Difference in 3H-thymidine incorporation after irradiation between murine B16 melanoma and squamous cell carcinoma in vivo.

The tumor growth and cell proliferation kinetics of B16 melanoma and transplantable squamous cell carcinoma (BSC tumor) in C57BL/6 mice were studied after single-dose X-ray and fast-neutron (2 MeV) irradiations. From tumor volume change studies, B16 tumor showed a high relative biological effectiveness (RBE) value (5.5) compared to that of BSC tumor (3.5). The tissue characteristics of the two tumors were not significantly different, but the post-irradiation changes in mitotic index (MI) and lebeling index (LI) were dependent on the tumor and on the nature of the radiation. After irradation, mitosis was immediately inhibited (G2 block) but recovered within several hours in BSC and X-ray-irradiated B16 tumors. The neutron-irradiated B16 tumors showed no recovery up to the end of the observation period. After X-ray irradiation, of B16 tumors, LI showed an immediate reduction, while the reduction was delayed several hours (equal to G1 + M) in BSC tumors. The extent of reduction was dose-dependent, and its recovery was coincident with the recovery of mitosis. The change in LI of neutron-irradiated BSC tumors was similar to that with X-rays, but no change in LI was observed in neutron-irradiated B16 tumors over 18 hr. From these results, it is assumed that the cell progression of B16 tumors is susceptible to ionizing radiation, and that G1 block is induced as a result of irradiation.