The response of skin to fractionated heat and X rays

The combination of X rays with raised tissue temperature may be a useful method of treating some types of cancer. At present most information on the topic is for single treatments, but as improved heating methods develop it is likely that treatments may be given in several fractions as is the practice in conventional radiotherapy. It is therefore desirable to understand how cells and tissues respond to fractionated heat and X rays. When cells in vitro are exposed to a combination of heat and X rays the survival curve obtained normally has an increased slope and a reduced shoulder relative to X-ray treatment alone. There is additional evidence from work with X rays at low dose rate that the accumulation and repair of sublethal damage is inhibited by heating (Ben-Hur et al., 1974). However, in some cases heat appears to cause an increase in the extrapolation number of the X-ray survival curve (Loshek, 1976). It is therefore difficult to predict the response of cells and tissues to fractionated treatments with combined heat and X rays. We have begun an investigation of this problem in vivo using the response of the ear (pinna) in the mouse. After irradiation of the ear there is a skin reaction beginning after about 12 days which reaches a peak at about 30 days with subsequent healing. If the ear is also heated the response to X rays is enhanced but there is no qualitative change from the X ray response providing the heat treatments are not sufficiently severe to cause early necrosis, i.e. less than 42.5°C for 90 minutes, or equivalent (Law et al., 1978). In our experiments the ear has been heated by immersion in a temperature-controlled water bath (±0.05°C).