Poole — On the bistrihutioii of Activity in Radium Therapy. 473 



assumed to be concentrated at 15 axial points one millimetre apart. The 

 stopping power of the wall of the serum needle is assumed to be equivalent 

 to 1 mm. of flesh for normal rays, and proportionally more for oblique rays. 

 For the latter the extra thickness of glass traversed must also be allowed 

 for. Weighing showed that O'l mm. is a fair figure to assume for the 

 thickness of wall of a typical emanation tube, as issued by the Eoyal Dublin 

 Society Eadium Institute. This is equivalent to 0'25 mm. of flesh. The 

 outside radius of the serum needle being 0'6 mm., the total absorption 

 between a point on the axis and a point P at a radial distance a from 

 in the tissue is equivalent to that of a + 1-25 - 0-6, i.e. a + 065 mm. of 

 flesh. This is equivalent to the bare tube plus a + 0'4 mm. of flesh. For 

 another axial point 0' distant h mm. from we have distance 



. O'P 



O'P = \/o? + V^ and " equivalent" distance = ^ (« + 0'65) - 0-25. 



We look up the corresponding transmission factor from the "leather" curve, 

 taking the unscreened activity as 1000, and multiply this by , g , where m 



is the number of millicuries per mm. of the tube. 



Summing the results for the 15 axial points, we obtain the activity for 

 any given point P. The units are such that the activity due to one 

 millicurie at one millimetre distance, with no other screening than the 

 normal screening of the emanation tube wall, is lOUO. The figures in the 

 table are calculated for points opposite the centre of the capillary, so that 

 the axial points considered, with the exception of the central one, occurred 

 in pairs, thus simplifying the arithmetic. 



jEffect of Distance. Thick Surface Apj^lieator. 



A type of applicator which is considerably used in Dublin consists of 

 a rectangular box made of brass, o mm. thick, the internal dimensions 

 being 30 x 20 x 0'6 mm. To the inner surface of this a number of 

 emanation tubes are fixed, with a little wax so as to give as uniform a 

 distribution of activity as possible over the available area of 6 sq. ems. 

 As the diameter of these tubes is about 0-8 mm., we may (making a small 

 allowance for the wax) assume that the centre of the tube is 3'5 mm. 

 from the nearer outer face of the box, and 8*5 mm. from the further 

 outer face. 



In estimating the activity at various depths due to such an applicator 

 in contact with the skin the following approximations are made to simplify 

 the calculation : — • 



(1) The box is assumed to be circular, of such a diameter so as to have 

 the same area, and the calculations made for axial points only. 



