OCULAR GLOBE TOPOGRAPHY IN RADIOTHERAPY

Purpose: Ocular lens, retina, and olfactory bulb exposure are common concerns in contemporary radiotherapy practice. Methods to clinically localize soft tissue structures (i.e., lens and retina) are varied and often imprecise. We hypothesized that eyelid markers constituted a better reference point than the commonly used lateral canthus marker for lateral beam simulations, unless diagnostic computed tomography or ultrasound examinations were available and/or used. Methods and Materials: Sixty-six pre-Magnetic Resonance Image, normal, orbital computed tomography scans from adult patients were used to measure (a) sagittal distances from eyelid to posterior lens surface, from lateral canthus to posterior lens surface and to the globe's posterior pole, (b) supero-inferior distances in the lateral projection from the lens to the cribriform 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 little if any supero-inferior separation between the lens and the cribriform plate lateral projections. Conclusions: The lateral canthus does not specify lens or retina locations. Eyelid markers of known size provide more accurate anatomical information. Lateral beam ocular globe shielding has to be individualized. Lens shielding is questionable if the olfactory bulb needs to be irradiated by a lateral beam.