Hyperbaric oxygen therapy is a medical treatment that delivers 100 percent oxygen at 2.0 to 2.4 atmospheres of pressure inside a sealed chamber. In delayed radiation injury, that high-pressure oxygen does something specific and measurable: it stimulates new blood vessels to grow back into tissue that radiation therapy permanently scarred years or decades earlier.
TL;DR: HBOT promotes angiogenesis in irradiated tissue by triggering a brief but repeated low-oxygen signal once each session ends. That signal recruits stem cells, releases vascular endothelial growth factor (VEGF), and over 20 to 40 sessions stimulates new blood vessels to grow into damaged areas. The new vessels are permanent and can rebuild oxygen delivery in tissue that has been quietly starved for years. This is the central mechanism behind HBOT’s role in delayed radiation injuries treated under provincial health insurance at hospital programmes across Canada.
What is angiogenesis, and why does irradiated tissue need it?
Angiogenesis is the medical term for the body’s growth of new blood vessels from existing ones. It happens during normal wound healing, exercise adaptation, and recovery from injury. In healthy tissue, the body matches blood supply to demand: muscles that work harder grow more capillaries, and skin that heals after a cut sprouts new vessels into the repaired area within days.
Radiation therapy disrupts this process in a particular and persistent way. The energy of therapeutic radiation damages DNA in tumour cells (the goal) but also damages DNA in the small blood vessels that pass through the treatment field. Endothelial cells lining those vessels accumulate scarring, the vessel walls thicken, the lumen narrows, and over months to years the local capillary network fails. By the time symptoms appear, the affected area can be running on a fraction of its normal blood supply, even though the surface might look intact.
Robert Marx, the maxillofacial surgeon who developed the standard hyperbaric protocol for irradiated jaw bone in the 1980s, summarised the chronic state of irradiated tissue as three H’s: hypoxia (low oxygen), hypovascularity (too few blood vessels), and hypocellularity (too few living cells to do the work of repair). All three feed each other. Without enough oxygen, cells cannot survive long enough to lay down new tissue. Without new tissue, blood vessels cannot be repaired. The tissue is locked into a slow decline.
How does HBOT trigger new blood vessel growth?
Hyperbaric oxygen therapy delivers 100 percent oxygen at 2.0 to 2.4 atmospheres absolute (ATA) for 90 to 120 minutes per session. The pressure forces large amounts of oxygen to dissolve directly in the blood plasma rather than only travelling on haemoglobin. Tissue oxygen tensions in the treated area rise from a baseline of around 30 mmHg to several hundred mmHg during a session.
The angiogenic effect comes not from the high-oxygen state itself, but from the contrast between the high-oxygen state during a session and the lower-oxygen state when the patient leaves the chamber. This intermittent hyperoxia-then-normoxia cycle is the signal. The cell responds as if it has just experienced a relative drop in oxygen, even though it is now back at ordinary atmospheric levels. Several pathways activate:
- VEGF release: Vascular endothelial growth factor is the master signal for new vessel sprouting. HBOT increases VEGF expression in irradiated tissue and in endothelial progenitor cells circulating in the blood.
- Bone marrow stem cell mobilisation: Repeated HBOT sessions release CD34-positive endothelial progenitor cells from bone marrow into the circulation. These cells home to oxygen-starved tissue and contribute directly to new vessel formation.
- Nitric oxide signalling: Nitric oxide induces VEGF and basic fibroblast growth factor (bFGF), which together drive both new vessel formation and the supporting cells around them.
- Fibroblast proliferation: Fibroblasts produce the collagen scaffold that new blood vessels grow along. HBOT stimulates fibroblast division in tissue that has been too oxygen-starved for normal cell replication.
- HIF-1 alpha modulation: Hypoxia-inducible factor 1 alpha is the cellular oxygen sensor. HBOT modulates HIF-1α activity in a tissue-specific way, suppressing it where excessive (such as in tumour-promoting hypoxia) and supporting it where helpful (such as during recovery from ischaemic injury).
This intermittent-hyperoxia mechanism is why a single dive does little for chronic radiation injury. The signal needs repetition. Twenty to thirty sessions over four to six weeks are the threshold at which most patients begin to show measurable changes in tissue oxygenation. Sixty sessions is the standard course for radiation-induced soft-tissue and bone necrosis treated under the Marx protocol. Some indications, including refractory cases of radiation cystitis and chest-wall radionecrosis, may require additional cycles separated by rest periods.
How long does the new vasculature last?
The new blood vessels are permanent. This is the most important point about HBOT-induced angiogenesis. Once a capillary network is rebuilt in irradiated tissue, the patient does not need to keep returning to the chamber to maintain it. Tissue oxygen measurements taken months and years after a completed course consistently show vascular density well above pre-treatment levels.
Marx’s foundational tissue-oxygen studies from the 1980s and 1990s established that 30 sessions could increase capillary density in irradiated mandible to about 80 percent of non-irradiated levels, with the effect plateauing after that and persisting indefinitely. More recent work in soft tissue and bladder mucosa has confirmed similar permanence in those tissues.
The clinical implication is that HBOT for delayed radiation injury is a course of treatment, not a chronic therapy. A patient finishing 30 to 60 sessions for radiation cystitis, proctitis, osteoradionecrosis of the jaw, or chest-wall necrosis is treating the underlying tissue defect once. Future symptom management may need additional treatments only if the radiation dose was very high or new injury appears in adjacent tissue years later.
What does the clinical evidence show?
Published clinical evidence for HBOT-driven angiogenesis in radiation-injured tissue is among the strongest in hyperbaric medicine. The Cochrane review on hyperbaric oxygen for late radiation tissue injury, the foundational Marx studies, and the more recent randomised trials in radiation cystitis (RICH-ART, 2025 long-term follow-up) and breast cancer late toxicity (HONEY trial, 2024) all support the core mechanism. Imaging studies using laser Doppler flowmetry and transcutaneous oxygen monitoring show measurable improvement in tissue perfusion after standard HBOT courses.
Important context for patients reviewing the evidence: the angiogenic response is most reliable when treatment begins while some viable tissue remains in the affected area. HBOT cannot regenerate tissue that has already been completely necrotic for years; the goal is to halt progression and rebuild the capillary network in marginal tissue. Clinical responses are typically evaluated at the end of the treatment course rather than mid-course, since most of the angiogenic effect appears in the second half of the regimen.
| Indication | Standard course | Typical response window |
|---|---|---|
| Osteoradionecrosis of the jaw (Marx protocol) | 20 to 30 sessions pre-surgery, 10 sessions post-surgery | Tissue oxygen improvement by session 20; healing visible by session 40 |
| Radiation cystitis | 30 to 60 sessions at 2.0 to 2.4 ATA | Bleeding reduction by session 15 to 20 in most responders; full effect at end of course |
| Radiation proctitis | 30 to 60 sessions at 2.0 to 2.4 ATA | Bleeding and pain reduction over the second half of the course |
| Chest-wall radionecrosis (post breast radiotherapy) | 30 to 40 sessions, with additional cycles for surgical reconstruction support | Pain and ulcer reduction by mid-course; full vascular recovery at completion |
| Soft-tissue radiation necrosis | 30 to 60 sessions at 2.0 to 2.4 ATA | Healing of refractory ulceration by mid-to-late course |
Where do the stem cells come from, and why does that matter?
Beyond VEGF release, HBOT recruits endothelial progenitor cells from bone marrow into the bloodstream. Stephen Thom and colleagues at the University of Pennsylvania showed in a series of human studies that a single 2-hour session at 2.0 ATA roughly doubles the circulating CD34-positive progenitor cell population, and that an eight-fold increase is achievable over a 20-session course.
Once in circulation, these progenitor cells home to oxygen-starved tissue using gradient signals like stromal cell-derived factor 1 (SDF-1) and the chemokine receptor CXCR4. They embed in the endothelial layer of newly forming vessels and contribute directly to vascular regeneration. This is one of the few clinical settings in which a non-pharmacological intervention reliably mobilises adult stem cells for therapeutic effect, and it explains why HBOT’s vascular effects extend beyond the tissue immediately under treatment to remote ischaemic areas as well.
What are the limits and contraindications?
HBOT is not a universal solution for every irradiated tissue problem. Several caveats apply.
- Tissue must retain viability. The angiogenic response builds capillaries into surviving tissue. Areas of complete bone or soft-tissue necrosis cannot be regenerated by HBOT alone and may require surgical debridement and reconstruction with HBOT as adjunct.
- Active malignancy is a relative contraindication. While long-standing concern that HBOT might accelerate cancer growth has not been borne out by clinical evidence, most hyperbaric programmes still defer treatment until the patient is in remission or until oncology gives clearance.
- Untreated pneumothorax is an absolute contraindication. The compressed gas trapped in a pneumothorax expands on decompression and can cause tension pneumothorax. Tube thoracostomy is performed first.
- Severe chronic obstructive pulmonary disease with bullous lung disease is a relative contraindication that requires individual assessment.
- Claustrophobia can interfere with multiplace chamber treatment and may require monoplace alternatives, sedation strategies, or behavioural support.
Treatment should be delivered at a hospital-based hyperbaric programme staffed by physicians with formal hyperbaric medicine training. The 11 hospital hyperbaric programmes in Canada (in Ontario, Quebec, British Columbia, Alberta, Nova Scotia, Newfoundland, and Saskatchewan) all treat radiation injury indications under provincial health insurance with a physician referral.
How is HBOT for radiation injury covered in Canada?
Delayed radiation injury is one of the 14 conditions Health Canada recognises as accepted indications for hyperbaric oxygen therapy. Coverage at hospital-based programmes flows through provincial health insurance: OHIP in Ontario (Toronto General, Hamilton General, The Ottawa Hospital), MSP in British Columbia (Vancouver General Hospital), AHCIP in Alberta (Misericordia Edmonton, Foothills/AJECCC Calgary), RAMQ in Quebec (Sacré-Coeur Hospital, Hôtel-Dieu de Lévis), MSI in Nova Scotia (QEII Halifax), MCP in Newfoundland (Health Sciences Centre St. John’s), and Saskatchewan Health (Wigmore Hospital, Moose Jaw, capacity permitting).
For patients in Manitoba, New Brunswick, Prince Edward Island, Yukon, Northwest Territories, and Nunavut, where there is no in-province hospital hyperbaric programme, treatment is coordinated through inter-provincial referral, most commonly to Edmonton or Ontario depending on geography. Manitoba patients accessing HBOT for radiation injury are typically referred to Misericordia Edmonton or to one of the Ontario programmes; New Brunswick and PEI patients are commonly referred to QEII Halifax. April Daffodil Month, Canada’s annual cancer survivorship awareness initiative led by the Canadian Cancer Society, is a useful prompt to ensure cancer survivors with delayed radiation effects know this option exists and how to access it. Canada Hyperbarics maintains a current directory of these programmes.
Frequently asked questions
How quickly will I see results from HBOT for radiation injury?
The angiogenic response builds gradually. Most patients report no obvious change in the first 10 to 15 sessions; measurable improvement in tissue oxygenation typically appears between sessions 20 and 30. Symptom improvement in radiation cystitis and proctitis often becomes noticeable by session 20, while bone-healing changes in osteoradionecrosis are typically seen by session 30 to 40. Complete vascular regeneration is generally evaluated at the end of the full course rather than mid-course.
Will the new blood vessels last after I finish treatment?
Yes. Tissue oxygenation measurements taken months and years after a completed HBOT course consistently show vascular density well above pre-treatment levels. The new capillaries are permanent. HBOT for delayed radiation injury is a course of treatment, not a chronic therapy.
Does HBOT help all forms of radiation injury equally?
The strongest evidence is in osteoradionecrosis of the jaw, radiation cystitis, radiation proctitis, soft-tissue radionecrosis, and chest-wall necrosis after breast radiotherapy. Evidence for radiation-induced brain necrosis is positive but more limited; for radiation enteritis it is encouraging but the data are mixed. The treatment course needs to be individualised by an experienced hyperbaric medicine physician.
Can HBOT regrow blood vessels in tissue that has been damaged for decades?
Yes, provided some viable tissue remains. Patients have been successfully treated 10, 20, and even 30 or more years after completing their original radiation therapy. There is no upper time limit. The angiogenic mechanism does not depend on how long the tissue has been hypoxic; it depends on whether enough living cells remain to respond to the oxygen and growth-factor signal.
Does HBOT for radiation injury increase cancer recurrence risk?
Multiple clinical reviews and the Undersea and Hyperbaric Medical Society position statement have addressed this question. The current evidence does not support an increased recurrence risk from adjunctive HBOT in cancer survivors. Most hyperbaric medicine programmes still confirm with the patient’s oncology team that the cancer is in remission before starting treatment. Patients with active untreated cancer in the field are typically managed differently.
How many HBOT sessions are needed before angiogenesis becomes measurable?
Twenty to thirty sessions over four to six weeks is the typical threshold at which tissue oxygen tensions and laser Doppler flow measurements show consistent improvement. The full course of 30 to 60 sessions builds the capillary network to 75 to 80 percent of non-irradiated baseline, after which the response plateaus.
Where can I find a Canada Hyperbarics-listed hospital programme that treats radiation injury?
Canada Hyperbarics maintains a verified directory at /facilities/. The 11 hospital programmes that accept radiation-injury referrals under provincial health insurance are listed there along with phone numbers and websites. The condition-specific page delayed radiation injury covers what to bring to your assessment, what to expect during the treatment course, and how the referral process works in each province.
References and further reading
- Tao R, Chen Z, Wu P, et al. VEGF and bFGF induction by nitric oxide is associated with hyperbaric oxygen-induced angiogenesis and muscle regeneration (2020).
- Yan Y, Wang T, Wang H, et al. Hyperbaric Oxygen Therapy Improved Neovascularisation Following Limb Ischaemia (2024).
- Lu Z, Yang J, Wei Y, et al. Hyperbaric oxygen therapy enhances osteointegration through angiogenesis-osteogenesis coupling (2024).
- Marx RE. Osteoradionecrosis: a new concept of its pathophysiology. Journal of Oral and Maxillofacial Surgery. 1983;41(5):283-288. (Foundational paper on the three-H pathophysiology and HBOT angiogenesis.) PMID 6572704.
- Bennett MH, et al. Hyperbaric oxygen therapy for late radiation tissue injury. Cochrane Database of Systematic Reviews. Cochrane review.
- Müller-Edenborn B, et al. Angiogenesis: all a radiation oncologist should know (2008). Internal study page.
- Health Canada. Notice on hyperbaric oxygen therapy.
- Canadian Cancer Society. April is Daffodil Month.
Disclaimer
This article is provided by Canada Hyperbarics for informational purposes only and is not medical advice. Do not delay or replace medical care based on what you read here. Always speak with your radiation oncologist, your family physician, and a hospital-based hyperbaric medicine specialist before starting hyperbaric oxygen therapy for radiation injury. Treatment courses, contraindications, and coverage details should be confirmed directly with the receiving hyperbaric programme. Canada Hyperbarics is an independent educational resource and has no commercial relationship with the hospital programmes referenced above.