Photoradiation therapy and its potential in the management of neurological tumors

Photoradiation therapy is a form of local treatment that depends on the selective retention of a photosensitizer, such as hematoporphyrin derivative (HpD), by the tumor followed by treatment with light of an appropriate wavelength to activate the sensitizer in the tumor. The selective uptake of HpD by cerebral tumors has been demonstrated both in laboratory animal model studies and in clinical studies, and selective destruction of intracerebral tumors has been demonstrated in animal glioma models. The biological basis for photoradiation therapy and, in particular, the mechanisms for the selective uptake of the sensitizer into the tumor and the destruction of tumor with photoradiation therapy are discussed. Current evidence suggests that singlet oxygen is the major intermediary leading to cell damage, although other radicals such as hydrogen peroxide and hydroxyl radicals may be involved. Other studies suggest that the initial damage is to the blood vessels, and the tumor subsequently undergoes ischemic changes. Sixty-four patients treated with photoradiation therapy have been reported in the literature. The initial clinical studies were disappointing in their therapeutic effect but these studies often included treatment of recurrent gliomas and low doses of light were used. Technical advances, particularly in laser technology, have enabled more effective photoradiation therapy and the clinical trials are reviewed.