[Response of digestive tract mucosa to single and fractionated irradiation. Practical implications for radiotherapy]

After a short description of the gastro-intestinal syndrome (for a single "acute" irradiation) and a discussion of the importance of fractionation in radiotherapy, the authors report their experimental results on intestinal tolerance in mice after fractionated irradiation. Intestinal tolerance was assessed from LD50 after abdomen irradiation. Variation of LD50 was first studied as a function of the fraction number N (keeping constant the overall time T). From these data iso-effect curves were calculated which can be used for clinical applications for correcting the total dose when the fraction number has to be modified (for T constant). In a second series of experiments, variation of LD50 was studied as a function of overall time T (keeping constant the fraction number N), this study confirms the large recovery capability of the intestinal mucosa, which is due to the combination of 2 mechanisms: shortening of the mitotic cycle of the surviving stem cells and increase of the size of the stem cell compartment. These 2 mechanisms were evaluated quantitatively for a fractionated irradiation. The data obtained for intestine are compared to similar data obtained for other tissues (skin, lung). The authors emphasize the risk of applying to late effects the conclusions obtained for early effets.