Conventional and virtual simulation in retrobulbar irradiation

Radiotherapy is commonly used in Grave's ophthalmopathy. The target volume encompasses the ocular muscles and the orbital tissue. The result of conventional simulation can be examined by means of CT simulation. Twenty-five planning CTs with Grave's ophthalmopathy were studied. The conventional simulation of 4 x 4 cm2 lateral portals confined anteriorly by the fleshy canthus was performed on a CT-simulator using the observer's eye view (OEV) and digitally reconstructed radiographs (DRR). The coverage of the target volume and sparing of the eye lenses were studied on axial CT slices and multiplanar reconstructions (MPR). The distances between the apex of the orbita and cornea, fleshy canthus, and bony canthus were measured as well as the distance between cornea and posterior face of the lens. The pituitary gland and the ocular lenses were spared in each case (25/25). The orbita was entirely covered in 24 cases (96%). However, anterior parts of the external eye muscles were not completely encompassed in 7 cases (28%). The distance from the apex of the orbita to the cornea was 54.6 mm (53.3 to 55.8 mm, 95% confidence interval), to the fleshy canthus 40.3 mm (39.4 to 41.2 mm), and to the bony canthus 31.4 mm (30.2 to 32.5 mm). The distance between cornea and posterior face of the lens was 8.3 mm (7.9 to 8.7 mm). The distance between cornea and canthus differed significantly from normal eyes while the distance between cornea and posterior face of the lens was very similar to normal eyes. Conventional simulation of orbital irradiation with lateral fields confined anteriorly by the fleshy canthus ensures protection of the ocular lenses and the pituitary gland. However, anterior parts of the eye muscles may occasionally not completely be covered. The fleshy canthus and the cornea are more reliable landmarks as compared to the bony canthus.