Ferromagnetic hyperthermia: Functional and histopathologic effects on normal rabbit ocular tissue

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Abstract
Ferromagnetic (FM) hyperthermia has previously been evaluated in a rabbit tumour model of ocular melanoma. To study the effect of focal heating in normal rabbit eyes, FM seeds were implanted into a 14-mm episcleral plaque and heated to operating temperatures of 48 or 54°C. Thermal induction was performed by placing rabbits in a uniform, oscillating (100 kHz) magnetic field operating at 1200 W and an H-field strength of 265A/m. Eyes were heated for 60min with continuous scleral temperature monitoring. Hyperthermic effects were monitored by direct ophthalmic examination, fundus photography, serial electroretinography and histopathology. Intraocular temperatures were mapped with direct fiberoptic thermometry. All treatment effects were confined to the area covered by the episcleral plaque. Direct ophthalmoscopic examination revealed early retinal whitening during heat induction followed by localized exudative retinal detachments, limited to the area of the retinal surface overlying the plaque, that resolved spontaneously. Serial electroretinography was virtually indistinguishable between the 48 and 54°C treatment groups. We noted a minimal alteration in a- and b-wave amplitudes with no changes in implicit times. Histopathology at 3 weeks post-treatment documented chorioretinal scarring overlying the thermal plaque treatment zone. No evidence of haemorrhage, infection, cataract or scleral thinning was noted. This study documents the apparent focal containment of thermal effects with FM heating utilizing operating temperatures as high as 54°C for 60 min, and discloses no evidence of diffuse ocular toxicity.