Why are blood-producing tissues (like bone marrow) considered highly radiosensitive?

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The main idea is that radiosensitivity is driven by how actively a tissue cells are dividing and how undifferentiated they are. Blood-forming tissues like bone marrow contain cells that are constantly proliferating to replenish blood cells and are relatively immature compared to fully differentiated cells. Radiation damages DNA, and cells that are in active division have a higher chance of sustaining lethal genetic damage because they are repeatedly replicating and less capable of accurate repair at these stages. This combination—high mitotic activity and low differentiation—makes bone marrow extremely susceptible to radiation, so it is considered highly radiosensitive. In contrast, tissues with mature, specialized cells or those that aren’t dividing rapidly resist radiation better, and location near the exposure field or a term like “high resistance” doesn’t determine the intrinsic sensitivity.