Conventional and CT simulation of radiotherapy in Grave's ophthalmopathy

Background: Radiotherapy is commonly used in Grave's ophthalmopathy. The ta rget volume encompasses the ocular muscles and the orbital tissue. The resu lt of conventional simulation can be examined by means of CT simulation. Patients and Methods: Twenty-five planning CTs with Grave's ophthalmopathy were studied. The conventional simulation of 4 x 4 cm(2) lateral portals co nfined anteriorly by the fleshy canthus was performed on a CT-simulator usi ng the observer's eye view (OEV) and digitally reconstructed radiographs (D RR). The coverage of the target volume and sparing of the eye lenses were s tudied on axial CT slices and multiplanar reconstructions (MPR). The distan ces between the apex of the orbita and cornea, fleshy canthus, and bony can thus were measured as well as the distance between cornea and posterior fac e of the lens. Results: The pituitary gland and the ocular lenses were spared in each case (25/25). The orbita was entirely covered in 24 cases (96%). However, anter ior 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). T he distance between cornea and posterior face of the lens was 8.3 mm (7.9 t o 8.7 mm). The distance between cornea and canthus differed significantly f rom normal eyes while the distance between cornea and posterior face of the lens was very similar to normal eyes. Conclusions: Conventional simulation of orbital irradiation with lateral fi elds confined anteriorly by the fleshy canthus ensures protection of the oc ular lenses and the pituitary gland. However, anterior parts of the eye mus cles may occasionally not completely be covered. The fleshy canthus and the cornea are more reliable landmarks as compared to the bony canthus.