The ability of hyperbaric oxygen (HBO) treatment to modulate the response of a rodent fibrosarcoma to interstitial photodynamic therapy (PDT) using 630 nm light and intravenous polyhaematoporphyrin (PHP) was examined. Application of HBO for 30 min immediately prior to PDT resulted in a light dose-dependent increase in tumour growth retardation, maximum effect (+227%) being observed with 100J light, and no enhancement being seen at or above 400J. Application of HBO during or immediately after PDT had no effect on response. Reducing the interval between giving PHP and administering light from the usual 48 h to just 1 h increased the efficacy of PDT even in the absence of HBO pre-treatment. Under these circumstances, application of HBO did not realise any further increase in tumour response at any light dose. Our findings suggest that manipulation of tumour oxygenation may be able to improve the outcome of PDT under some circumstances, but that careful consideration of other treatment variables, such as light dose and drug-to-light interval, may provide similar, more easily achievable, improvements in efficacy.

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