The left side shows that, as oxygen diffuses from the vessels, it is used by the 'normoxic' (not radiobiologically hypoxic, but often still hypoxic to a certain degree) tumour cells, which results in a decrease in oxygen partial pressure (pO2) as shown in the insert.16 This decrease in oxygen pressure eventually triggers the development of chronically hypoxic cells at the edge of the cord. On the right side perfusion through the vessel has been transiently compromised and all the cells around that occluded vessel become acutely hypoxic.11 Suggestions for causative factors for this shut-down in flow include vessel plugging by blood or circulating tumour cells, collapse of vessels in regions of high interstitial pressure, or spontaneous vasomotor activity in normal vessels incorporated in the tumour that then affects downstream tumour vessels.110 Representative examples of the flow and associated pO2 changes reported during a 60-min period are shown in the four panels at the right, which illustrate a rapid and complete shutdown (panel 1) and recovery (panel 4), a gradual decrease with time (panel 3), and repeated fluctuations (panel 2).20, 21 Modified with permission from Elsevier Ltd © Horsman, M. R. Int. J. Radiat. Oncol. Biol. Phys. 42, 701–704 (1998).111