Noninvasive Monitoring of Radiation-Induced Early Therapeutic Response Using High-resolution MR Imaging and Proton MR Spectroscopy in VX2 Carcinoma

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The purpose of this study was to evaluate the usefulness of high-resolution MRI (HR-MRI) and proton MR spectroscopy (¹H-MRS) for monitoring the early therapeutic response to radiotherapy. Twenty rabbits with VX2 carcinoma were divided into control (n = 8) and irradiation (n = 12) groups. The irradiation group underwent HR-MRI and ¹H-MRS using a microscopy coil at 1, 3, 7 or 14 days after irradiation. Rabbits in the control group were subjected to HR-MRI and ¹H-MRS at the same time intervals. All rabbits were killed after imaging and subjected to histopathologic examinations. The diameter of necrosis by HR-MRI was then compared to that on the gross specimens. The ratios of choline/creatine (Cho/Cr) and lactate/creatine (Lac/Cr) on the tumor and necrotic area detected by in vivo ¹H-MRS were compared between the control and irradiation groups, respectively. In addition, the ratios of Cho/Cr and Lac/Cr were compared between the tumor and necrotic area in each irradiation group. A significant correlation was found between the diameter of necrosis in each sequence of HR-MRI and that in the gross specimens (r = 0.84–0.91, p = 0.03– < 0.003). The ratios of Lac/Cr in the tumors of the irradiation groups were significantly higher than those in the control groups after 1 day and 3 days of irradiation (p = 0.04, and p = 0.02). Histological analysis showed necrosis and swelling of the endothelia of capillaries and arterioles at 1 day and 3 days after irradiation. It was suggested that HR-MRI and ¹H-MRS are useful methods for monitoring the early therapeutic response to radiotherapy.