Impact of pulmonary metastases of the R1H-tumour on radiation tolerance of rat lung

Purpose : The aim was to investigate the influence of pulmonary metastases of the rhabdomyosarcoma R1H on the radiation response of the lung of the WAG/Rij rat. Material and methods : Three groups of animals were investigated: metastases-free animals treated with fractionated irradiation of the lungs; metastases-bearing animals receiving no irradiation; and metastases-bearing animals treated with fractionated irradiation initiated 14, 21 or 28 days after induction of pulmonary metastases of the R1H-tumour by i.v. injection of viable tumour cells. Metastases were thus treated at various well-defined sizes. Total doses of 20-60Gy were applied in fractions of 2 Gy within 11 days. Complication rate and survival time were used as endpoints. Results : About 2 months after onset of irradiation treatment, animals had to be sacrificed because of severe respiratory distress either caused by irradiation-induced lung damage (median 57 days, range 36-77 days), or because of development of lung metastases (65, 20-160 days). A decrease of the ED 50 (dose required to induce lethal lung damage in 50% of irradiated animals) was determined for metastases-bearing animals. This effect increased with metastatic volume. Conclusions : The results suggest that the presence of tumours in the lung decreased the lung tolerance to radiation. This effect can hardly be explained by a reduction in functional lung volume by metastatic volume.