Rhabdomyosarcomas of the rat were irradiated with 15 Gy of X-rays and the changes in several parameters were measured as a function of time. In the course of 10 days after irradiation, the percentage of tumour cells changes by a factor of 10, the fraction of clonogenic cells by a factor of 20, and the volume of tumour cell nuclei by a factor of nearly 3. During the same time interval, tumour volume increases only by 50%, indicating that volume curves yield little information about the drastic changes occurring in tumours during the first 2 weeks after irradiation.
