The quality of evidence is downgraded by indirectness (differences in studied patients) and inconsistency (variability in results across studies).
A Cochrane review included 19 studies with a total of 2 286 subjects. With hyperbaric oxygenation therapy (HBOT), there was a reduction in mortality for head and neck cancers at both one year and five years after therapy (RR 0.83, 95% CI 0.70 to 0.98, number needed to treat (NNT)=11 and RR 0.82, 95% CI 0.69 to 0.98, NNT=5 respectively), as well as improved local tumour control immediately following irradiation (RR with HBOT 0.58, 95% CI 0.39 to 0.85, NNT=7). There was also a lower incidence of recurrence of tumour when using HBOT at both one and five years (RR at one year 0.66, 95% CI 0.56 to 0.78, NNTB=5; RR at 5 years 0.77, 95% CI 0.62 to 0.98, NNTB=6). The effect of HBO varied with different fractionation schemes. There was a significant increase in the rate of severe radiation reaction (RR 2.64, 95% CI 1.65 to 4.23), but not an increased risk of seizures from acute oxygen toxicity (RR 4.3, 95% CI 0.47 to 39.6).
For carcinoma of the uterine cervix and urinary bladder cancer, no benefits were observed with HBOT. When all cancer types were combined, there was an increased risk of severe radiation tissue injury during the course of radiotherapy (RR 2.45, 95% CI 1.85 to 3.24, NNTH=8) and of oxygen toxic seizures during treatment (RR with HBOT 6.8, 96% CI 1.2 to 39.3, NNTH = 22).
A Cochrane reviewNote: The observed benefits may only occur with unusual fractionation schemes. In general, HBO exposure during irradiation was more beneficial when the total dose of radiation was delivered in low numbers of fractions (12 or fewer) than when a more conventional 20- to 25-fraction scheme was used.