Effect of irradiation with fast neutrons or X-rays on the incidence of metastasis of transplanted B16 melanoma in mice.

The effects of X-rays and fast neutrons on the incidence of metastasis of B16 melanoma were investigated. Tumors were irradiated with the iso-effect dose (380 rad of 2 MeV fast neutrons, and 1900 rad of 200 kV X-rays) for regression and regrowth of the tumor, and then were excised 3 days after irradiation. Metastasis was examined macroscopically and microscopically in all organs and tissues of mice when death occurred or after sacrifice of surviving mice at 130 days after tumor inoculation. Metastasis was found in 16 to 22% of tumor-bearing mice irrespective of whether or not the tumor had been irradiated before excision. Almost all the mice with metastasis developed pulmonary metastasis accompanied by lymphogenous spread to the thoracic lymph node. The overall incidence of metastasis in the X- or fast neutron-irradiated mice was slightly but not significantly higher than that in the control mice. Irradiation significantly enhanced the incidence of metastasis when the tumor volume was smaller than 200 mm3, but not when the tumor volume was larger than 200 mm3. The incidence of metastasis was significantly enhanced in mice with tumors invading the muscle. The metastasis-enhancing effect of irradiation was noticeably greater with X-irradiation than with fast neutron irradiation in mice with localized tumors of B16 melanoma, whereas in mice with invasive tumors the effects were similar.