Quality of Radiation in Roentgen Therapy

I first met Russell Carman when he was President of the Radiological Society of North America and I was just learning my way about in radiology. I shared your admiration for him, those of you who knew him, and shared your depression and sense of loss when he died, only a few years later. There have been ten Carman Lectures, delivered by a selection of the best minds of our Society, who have often expended their best efforts in honoring his memory. Need I say that I feel keenly the honor of joining this illustrious company? Although Russell Carman's career was in diagnostic radiology, yet I have chosen a therapeutic subject for this lecture. The invention of 200-kv. x-ray machines created such a revolution in therapy that we all became very quality conscious. Dr. Chamberlain and I were deeply concerned to choose between 0.5- and 0.75-mm. copper filtration—this in spite of our experience in measuring all the deep-therapy outfits in California (1) and finding their 10-cm. depth doses all alike (regardless of large differences in output). Later we tried two different qualities on small skin areas, and found that the erythemas from equal doses looked alike (2). The conviction that the importance of differences in quality is apt to be overestimated and over-emphasized has grown rather than lessened with added clinical experience. I have tried faithfully to choose the quality according to the case, but remain unconvinced of my clinical ability to distinguish the effects of small differences, easily measurable by physical means. I have had opportunity to talk about the biologic and clinical implications of quality before (3). This time I wish to start with the more practical engineering and physical aspects. We owe a great debt to the engineers who have built machines to give a range of quality from grenz rays at 10,000 volts through one and two megavolts and on, with the betatron, to 30 megavolts and beyond. Surely, if we can only know what we need, we can have it, just so we can pay for it! I am overcome by admiration for the inventiveness and skill of the men who have driven x-ray production to such heights. And presumably the end is not yet! We owe a great debt, also, to the physicists who have taught us to measure x-ray precisely and have devised a unit of most peculiar “dimensions” exactly suited to our clinical needs. It may require a 20 per cent increment in dose to be observable clinically, but we can do the physical measurement with an accuracy of 2 per cent. Most of us, in fact, do this as an everyday matter. The physicists have given us standards of quality, too. We have the international agreement to designate quality by halfvalue layer (h.v.1.), naming a suitable absorber. Now this is a very delicate method, capable of detecting a change of a few kilovolts on the tube or a few per cent increase in filtration. I am not sure that this was so great a service.