Radiation optic neuropathy after external beam radiation therapy for acromegaly.

Acromegaly is an uncommon disease, mostly caused by a growth hormone (GH)-secreting pituitary adenoma. Its incidence has been estimated at 2.8–6 cases per million and its prevalence at 38–68 cases per million [2,10,64]. A GH-secreting pituitary adenoma does not only lead to clinical signs and symptoms, like acral enlargement and soft tissue swelling, but may also be accompanied by visual field defects and acuity loss caused by tumour compression on the optic nerves or chiasm, often seen in combination with pituitary hormone insufficiencies. Surgery, drug therapy with somatostatin analogs and external beam radiation therapy are currently the available treatment options [61]. External beam radiation therapy is available since the beginning of the 20th century [12,35]. It became clear, however, that radiation therapy alone often results in insufficient biochemical control, which means a decline but not a normalisation of GH hypersecretion [3,4, 24,29,56]. Consequently, surgery became the initial treatment of choice. According to some experts drug therapy may be the first treatment of choice for selected patients [61]. Postoperative radiation therapy is performed in many centres to reduce the postoperative time span of medical treatment, to normalise remaining GH hypersecretion, and to prevent regrowth of residual tumour [75]. Radiation optic neuropathy (RON) was for the first time reported by Forrest et al. in 1956 [30]. They defined RON as a sudden and profound irreversible vision loss due to damage of the optic nerves or damage of the chiasm caused by radiation therapy. Kline et al. and Parsons et al. defined the following criteria for diagnosing RON [49,65]: (a) irreversible visual loss with visual field defects, indicating optic nerve or chiasmal dysfunction; (b) absence of visual pathway compression due to recurrence or progression of tumour, radiation-induced neoplasm, arachnoidal adhesions around the chiasm, radiation retinopathy or any other apparent ophthalmological disease; (c) absence of optic disc edema and (d) optic atrophy within 6–8 weeks after onset of symptoms. In the past decades many reports on RON have appeared in the literature. The last review of RON in acromegaly was published by Eastman et al. in 1992 [25]. The radiation dose level for the occurrence of RON is not known. Moreover the dose–response relationship for RON has not been firmly established due to the small numbers of events in most series [46,65]. It has been suggested that the maximal steepness of the sigmoid dose–response curve for RON is between 50 and 60 Gy [46,65]. The occurrence of RON after doses as low as 45–50 Gy administered in fractions of 1.67–2 Gy seems to be a problem unique to patients with pituitary tumours, and probably reflects pre-existing optic nerve and chiasm compression and vascular compromise secondary to a mass effect or due to surgery [46,65]. Some authors propose that the optic system in acromegalic patients might be more sensitive to radiation damage compared to patients irradiated for non-functioning pituitary adenomas [5,6,13,39]. This may be caused by vascular and hormonal changes in relation to acromegaly [5], but this opinion is not uniformly accepted [23,25].