PURPOSE: The aim of the present study was to determine the radiosensitivity and the growth kinetics of pulmonary micrometastases of the R1H-tumor of the rat. MATERIAL AND METHODS: Lung metastases were induced by intravenous injection of viable tumor cells. At different time intervals (3 to 32 days) after injection, lungs were locally irradiated, using 1.5 Gy per fraction. Total doses of 6 to 33 Gy were administered within eleven days. Animals exhibiting a dyspnoea were sacrificed and the number of metastases in the lungs were determined. The data were evaluated using the multi-target model. RESULTS: Beginning in the fifth week after tumor cell inoculation the animals started to exhibit a pronounced dyspnoea and were sacrificed. Sections revealed an extensive metastatic infiltration of the lungs. The number of metastases found in the lungs decreased with increasing total dose. The irradiation showed to be more effective in case of short intervals between induction of metastases and start of treatment. CONCLUSIONS: Metastatic growth does not start earlier than three days after tumor cell inoculation. The doubling time of the tumor cells in the micrometastases decreases continuously from 5.2 to 1.7 days between the third and the 32nd day. During the first four weeks of metastatic growth the radiosensitivity of metastatic R1H-cells in the lungs is the same as in vitro.
