OCULAR GLOBE TOPOGRAPHY IN RADIOTHERAPY

Purpose: Ocular lens, retina, and olfactory bulb exposure are common c oncerns in contemporary radiotherapy practice. Methods to clinically l ocalize soft tissue structures (i.e., lens and retina) are varied and often imprecise. We hypothesized that eyelid markers constituted a bet ter reference point than the commonly used lateral canthus marker for lateral beam simulations, unless diagnostic computed tomography or ult rasound examinations were available and/or used.Methods and Materials: Sixty-six pre-Magnetic Resonance Image, normal, orbital computed tomo graphy scans from adult patients were used to measure (a) sagittal dis tances from eyelid to posterior lens surface, from lateral canthus to posterior lens surface and to the globe's posterior pole, (b) supero-i nferior distances in the lateral projection from the lens to the cribr iform plate, and (c) common dimensions to establish internal validity of the measurements. Results: The eyelid to lens and retina topography is individually more constant than that from the canthus. There is li ttle if any supero-inferior separation between the lens and the cribri form plate lateral projections. Conclusions: The lateral canthus does not specify lens or retina locations. Eyelid markers of known size pro vide more accurate anatomical information. Lateral beam ocular globe s hielding has to be individualized. Lens shielding is questionable if t he olfactory bulb needs to be irradiated by a lateral beam.