External beam radiotherapy (RT) has been used for thyroid eye disease for over 60 years and is presumed to work by directly targetting lymphoctyes and fibroblasts in the involved orbital tissue. Typically 20 Gy (2000 Rads) divided over 10 days are delivered to each orbit avoiding ocular or intracranial exposure.
V: Optic neuropathy: Some studies have shown that it may sufficiently reduce the width of distended ocular muscles near the orbital apex to prevent the need for surgery in patients with optic neuropathy. XRT may also be useful in preventing progressive expansion of muscles and recurrence of vision loss following surgical orbital decompression in patients with compressive optic neuropathy.
I: Inflammation/congestion: An analysis of many retrospective series over several decades has found RT may help reduce inflammatory changes on the surface of the eye and on the eyelids in approximately 60% of patients. A randomized controlled prospective trial found RT was equally helpful as oral prednisone in reducing inflammatory periocular soft tissue changes. The benefit of prednisone was more rapid, but the benefit of RT appeared to be longer lasting. Prednisone combined with RT appeared to be more beneficial than treatment with prednisone alone.
S: Strabismus / motility: Two randomized controlled studies from the Netherlands found that patients with TED treated with RT ultimately had better ocular excursions than patients treated with sham (no) RT.
A: Appearance / exposure: Radiotherapy appears to have little benefit in reducing ultimate proptosis or lid retraction in patients.
Also, as with medical therapy, RT is most effective for patients with active or progressive disease, and is unlikely to reverse orbital changes in stable, post-inflammatory thyroid eye disease.
Safety and Complications:
Modern linear accelerator radiotherapy units have an excellent safety record. Several large reviews demonstrate no increased risk of cataract and no reports of radiation induced tumors. Because of a small theoretical lifetime risk of developing tumors, its use is usually avoided in people less than 35 years old.
There have been reports of a slight increased risk of retinal vascular disease in patients with diabetes or hypertension, and its use is often avoided in patients with diabetes.
Radiotherapy may cause some temporary redness and hair loss in the temple area near where the beam is focussed. Orbital inflammation may temporarily be worsened during the two week therapy and oral prednisone may be given to help reduce this effect.