Radiobiological considerations in multifraction irradiation.

The initial shape of a cellular dose-survival curve is a factor of major importance in determining radiotherapy responses. It is probable that survival curves for mammalian cells exposed to X- and gamma-rays, as well as higher LET beams have an initial exponential region after which they bend downwards. The slope of this initial segment of the curve, and more importantly, the width of the shoulder before it bends significantly are the subject of the investigations to be reported. Because of the difficulty in measuring the biological effects of low doses of radiation--particularly in vivo--we have used multifraction experiments to determine the effects of the fractional doses. Such studies help determine the width of the shoulder and, with less precision, the slope of the initial part of the curve. The tissues we have studied include the bone marrow, testis, fibrovasculature, transplantable tumors, and the gastrointestinal tract. The results will be presented and some of the difficulties in their interpretation will be discussed.