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)

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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

Screening for breast cancer.

Correspondence that "the change of RBE [relative biological effectiveness] with depth is generally not seen for high energy neutron b e a m s . . . wh...
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