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A review of the first year's clinical work on the M.R.C. 8 MeV linear accelerator is given. The biological efficiency of pulsed 8 MeV X rays is discussed. The absorption in tissue of 250 kV and 8 MeV X rays is compared and the clinical applications of megavoltage therapy surveyed. The planning techniques employed and field arrangements used for the 257 patients treated in the year's work are described.

Calculated values of integral doses delivered during radiotherapy at 8 MeV and with radiation of H.V.L. = 2 mm of Cu are presented. These values have been checked experimentally. Meredith and Neary's method is found to give reliable values of integral dose at H.V.L. = 2 mm of Cu. The experimental measurements were made with a celluloid model of the human body, the model being composed entirely of ionization chambers. A description of the model is given, together with an analysis of the derivation of integral doses from measured ionization currents. The integral doses delivered during typical treatments at the two qualities are compared. It is found that in practice integral doses are not always greatly reduced at 8 MeV, owing to the differing field arrangements used.

The 8 MeV linear accelerator of the Medical Research Council, which is installed at Hammersmith Hospital, London, is described. An account is given of the physical measurements which have been made to enable the best use to be made of the machine for X-ray therapy. These measurements include work on protection, beam flattening, output calibration and isodose curves. The machine normally gives a stabilised dose-rate of 100 rads/min at 2 cm deep in a water phantom with its surface at 1 m from the target. Four hundred patients were treated between September 1953 and December 1954. A report is made on the operation of the machine, which has been very satisfactory.

The X-ray set is mounted at the centre of a strong gantry supported at each end by arms which are mounted on bearings and carry counterweights. The gantry and the treatment couch are free to rotate about axes which are arranged to pass through the treatment area on the patient's skin. With these movements the direction of the X-ray beam can be adjusted independently of other movements, so that patients can be set up quickly and accurately. This mounting is suitable for equipment that is large and heavy.

Before the discovery of the nuclear chain reaction, the cyclotron was the only important source of radioactive isotopes for medical purposes. The development of the nuclear reactor made large scale isotope production possible and a wide range of radioactive isotopes is now readily available at a reasonably low cost. In consequence the demand for cyclotron-produced isotopes has decreased. However, there is now an increasing requirement for isotopes which are not available from the reactor for one reason or the other but which can be produced in a cyclotron. The true magnitude of this requirement is as yet unknown and can only be properly assessed by making such isotopes more freely available. Isotopes are produced in the reactor by neutron capture, by fission, or when the thresholds are suitable, by (n, p) or (n, α) reactions. It can be seen from Fig. 1 that (n, γ), (n, p) or (n, α) reactions tend to produce nuclides with a surplus of neutrons. Such isotopes decay towards the stability line by emission of ...