Review question
For people with solid cancers, we asked if the combination of radiotherapy and hyperbaric oxygen (HBO) breathing could reduce mortality and the chance of cancer spread when compared to radiotherapy alone or to radiotherapy and an alternative approach to reducing mortality and cancer spread.

Background
Invasive cancer is a major health problem and results in the death of millions of people each year. Many solid cancers are low in oxygen (hypoxic), which means they are resistant to the effect of radiotherapy treatment. For this reason, it has been suggested that raising the oxygen levels in the tumours by administering HBO breathing could make treatment with radiotherapy more effective.

Study characteristics
We found 19 randomised trials that together included 2286 participants. The dose of oxygen per treatment session in the HBO arm was remarkably uniform, with all trials except one administering external beam radiation therapy at 3 atmospheres absolute (ATA). However, the number of treatments given ranged widely, from two sessions only, separated by three weeks, up to 40 sessions over eight weeks.The total dose of radiation was generally reduced in the HBO participants in order to reduce side effects. The follow-up period varied between trials, from six months to 10 years, although most studies followed participants for between two and five years.

Key results
Adding HBO to the treatment of head and neck cancers reduced mortality at both one year and five years after therapy. Local tumour recurrence was also less likely with HBO at one year and five years in head and neck cancer. However, these advantages are achieved at the cost of some adverse effects. There was a significant increase in the rate of severe radiation tissue injury and the chance of seizures during HBO therapy.

Quality of the evidence
The quality of evidence was generally high with close agreement between several different trials. Similarly, there was high-quality evidence of an increased risk of having a severe reaction to the radiation while breathing HBO. The evidence for an increased risk of seizures during treatment when using HBO was of moderate quality, mainly because of the small numbers of seizures seen in the included studies.

Conclusions
There is some evidence that breathing oxygen while at raised pressure may improve mortality and reduce tumour regrowth in cancers of the head and neck, but at the cost of increased side effects.