORREST
Scottish Cancer Trials Office, University of Edinburgh, Medical School, Teviot Place, Edinburgh EH8 9AG (Received October 1989) The British Journal of Radiology, February 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
Correspondence that "the change of RBE [relative biological effectiveness] with depth is generally not seen for high energy neutron b e a m s . . . where the neutron spectrum produced has a single peak". I should like to remind your readers of a study using monoenergetic neutrons from a D - T generator (Nias et al, 1971). We found a 1\% reduction in the RBE value when HeLa cells were irradiated at 10 cm depth compared with those irradiated at the surface. Yours, etc.,
References HALL, E. J., 1989. Changes in relative biological effectiveness with depth of neutron beams. British Journal of Radiology, 62, 765.
A. H. W. NIAS
biological parameters in a 14 MeV neutron field. International Journal of Radiation Biology, 20, 145-151.
Richard Dimbleby Department of Cancer Research, United Medical and Dental Schools, St Thomas' Hospital, London SE1 7EH {Received August 1989)
(Author's reply) THE EDITOR—SIR,
(1) The point at issue in my letter (Hall, 1989), and the paper on which it was a comment (Hornsey et al, 1988), was the biological consequences of the difference in neutron spectra characteristic of the reactions involving protons on beryllium versus deuterons on beryllium; 14 MeV D-T neutrons are quite different. They can hardly be regarded as high-energy neutrons, and are nominally monoenergetic except for scattered neutrons from the collimator, and a large y-ray component. (2) In reading Professor Nias's reminder that he and his colleagues found a reduction in neutron RBE when HeLa cells were irradiated with 14 MeV neutrons at 10 cm depth compared with those irradiated in air, I was intrigued to note that their paper describing this work is entitled "Constancy of biological parameters in a 14 MeV neutron field". This paper concludes that " . . . the neutron therapist can be assured that however much the proportion of primary neutrons, scattered neutrons and y-radiation may vary at different positions in a 14 MeV monoenergetic neutron beam, variations in the biological parameters are small". Between writing this paper in 1971 and his present letter in 1989, Professor Nias appears to have changed his mind and now thinks the data show a change of RBE with depth. On a quick check on the constancy of the neutrons RBE with depth, we have fitted his survival measurements for oxic cells to a linear quadratic form where i refers to either in air or at 10 cm depth of water. Using the standard method of iterative reweighting, our maximum likelihood estimates for a//? are 5.5 + 3.0 Gy in air and 4.0 + 2.1 Gy at depth in water. From these numbers, the RBE in air relative to that in water can be estimated, as well as its statistical uncertainty. For example, at 2 Gy (air does), the RBE is 1.2 + 0.9. These results would seem to confirm Professor Nias's original statement that "the difference is of doubtful significance". Yours, etc., ERIC J. HALL
Center for Radiological Research, College of Physicians and Surgeons of Columbia University, 630 West 168th Street, New York, NY 10032, USA (Received August 1989)
150
HORNSEY, S., MYERS, R., PARNELL, C. J., BONNETT, D.
E.,
BLAKE, S. W. & BEWLEY, D. K., 1988. Changes in relative
biological effectiveness with depth of the Clatterbridge neutron beam. British Journal of Radiology, 61, 1058-1062. NIAS, A. H. W., GREENE, D. & MAJOR, D., 1971. Constancy of
Screening for breast cancer THE EDITOR—SIR,
It is a pity that in the otherwise excellent review of the UK Breast Screening situation by Professor Forrest he perpetuates two errors of his working party. The first is that a single view is adequate for the initial breast screen. The evidence from Sweden and elsewhere at the time of the publication of the Working Group Report (1979) was quite clearly that the two views were required for the initial screen. The second and more serious error is the recommendation of a 3 year screening interval for which there was, and is, no justification apart from a financial one. To plan propsective studies to prove what is manifestly obvious to every specialist in the field is not only a waste of money and time but is dubious ethically. Yours etc., A. F. MACDONALD
Neuroradiology, In-patient X-ray Department, Foresterhill, Aberdeen AB9 2ZB (Received August 1989) (Author's reply) THE EDITOR—SIR,
Dr MacDonald's concern is well known, but it was not one which was shared by his colleagues at the time of the working group's report. Our recommendations for single-view mammography for the basic screen and an interval for screening of 3 years were based on the facts then available. The most recent report of the two-counties study from Sweden is relevant, as it indicates that the 30% reduction in mortality in women invited to be screened by single-view mammography at intervals of 33 months is still maintained at 8 years (Tabar et al, 1989). Dr MacDonald fails to acknowledge that our recommendations for single-view mammography were "initial" and that the frequency of 3 years was a starting point which should be kept under review. We must know the truth on these matters but this can come only from properly conducted trials, such as those now being set up under the aegis of the UK Coordinating Committee on Cancer Research, and not from the subjective opinions of individual radiologists. To say that prospective well-controlled trials are unethical is but to deny the scientific basis of modern medical practice. Yours etc., P. FORREST
Scottish Cancer Trials Office, University of Edinburgh, Medical School, Teviot Place, Edinburgh EH8 9AG (Received October 1989) The British Journal of Radiology, February 1990
