What is commonly used as a threshold parameter to assess radiation sensitivity in cells?

The cell cycle phase is indeed a commonly used threshold parameter to assess radiation sensitivity in cells. Different phases of the cell cycle exhibit varying degrees of sensitivity to radiation. For instance, cells are typically more sensitive to radiation during the mitotic phase (M phase) when they are actively dividing and undergoing significant genetic and cellular changes. Conversely, cells in the G1 phase (pre-DNA synthesis) or G0 phase (resting state) may demonstrate reduced sensitivity to radiation because they are not in an active state of division or immediate DNA replication. This relationship between the cell cycle and radiation sensitivity is a key aspect of radiation biology, as it informs treatment strategies in radiation therapy, where timing might be crucial to maximize damage to tumor cells while minimizing harm to normal cells.

Although other factors such as cellular DNA structure, oxygen levels, and cell membrane integrity can influence a cell's response to radiation, they do not serve as primary threshold parameters in the same way that the cell cycle does. Each of those aspects may contribute to the overall sensitivity but are not as directly related to the time-dependent context of cell division that characterizes radiation response.