Quantitative aspects of tumor radioresistance related to the presence of hypoxic cells

With respect to the influence of hypoxic cells on responses of tumors to fractionated radiation treatments, the phenomenon of reoxygenation after individual dose fractions as observed in many tumors is of crucial importance. The extent of reoxygenation and the rate at which it proceeds are likely to demand on several factors. At least three mechanisms can be envisaged in this respect. The first holds that, at specific distances from blood vessels, tumor cells are deprived of oxygen due to the limitation of oxygen diffusion and inadequate vascularisation. In recent years attention has been drawn to the possibility that insufficient oxygen supply to tumor cells might be caused by a second mechanism-abrupt changes in blood flow in capillaries. This would result in temporary transient hypoxia of cells in many areas in tumors, not all of them associated with necrotic zones. A third mechanism which might cause tumor cells to become hypoxic would be an increased metabolism during specific parts of the cell cycle