B-16 melanoma transplanted into C57BL/6 mice was used to investigate the antitumor effect of Schizophyllan (SPG) combination with local irradiation by X-rays using 4 fractions of 3 Gy or 9 Gy and single fractions of 12, 19 and 26 Gy. In the groups which received a single fraction of 26 Gy and 4 fractions of 9 Gy, significant differences were found between the groups with/without SPG in terms of tumor volume change and lymph node and pulmonary metastases. In terms of survival, significant difference was found only in the 4 fraction of the 9 Gy group. SPG had no antitumor effect nor life-prolonging effect, when it was applied to tumors irradiated with single fractions of 12 and 19 Gy or 4 fractions of 3 Gy, except for some metastasis-suppressing effect in the group given 19 Gy. It seems that SPG has some adjuvant effect only where a limited number of tumor cells remain following rather large dose of X-irradiation. Furthermore, the gain produced by host-mediated immune response augmented by SPG seems to be larger in the 4 fraction group. Therefore, combination radiotherapy using SPG may be advantageous for patients with complete response or a good response after multifractionated irradiation.

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