Figure 1.

Distribution of hypoxia levels in tumors. (A) Illustration showing distribution of hypoxia levels from mild to severe in a section through a solid tumor. Non-hypoxic levels are seen close to capillaries (in red). (B) Close-up of a region in (A), showing the gradient in hypoxia levels from a capillary towards severe hypoxia. Symbols for different cell types and extracellular matrix (ECM) are indicated. (C) Illustration of how oxygen enhancement ratio (OER) increases with increasing oxygen concentration or tension in tumors, where OER is defined as the ratio between the dose needed to cause the same harmful effect to cells under anoxia and when oxygen is present. Data relative to maximum OER are shown and the curve is based on the study by Koch and coworkers described in [3]. The ranges indicated refer to the scale of the x-axis and are median pO₂ values reported across tumor types, median pO₂ cutoff for hypoxic fractions associated with poor radiotherapy outcome across tumor types, and median oxygen concentration reported for activation of hypoxia inducible factors (HIF). The pO₂ data are collected from [5] and the HIF data are based on [6,7]. (A–C) Hypoxia levels are indicated by the color code, with approximate oxygen concentrations (% O₂) and tensions (mmHg) provided by the x-axis in (C).