Radiosensitivity of hypoxic and proliferating clonogen in a human lung cancer grown in nude mice.

Using cultured and nude mouse tumor cells (IA) derived from a human lung cancer, we studied their radiosensitivity by focusing attention on the dynamics of tumor clonogens. The movement of clonogens in the regrowing IA tumor after irradiation can be divided into three phases: first, the early and rapid survival recovery (PLD repair) phase; second, the delay phase involving a certain lag in survival change; and third, the repopulation phase consisting of two stages: the anoxic repopulation before angiogenesis and the hypoxic repopulation in the presence of a poorly developed vascular network. Clonogens in a regrowing tumor after irradiation were found to actively proliferate even in an anoxic environment before angiogenesis and under the hypoxic conditions prevailing after the formation of a tumor with a poorly developed vascular system. This re-grown tumor was found to be more radioresistant than a sham-treated control tumor. It is believed that these clonogens are genetically selected under hypoxic conditions throughout the process of tumor growth and regrowth, and may be primarily involved in tumor recurrence or accelerated repopulation in fractionated irradiation.