Effects of fractionated irradiation on mouse lung and a phenomenon of slow repair

Abstract

The LD/50 between 40 and 180 days after irradiation of the thorax has been taken as a measure of lung damage in mice. The method has been used to derive the relationship between total dose, number of fractions (N) and overall treatment time (T). For X rays the results fit an Ellis type of relationship, i.e. total dose alphaN0-25 T0-07. This only applies for more than 8 fractions, below which N0-39 becomes a good fit to the results. For neutrons, the best fit is total dose alphaT0-07. Various possible explanations for the T factor are discussed, but for lung damage we attribute it to a slow repair process which possibly may only be observed in slowly proliferating tissues. Slow repair is about 100 times less rapid than Elkind-type of repair of sublethal damage and whereas Elkind repair is greatly reduced for irradiation with fast neutrons, relative to X rays, slow repair is not.

Late Radiation Injury of the Small Intestine Clinical, pathophysiologic and radiobiologic aspects: A review

“…Similar observations have been reported from a study of chronic rectal radiation injury in the rat (172). In addition to repair of sublethal and potentially lethal damage, the presence of a third repair mechanism, so-called slow repair, may influence the response to fractionated irradiation (173). The precise nature of this mechanism has not yet been determined but seems to be clearly different from repair of sublethal and potentially lethal damage.…”

“…In contrast to repair of sublethal damage, the amount of slow repair seems to be relatively independent of the RBE of the radiation. Although most prominent in the lung, evidence of slow repair has been found in several other tissues, including the gastrointestinal tract (173,175,176). Slow repair may be of particular importance in slowly proliferating tissues ( 177).…”

Re-Treatment After Full-Course Radiotherapy: Is It a Viable Option?

“…Experimental studies in mice demonstrate very good recovery and re-irradiation tolerance in the lung for the end point of lethal pneumonitis (22,23). Mice given initial doses equivalent to 30 to 50% of BED t could be re-irradiated with doses equivalent to 100% BED t at 4 to 8 weeks.…”

‘Fading Times’ Required for Apparently Complete Repair in Irradiated Tissues Assuming the Linear Quadratic Model of Dose Response

“…True values for T 1 / 2 are uncertain and are only now being sought in experiments designed for that purpose (Thames 1984(Thames , 1985. It is conceivable that a slower component of repair will be found in some tissues, although there is as yet no experimental evidence for it except in lung, where slow repair was reported with a half-life of several days (Field et al 1976).…”