Radiosensitivity of rat mammary tumors correlates with early vessel changes assessed by power Doppler sonography

Objective. To investigate the changes occurring in the vascularization of tumors during irradiation, we used a model of autochthonous mammary tumors in rats and assessed early vascular changes after irradiation by power Doppler sonography. Methods. Mammary tumors were induced in 24 female Sprague Dawley rats by a single subcutaneous injection of N-nitroso N-methyl urea. After tumor areas reached 1 cm(2), the animals received a single fraction of 18-Gy radiation or intraperitoneal saline injection. Power Doppler sonographic quantification of detected vessels was performed 1 day before irradiation and 7 days after the use of a power Doppler index of 5 different tumor imaging planes. Final tumor shrinkage was compared with early changes in the power Doppler index. Not all tumors regressed in a similar fashion. Radiosensitive tumors were defined as tumors with a greater than 50% decrease in baseline area 28 days after irradiation, whereas radioresistant tumors were tumors with a less than 50% decrease in baseline area. Statistical analysis was performed by the Mann-Whitney U test. Results. Tumor area changes were similar in radioresistant and radiosensitive tumors 7 days after irradiation (-41% and -35%, respectively; P >.05, not significant), whereas reduction in the power Doppler index was significantly greater in radiosensitive tumors (mean value, -63%) than in radioresistant tumors (mean value, -12%) (P =.001). Late tumor regrowth was correlated with day 7 power Doppler index changes (P=.009). A 40% reduction in the power Doppler index at day 7 distinguished 8 of 9 radiosensitive tumors and 8 of 9 radioresistant tumors (P =.003). Conclusions. This study suggests that early changes in tumor perfusion as assessed by power Doppler sonography after tumor irradiation may precede the long-term tumor regression.