A review is given of data published on the response of experimental tumours to fractionated irradiation including ten or more fractions. In addition experiments performed with the R1H rhabdomyosarcoma of the rat including regimens of six to 126 fractions (dose range 0.43 to 12.5 Gy per fraction) with an overall treatment time of six weeks are reported. Tumour response was assessed by: in vitro colony assay, tumour control probability, tumour growth delay, and flow cytometric determination of DNA-index. While a clear reduction of normal tissue damage is observed with increasing number of fractions, the response of the R1H-rhabdomyosarcoma is essentially independent of the number of fractions. This result encourages clinical research on the use of hyperfractionation for curative radiotherapy. The results also indicate that proliferation is decelerated during treatment. During irradiation the DNA-index of the highly hyperploid R1H-tumour cells decreases. This reduction is proportional to the total dose applied. Cells that are separated from the tumour and from metastases have defined DNA-indices. Thus, this special tumour system allows to determine the time point of onset of metastatic spread from flow cytometric DNA-index measurements.
