Depth measurements and histopathological characterization of photodynamic therapy generated normal brain necrosis as a function of incident optical energy dose.

The response of normal brain to photodynamic therapy (PDT) was investigated in 62 Fisher rats. The animals were injected i.p. with Photofrin II (12.5 mg/kg). Forty-eight hours following injection, an area of dura 5 mm in diameter over the frontal cortex was photoactivated with red light (632 +/- 2 nm) at 100 mW cm-2, with no contributing thermal increases, at optical energy doses ranging from 1-140 J cm-2 from an argon-pumped dye laser. Appropriate controls were also prepared. Brain tissue samples for histological analysis were taken 24 h following PDT treatment. Maximum lesion depth perpendicular to the pial brain surface, was measured using an eyepiece micrometer. Lesions of increasing depth were generated as the incident optical energy dose was increased. Fitting the depth of necrosis to a natural log dependence of incident optical dose yielded a slope of 0.83 mm/ln J cm-2 (r2 = 0.99). The intercept of 1.47 J cm-2 indicated the energy dose below which no normal tissue damage would occur at the incident laser intensity of 100 mW cm-2. The smallest lesions consisted almost exclusively of isolated neuronal injury and neuropil vacuolation, suggestive of an early ischemic lesion. Damage at the upper energy levels (35-140 J cm-2) consisted of complete coagulative necrosis identical to that induced by an arterial occlusion. The existence of viable tissue alongside neurons in various stages of necrosis at low energy levels (less than 35 J cm-2) is suggestive of reversible injury and possibly clinically relevant treatment levels.