The effect of several electron affinic compounds on the radiation response of Chinese hamster cells irradiated in both dilute cell suspension and centrifugation pellets was investigated. p-Nitroacetophenone (500 μM), 175 μM nitrofurazone, and 200 μM dimethylfurmarate, when present during the irradiation of anoxic, dilute cell suspensions, produced dose-modifying factors of 1.5, 1.8, and 1.8, respectively. Anoxic cells irradiated at high cell concentrations and in the presence of a sensitizer experienced a two-stage loss of much of the sensitization normally seen in dilute cell suspension. A large component of sensitization was lost upon formation of the cell pellet, reducing the dose modifying factors to values approximately 70% of those obtained in dilute cell suspensions. This was followed by a slow disappearance of the remaining component of sensitization. Some possible mechanisms for this cell concentration effect are discussed. It is suggested that the cell pellet experiment may prove to be a useful indicator of the effectiveness of radiosensitizers in vivo.

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