section 2 . x rays : half-value thickness range 1.0-4.0 mm of Cu . ( 200-400 kV ) . closed-ended applicators . compiled by . R G Wood , M.Sc , F.Inst.P , A.M.I.E.E , and W H Sutherland , B.Sc . the royal Infirmary , Cardiff . and M Cohen , Ph.D , A.R.C.S , A.Inst.P , the London Hospital , London , E.1 . the tables published in the corresponding section in supplement no 5 were compiled by Clarkson from measurements made by a number of hospital physicists in Great Britain . in common with other tables derived from a number of sources , they suffered from the disadvantage that several of the conditions ( e.g thickness of applicator end-plate ) to which the data referred were not closely defined . furthermore , if smoothing of survey data is based on values calculated by an empirical formula to fit the figures available , some inconsistency will inevitably remain since no formulae have been found which ensure smoothness in each of the three ways in which depth-dose data can be plotted ( see below ) . it was therefore decided to replace the survey data with tables derived from a single centre , thus bringing this section into line with the two other sections of this supplement concerned with low and medium energy radiation . the new tables have , however , been compared with data made available from a number of other centres , and with the tables for diaphragm-limited fields in section 3 . source of data . the present tables are based on water-phantom measurements made in Cardiff by Wood and Sutherland using an ionization chamber of external diameter 3 mm so arranged that its centre could approach to 1.5 mm from the applicator end-plate with no intervening tank wall . as an independent check an experimental comparison of this technique with that of the London Hospital ( Oliver and Kemp , 1949 ) was carried out in 1955 in conjunction with Cohen . measurements were made on the same x-ray set using alternately a Kemp ionization current comparator with a chamber of external diameter 6 mm ( Kemp , 1945 ; Kemp and Banfield , 1957 ) , and the apparatus of Wood and Sutherland . this comparison showed that when allowance was made for small differences near the surface , no significant disagreement existed between the results obtained by the two techniques . the final measurements of percentage depth dose from which the tables were derived were made on a Westinghouse Quadrocondex machine under the following conditions : the measurements were made with a series of square Fulfield applicators of 50 cm F.S.D , the ends being closed with flat Perspex of thickness 1/8 inch ( approximately 3 mm ) . strictly , the data refer only to these applicators , but for clinical purposes the tables may be used for any applicator of similar design provided the thickness of the end-plate is the same and it is made of similar material . the effect on the data of using applicators of different design or end-plate thickness will be considered further in a separate publication . no measurements were made for zero area , but this information is provided in section 3 . smoothing and extrapolation of data . smoothing of the experimental data , for square fields , was carried out graphically by plotting ( 1 ) individual depth-dose curves on log &amp;sol; linear paper , ( 2 ) percentage depth dose versus square root of area on linear paper for individual depths , and ( 3 ) percentage depth dose versus half-value thickness on linear paper for individual depths . values for depths from 16 to 20 cm were obtained by extrapolation . this is justified since the logarithmic plots of depth dose are straight lines from 10 cm downwards . the whole table for 2.5 mm of Cu H.V.T was obtained by interpolation , while that for 4.0 mm of Cu H.V.T was obtained by extrapolation . for the latter purpose guidance was provided by some additional experimental data from the royal Victoria Infirmary , Newcastle upon Tyne , but owing to the uncertainties of extrapolation this table must be regarded as somewhat less reliable than the others . data for rectangular fields . Johns and his colleagues have used the data for square fields to calculate tables for a series of rectangular areas , using Clarkson &apos;s ( 1941 ) method , with the help of the digital computer of the University of Toronto . depth doses for the primary radiation were assumed to be the same as those given in section 3 . the computed values of percentage depth dose were smoothed graphically , by methods ( 1 ) and ( 3 ) above , prior to tabulation . some of the rectangular fields included in these tables are different from those given in section 3 , since it is intended that the data in this section shall correspond to the applicators most commonly used in Great Britain . data for circular fields and for other rectangles may readily be computed by the equivalent field method ( see appendix A ) . comparison with previous tables ( supplement no 5 ) . in this context deviations of the new tables from the old are expressed as percentages of the local dose . ( 1 ) . in the range 1.5-3.0 mm of Cu H.V.T , for areas of 100 cm2 and above , the new tables agree with the old to within 3 per cent on average , with occasional divergencies up to 6 per cent . on the whole the new values are lower than the old , except when both area and depth are large . ( 2 ) . in the range 1.5-3.0 mm of Cu H.V.T , for areas less than 100 cm2 , the new values are significantly lower than the old , confirming the findings of Cohen ( 1955 ) . the average differences amount to 5 to 7 per cent , but at some depths reach 10 to 12 per cent . ( 3 ) . at 1.0 mm of Cu H.V.T the new values are lower than the old for all areas , the average difference being 4 to 5 per cent . comparison with data from other centres . in addition to the data from Cardiff covering the range of qualities from 1.0 to 3.0 mm of Cu H.V.T presented in the tables , recent measurements at some qualities have been made at the royal Infirmary ( Bradford ) , western general Hospital ( Edinburgh ) , Lambeth Hospital ( London ) , Christie Hospital ( Manchester ) and war memorial Hospital ( Scunthorpe ) . these measurements have been intercompared , revealing close agreement between the various centres , provided allowance is made for differences occurring in the first 2 or 3 cm by matching at some arbitrary depth , say 5 cm . these differences are partly real , arising from the use of applicators of different design , but mainly only apparent , arising from variations in the methods of assessing the surface dose . in view of the very small diameter of the chamber used and its close approach to the surface it is thought that the values tabulated represent a very close approximation to the variation in dose near the surface . comparison with data in section 3 . the depth-dose data in this range of qualities measured by Johns and his colleagues in Saskatchewan are presented in section 3 . the Canadian measurements were made with open applicators of special design ( see introduction to section 3 ) and differ from the British data in that there is no scatter component from the walls or end of the applicator . the two sets of data have been compared after applying the method of transformation suggested by Johns , Fedoruk , Kornelsen , Epp and Darby ( 1952 ) , making use of data for the range of depths 0 to 1 cm kindly supplied privately by Miss Fedoruk . agreement is obtained within experimental error provided an appropriate equivalent water thickness , which allows approximately for the effects of both the end-plate and the applicator walls , is used in place of the nominal thickness of the applicator end-plate . the equivalent water thickness ( for Fulfield applicators , 1/8 inch flat Perspex end-plates ) is independent of area , but varies with H.V.T as follows : since the equivalent water thickness of the end-plate alone is approximately 3.8 mm , it is seen that the allowance which must be made for the scatter contribution from the applicator walls is substantial . thus the simple correction factors for the end-plate only , measured by Johns , Hunt and Fedoruk ( 1954 ) , are insufficient for applicators of the type considered in this section . surface back-scatter factors . these are taken from the survey values published by Greening ( 1954 ) , which were based on measurements made at 11 centres with seven different types of x-ray generator . section 4 . gamma rays : caesium 137 teletherapy units . reviewed by . J E Burns , M.Sc , A.Inst.P , Westminster Hospital , London , S.W.1 . sources of data . at the time the work on this section had been completed , there were to the knowledge of the reviewer , seven caesium units which were in clinical use , four in England and three in north America . data were obtained from six of these centres : Addenbrooke &apos;s Hospital ( Cambridge ) , royal Marsden Hospital ( London ) , royal south Hants Hospital ( Southampton ) , Westminster Hospital ( London ) , Ontario cancer institute ( Canada ) , and Alice Lloyd radiation therapy centre ( Michigan , U.S.A ) . energy of radiation . the caesium source at Michigan was manufactured at Oak Ridge national laboratory ; all the other sources were manufactured by the United Kingdom atomic energy authority . it is well known that one of the main difficulties in the preparation of caesium 137 is to obtain freedom from contamination by other radioactive materials . thus sources may differ in their degree of contamination , and the effective quality of their radiation may be different . it is of importance , therefore , to know whether the data given here , being mainly from British sources , applies also to sources manufactured elsewhere , in particular to American sources . comparing the data received from the caesium unit at Michigan , it appears that it probably does apply . the half-value thicknesses from the five British sources were reasonably consistent , varying from 5.3 mm to 5.7 mm ( mean 5.4 mm ) in lead , from 10.6 mm to 11.0 mm ( mean 10.8 mm ) in copper , and from 7.8 to 8.0 cm ( mean 7.9 cm ) in water . comparing the American source , the percentage depth doses differ from the average ( see later ) by no more than &amp;plusmn;1 1/2 per cent of the local dose , the half-value thickness in lead is in good agreement , 5.35 mm , but the half-value thicknesses in copper and water are rather higher , 11.25 mm and 8.35 cm respectively . using the calculated attenuation coefficients of white Grodstein ( 1957 ) , the average half-value thicknesses of the British sources have been used to calculate the effective photon energy of the radiation . these are as follows : and can be compared with the accepted value of 0.66 MeV for caesium 137 g rays . back-scatter factors . data were received from four centres , and a smoothed average of values was taken . individual values of back-scatter factors differed from the average by not more than 1 per cent . in estimating the dose-rate at the maximum level from the dose-rate in air , it should be remembered that variations of air dose with area are at least as great as ( and are additional to ) the variation of back-scatter with area . the values will of course depend very much on the collimator system of the unit . for the only unit for which information is available , the dose-rate in air at the normal working S.S.D increases by 15 per cent from a 4 x 4 cm field to a 16 x 16 cm field ; as the back-scatter factor varies by 4 1/2 per cent over the same range , the skin dose will vary by a total of 20 per cent . percentage depth doses . owing to the fact that caesium sources are usually several centimetres long it is necessary to define the term source-skin distance ( S.S.D ) for these units . the definition of S.S.D adopted for this section is the distance of the skin from the front of the source container . four of the six centres had already chosen this definition for their own units . the other two centres were using different definitions but when their values for S.S.D were converted to the more common definition their percentage depth doses showed improved agreement with those from the first four centres . on the basis of the general definition , the S.S.D.s at which measurements were taken were as follows : for the purpose of comparison , all percentage depth doses were converted to 40 cm S.S.D using the method described by Johns , Bruce and Reid ( 1958 ) and Burns ( 1958 ) ( see appendix B ) ; 40 cm was chosen as being midway between the extremes , so as to minimise any errors that the method of conversion may introduce . 