Tumor metastasis exposed to high-LET radiations

The aim of this study is to clarify the effect of carbon ion beams (C-ion) on metastatic potential of melanoma in vitro and in vivo. A highly metastatic mouse malignant melanoma cell line B16/BL6 were maintained in RPMI-1640 medium supplemented with 10% FBS and antibiotics. [in vitro] Samples were prepared 2 days before and then irradiated with C-ions or X-rays. Surviving fractions were obtained using colony formation assay. Migration and invasion activity as metastatic potentials of the cells were examined using the Boyden-chamber method and the Matrigel invasion assay. [in vivo] The cells were implanted in right-leg of C57BL/6J mice at 1 x 10⁶ cells/mouse 9-10 days before irradiation. Tumors were irradiated at the center of 6 cm-spread-out Bragg peak (SOBP) of C-ions, or γ-rays. Radiosensitivity for whole tumor was obtained by the tumor growth delay method, and that for individual cell in a tumor was obtained by an in vivo-in vitro assay. The metastatic effects were analyzed with the spontaneous lung metastasis model. The dose averaged linear energy transfer (LET) values of carbon beams were approximately 50 keV/μm. [in vitro] Survival curves showed higher cytotoxic effects of C-ions compared with X-rays, and the relative biological effectiveness (RBE) values were 1.96. The potential of migration and invasion were suppressed by C-ions at all dose points (0.50 to 8.0 Gy) tested, however it was enhanced by X-rays at low dose points (0.50 and 1.0 Gy) than non-irradiated controls. The RBE values obtained from migration and invasion test were higher than that from cell killing. [in vivo] C-ions significantly suppressed the tumor growth, and the RBE was 2.64. The numbers of lung metastatic nodules after tumor-irradiations decreased with the dose, and C-ions were more effective compared with γ-rays. The metastatic potentials of survived cells in a tumor after irradiation was analyzed with the number of metastatic lung colony from implanted tumors and survival of irradiated and explanted cells from a tumor. Smaller number of metastasis was found for C-ions than γ-rays when the numbers were compared with biological equivalent dose. It might suggest that C-ion inhibit metastasis at radiotherapy compared with low-LET photons. (author)