Experimental study on the evaluation of early therapeutic effect after irradiation using high-resolution MRI and 1H-MRS

An experimental study using VX2 carcinoma in rabbits was performed to clarify whether high-resolution MRI and 1H-MRS can reflect early histolopathologic changes after irradiation and whether high-resolution MRI and 1H-MR spectroscopy (MRS) can become useful tools for the evaluation of the early therapeutic effect after irradiation. VX2 carcinoma was implanted in the bilateral thighs of twelve Japanese white rabbits and irradiated using 9-MeV electron beam with a dose of 30 Gy/fr. The irradiated group was divided into four subgroups according to the time after irradiation. At 1, 3, 7, 14 days after irradiation, MRI and 1H-MRS examinations were performed, and rabbits were sacrificed for histopathological examinations. Intermediate to liquified necrotic areas were identified as high signal intensity on T2-weighted images. These necrotic areas were not enhanced on contrast-enhanced images. On the other hand, viable tumors as well as pseudocapsules were enhanced. Lactate (Lac)/Creatine (Cr) of viable tumors in the irradiated group showed higher values than those in the control group at 3 days after irradiation. Lac/Cr of viable tumors showed higher values than those of necrotic areas in the irradiated group at 1 and 3 days after irradiation. Choline (Cho)/Cr of viable tumors showed higher values than those of necrotic areas in the irradiated group at 7 days after irradiation. T2-weighted images and contrast-enhanced images were both useful for the evaluation of necrotic areas, and their utility in determining the early therapeutic effect after irradiation was validated. 1H-MRS could thus capture early tissue injury after irradiation, and its efficacy in the prediction of the therapeutic effect was suggested. (author)