This is an interesting, well-written and comprehensive report dealing essentially with the treatment of cancer of the uterus by radiotherapy and the extension of such services in developing countries. The main report, consisting of ten chapters (52 pages), is supplemented by a number of annexes written by individual authors and intended to provide additional information on particular aspects of the subject.

It is only because I have such a high regard for Dr. Ellis's concept of establishing a unit of biologically effective dose, that I have taken the trouble to criticize his suggestions in such detail. The better the idea the more carefully it should be considered, and this perhaps is why some of the criticism in my article, “A critical look at the ret” (Liversage, 1971) appears to Dr. Ellis to be of a trivial nature. Dr. Ellis replies to my criticism in his paper “Nominal standard dose and the ret” (Ellis, 1971) and on page 102 of this paper he disagrees with my findings on five counts which I wish to discuss further in the light of his comments and in the same numerical order.

The Ellis (1968) formulae for a unit of biologically effective dose are derived by applying doubtful assumptions to questionable data. Clinical and experimental evidence suggests that these formulae are not true for tumours in general, not true for all normal tissues and only approximately true for skin. A later version of the formula for normal tissues proposed by Winston et al. (1969) suffers from an additional disadvantage, in that it contains a quantity (NSD) which varies from one centre to another, so that identical treatments would be considered by different centres to be equivalent to different ret doses. An international unit of biological effect based on the concept of an equivalent single dose, as suggested by Ellis 1968), would be useful, but for any given treatment regime the equivalent single dose is liable to vary from tissue to tissue and therefore needs to be defined in terms of one specific tissue. The data of Fowler (1965) applied to skin reaction are probably the most reliable fractionation data available for any tissue. It is therefore proposed that a biological unit should be defined in terms of an equivalent single dose for skin. In the interests of uniformity it is essential that this be calculated by a standard procedure and, for this purpose, a new formula is proposed. The formula is based on the data of Fowler (1965a), corrected for variation in over-all time by the method of Cohen (1968). It is claimed that the new formula is more accurate, is very much easier to use and should form a sounder basis for a unit of biologically effective dose than the ret.

Suppose that a continuous low dose-rate radium treatment lasting t hours is replaced by a high dose-rate after-loading treatment employing N fractions. Theory, based on the recovery of intracellular sub-lethal damage, suggests that the total fractionated dose necessary to produce an equivalent effect on each irradiated tissue will be the same as the protracted dose, provided N and t are related by a formula which, for values of t>12 hours, approximates to N=t/4. If, in order to give Nfractions without spacing them too close together, the overall treatment time is increased to (N—1) days, the total fractionated dose should, however, be increased by C rads for each day by which the overall time is increased. It is suggested that for skin C is approximately equal to 25 rads per day. The formula is shown to be valid for skin reactions in patients and for LD50(3O) in mice. Theory suggests that the formula is likely to be approximately true for most mammalian tissues irradiated at normal body temperature, irrespective of their recovery regression coefficients. Thus, changing to a high dose-rate regime should produce no deterioration in the therapeutic ratio provided the treatment is fractionated in accordance with the proposed formula. If the number of fractions is reduced from N to some lower number, the corresponding reduction in required dose may be found using the formula of Ellis (1967) or the nomogram of Shuttleworth and Fowler (1966). Such a change may, however, be accompanied by a change in therapeutic ratio.

An account is given of the physical measurements which have been made prior to and during the clinical use of the prototype Cathetron unit for the treatment of uterine carcinoma. The measurement of source strength and effective screenage, the calculation of tumour dose, the production of isodose curves, the measurement and calculation of exposure during source transfer, the source position accuracy, radiographic reconstruction technique and protection measurements are described.

The Cathetron is a high dose-rate, remotely controlled, “afterloading” intracavitary radiation unit. The prototype unit installed at the Charing Cross Group of Hospitals and the clinical technique evolved for the treatment of uterine carcinoma using this machine are described.

Given an isodose chart for any radium needle, the following formulae enable one to determine the dose-rate at any point in the vicinity of a radium container having the same filtration but different active length and content. The formulae have been used to construct isodose curves for radium needles for which no isodose curves were available and have been found useful in cross-checking new isodose curves obtained from other sources.