The B-16 melanoma transplanted into C57BL/6 mice was used to investigate the antitumor effect of Schizophyllan (SPG) when applied alone and in combination with local irradiation using pions with 4 dose fractions of 2 Gy or 6 Gy each. SPG was given intramuscularly in a daily dose of 10 mg/kg body weight for 25 consecutive days from day 4 after the initiation of irradiation, and thereafter three times a week up to day 45. The antitumor effect was evaluated by the changes in tumor volume. survival, the size of metastatic lymph nodes grossly involved, and the number of pulmonary metastatic nodules on the surface of the lungs. After 24 Gy, significant differences were found between the group treated with combined pions and SPG, and the group treated with pions alone in terms of tumor volume change. survival and lymph node, and pulmonary metastases. However, when SPG was applied to non-irradiated tumors or to tumors irradiated with only 8 Gy, it had neither antitumor nor life-prolonging effect. From the present study, it seems that a SPG, as a Biological Response Modifier, has some adjuvant effect only where a limited number of tumor cells remain following pion irradiation. Combination therapy using SPG may, therefore, be advantageous for patients with complete response or good partial response to pion irradiation.

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