<i>Radiology now</i>. Fast neutrons—clinical requirements

Abstract

For decades, there have been numerous treatments for cancer and recently a plethora of drugs has added almost limitless possibilities to these. When fast neutrons became available clinically, there was, understandably, a demand for their effects to be compared objectively with conventional radiation. The precision and efficiency of modern megavoltage X- and γ-ray machines, and their ability to deliver an exact dose of radiation to the volume delineated, probably cannot be improved upon, and usually exceeds the ability of the clinician to localize the extent of the malignancy. Output of radiation is so good that treatment times are of the order of one to three minutes, and well-defined beams can be directed in any direction and in any plane. Patients can be treated in the most suitable of many possible positions and do not usually suffer skin reactions. Megavoltage machines are situated within hospitals and are available in most centres throughout the world where physicists and clinicians have wide experience in their use. In contrast, all existing neutron sources suffer from serious disadvantages. For instance, in the U.S.A., patients from the M.D. Anderson Hospital are transported 100 miles into Texas for each treatment. The M.R.C. Cyclotron in Hammersmith Hospital has a neutron beam which is fixed in the horizontal position and is collimated in squares or rectangles of fixed sizes. The energy (7·5 MeV) gives a dose distribution only slightly better than 250 kV X rays and the isodoses are rounded at the ends and show a clinically significant penumbra.