On the one hand, hypoxic tissues are more resistant to radiation effects. On the other hand, it is known that tumoral cells are in general in hypoxic conditions.

In the process of radiotherapy, after a tissue irradiation, tumoral cells that were in hypoxic conditions are better oxygenated due to an easier access to oxygen sources because of the death  of surrounded cells. As a consequence, tumoral cells that were resistant to radiation become more and more radiation sensitive due to this process of Reoxygenation.

Reoxygenation is the phenomenon where cells that are hypoxic at the time of irradiation become oxygenated after irradiation. The rapid reoxygenation is attributable to effects on acutely hypoxic cells, the “slow” reoxygenation being attributable to effects on chronically hypoxic cells.

Observations come from animal studies, and the role of reoxygenation in human tumors remains uncertain.

It is also important to discuss reoxygenation in parallel with other 4 of 5Rs in radiotherapy and radiation biology: i.e., repair, repopulation, redistribution, and radiosensitivity.

Hypoxia (intratumor hypoxia of solid tumors) is a microenvironmental factor facilitating tumor radioresistance, chemoresistance, progression to aggressive phenotypes, and metastasis. Various efforts have been made to radiosensitize hypoxic tumors.

https://www.ncbi.nlm.nih.gov/pubmed/28741527

https://www.ncbi.nlm.nih.gov/pubmed/28741521

https://www.ncbi.nlm.nih.gov/pubmed/28721903

https://www.ncbi.nlm.nih.gov/pubmed/26927177

https://www.ncbi.nlm.nih.gov/pubmed/22268387

https://www.ncbi.nlm.nih.gov/pubmed/16414370

For brain, there is a dose-dependent association between hypoxia, upregulation of vascular endothelial growth factor (VEGF), and dysfunction of blood-spinal cord barrier following ionizing irradiation of the spinal cord. Bevacizumab (a humanized monoclonal anti-VEGF antibody) significantly reduced brain edema.

https://www.ncbi.nlm.nih.gov/pubmed/11309291

https://www.ncbi.nlm.nih.gov/pubmed/17236958

https://www.ncbi.nlm.nih.gov/pubmed/26148625

For the lens of the eye, hypoxia does not prevent onset or progression of cataracts, although reduction of ocular blood flow (by a monolateral carotid artery ligation) promoted radiation cataractogenesis.

https://www.ncbi.nlm.nih.gov/pubmed/13089110

https://www.karger.com/Article/PDF/264700

For skin (as part of the thermoregulatory system), low skin temperature reduces blood flow, induces tissue hypoxia leading to slight radioresistance, which can be sensitized by hyperbaric oxygen.

The human dermis is well-oxygenated, the epidermis is modestly hypoxic and portions of some sebaceous glands and hair follicles are moderately to severely hypoxic. Normal and irradiated skin has similar oxygenation patterns.

Enhanced radiogenic early skin reactions due to use of misonidazole have not been observed.

https://www.ncbi.nlm.nih.gov/pubmed/16810299

https://www.ncbi.nlm.nih.gov/pubmed/5842583

https://www.ncbi.nlm.nih.gov/pubmed/389904

Thank You so much Sir!!!

My pleasure, Dr Shrikant Patil