The Distribution of Dose-rate across the Field of an X-ray Therapy Tube

The permissible variation of dose-rate across the largest field of an X-ray therapy tube should be specified and it should not exceed avout 10 per cent across a 25 cm diameter field at 50 cm F.S.D. This can be achieved at a given H.V.L. by a good design of tube shield and by a proper choice of target angle. The optimum target angle is about 45 deg. at 3 mm Al H.V.L., 30 deg. at 0·5 mm Cu and somewhat smaller at 1·7 mm Cu. In these respects each of the tubes investigated was capable of improvement. The use of a re-entrant glass window to the tube housing and of a target angle as large as 35 or 45 deg., or as small as 20 deg., is deprecated. A compensating filter for a 20 deg. target is described which, without loss of output, makes the field satisfactorily uniform in dose-rate and quality. The improvement in the distribution in air is reflected as a similar improvement in that in phantom. In nearly four years of use no significant change has been observed accompanying ageing of the target or replacement of the insert.