Accurate dose delivery in x irradiation with some practical designs of monitoring instruments.

Abstract

Accurate dose delivery in X-ray therapy, or in experimental irradiation, is best ensured by the use of a transmission chamber built into the tube housing. The ionization current may be indicated on a dose-rate monitor, stabilised or integrated. These methods are discussed, and a new method of dose control is proposed in which this ionization current is used to drive round the treatment timer. The exposure time is calculated in the usual way (prescribed dose/output of the field used) and set upon the dials of the “dose-rate controlled timer”. During the exposure, the output of the tube may vary but, since the timer rotates at a speed proportional to the instantaneous dose-rate, the prescribed dose is delivered—in practice, with an accuracy of 1 per cent. In this way, any consideration of in-air dose is avoided, as are the complications of stabilisers. The effects of field size and F.S.D. are taken into account in the preliminary calculation, available for later reference, and not as an ephemeral setting upon the dial of an integrating dosemeter. The instrument incorporates (1) a robust ionization chamber, built into the tube housing, connected to (2) a simple and reliable amplifier and dose-rate monitor (which may be used alone) driving (3) an integrating motor, which integrates dose upon a revolution counter, and which regulates the magnetic escapement of (4) a clockwork timer, which controls the X-ray exposure.