A longitudinal study of radiation-induced changes in tumor vasculature by contrast-enhanced magnetic resonance imaging.

Dynamic contrast-enhanced MRI with two different-sized contrast agents, Gd-DTPA and Gadomer-17, was used to study the effects of radiation on the pharmacokinetics of the paramagnetic enhancement of water relaxation in the rat R3230 AC adenocarcinoma tumor model. The kinetics of enhancement was analyzed by a two-compartment pharmacokinetic model to derive parameters related to vascular volume (V(b)) and permeability (K(2)). Rats implanted with tumors were divided into two groups; one received 5 Gy and the other received 20 Gy (137)Cs gamma rays. Sequential dynamic contrast-enhanced MRI studies were performed, one before irradiation, one at day 1 after irradiation, and another at day 3 after irradiation, to investigate the effect of the radiation dose and the changes that occurred over time. The analysis was performed on a pixel-by-pixel basis to study the heterogeneity within the tumor. The pixel distribution profiles of V(b) and K(2) from each tumor were obtained to assess the regional radiation-induced effects on vascular volume and permeability. No significant change in vascular volume was detected with either Gd-DTPA or Gadomer-17 after irradiation of the tumor; however, a small dependence of K(2) on the radiation dose was observed. After low-dose (5 Gy) irradiation, the mean value of K(2) decreased by 46% at day 1 compared to the baseline, presumably due to cell swelling, and decreased further by 67% from the baseline on day 3. When the dose was increased to 20 Gy, the mean value of K(2) measured with Gadomer-17 did not show any significant changes at either day 1 or day 3 after irradiation. The value of K(2) measured with Gd-DTPA did not show any significant changes after either the low or the high radiation dose.