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The photochemical reactions during PDT are limited to neoplastic or vascular cells accumulating photosensitizer and proceed photoenzymatically through interaction with oxygen molecules (the maximum electron receptors presenting in the viable tissues) through a process dependent on the energy transfer by the excited photosensitizer.

Once the photosensitizer is illuminated, it is converted from the stable electronic ground state into an excited singlet state. Its subsequent re-excitation and return back to the ground state may be realized either directly or via excited triplet states in an intersystem crossover [4]. The relaxation from the intermediate intersystem crossover states initiates a sequence of photochemical reactions. In the Type I reaction, which is considered typical for less oxygenated or hypoxic environments, a triplet state photosensitizer couples directly with the cellular substrate via electron transfer, which produces superoxide ions and ultimately leads to the creation of highly reactive free hydroxyl radicals. In the Type II reaction appearing in oxygenated tissues, the excited photosensitizer transfers its energy down to the ground state molecular oxygen, which initially results in the production of triplet and subsequently of extremely reactive singlet oxygen [4]. Both pathways lead to oxidations of biomolecules ultimately damaging the cell [2, 3], but it is believed that the Type II reaction dictates the outcome of PDT. Because singlet oxygen has a short lifetime (0.04–4 μs) induced cell death is realized only locally in the areas undergoing photo-irradiation [3, 7].

In general, oxygen levels within cancer cells tend to be low, and this can be an important issue, which may decrease the effectiveness of PDT. While there have been attempts to apply such treatment in a hyperbaric oxygen environment, no significant improvement in results was attained.