Hypoxia is a critical hallmark of solid tumors and involves enhanced cell survival, angiogenesis, glycolytic metabolism, and metastasis. Hyperbaric oxygen (HBO) treatment has for centuries been used to improve or cure disorders involving hypoxia and ischemia, by enhancing the amount of dissolved oxygen in the plasma and thereby increasing O(2) delivery to the tissue. Studies on HBO and cancer have up to recently focused on whether enhanced oxygen acts as a cancer promoter or not. As oxygen is believed to be required for all the major processes of wound healing, one feared that the effects of HBO would be applicable to cancer tissue as well and promote cancer growth. Furthermore, one also feared that exposing patients who had been treated for cancer, to HBO, would lead to recurrence.
Malignant tumours often display hypoxic tissue areas where the oxygen tension is < 7 mm Hg. Studies in this field have proved that the hypoxic state boosts tumour progression and aggressive behaviour. In tissue culture experiments "in vitro" oxygenation was found to inhibit in itself the proliferation of cells of healthy tissues as well as benign and malignant tumours. It is a very important observation from oncotherapeutic point of view that in the presence of partial oxygen pressure < 2.5 mm Hg the radiosensitivity decreases (intrinsic radioresistance). Most of the anticancer drugs (cytostatics) are also ineffective in hypoxic tumours (chemoresistance).