The relationship between lung colony and in situ assays

The relationship between three different assays: tumor control, tumor growth delay and lung colony formation, was examined after fast neutron and gamma ray irradiations. Fibrosarcomas (NFSa) in syngeneic C3Hf mice were irradiated locally with 60Co gamma rays, fast neutrons or mixed beams (gamma rays and fast neutrons). A comparison between the lung colony assay and the TRT50 (50% tumor growth delay time) assay when cells were exposed to single doses of fast neutrons or gamma rays, resulted in identical growth delay times. The fraction of cells surviving a single dose of fast neutrons, was 10 times higher than the surviving fraction of cells after a single dose of gamma rays. Both doses resulted in the same tumor control probability (TCD50 assay). Neither repair of potentially lethal damage nor tumor bed effect was sufficient to explain the difference between cell survival and tumor control probability. The surviving fraction of cells following fractionated irradiations of gamma rays and fast neutrons were identical at 50% tumor control probabilities.