RADIATION-INDUCED INJURY TO THE VISUAL PATHWAY

Radiation-induced injury to the visual pathway was reviewed in a cohort of patients treated for various cancers of the nasal cavity and paranasal sinuses between 1969 and 1985. The study subjects consisted of 219 patients, 137 men and 82 women, in whom detailed records were available on the extent of orbital shielding, treatment plan for estimating doses delivered to various optic structures, and visual acuity follow-up information. There was a wide range in doses administered to various optic structures because patients with different primary lesion types were included and the radiotherapy techniques used varied during this era. The endpoint of the study was visual acuity <20/100. The Cox proportional hazard model was used to assess the influence of various factors on the latent time to visual impairment and to fit the LQ model to the failure-time data. Actuarial curves showing the proportion of patients with visual impairment as a function of dose and time were generated. Corneal injury occurred in 24 of the 49 patients treated with the 3-field technique without any orbital shielding; variables affecting the incidence of cornea injury were total radiation dose and chemotherapy. Symptomatic retinopathy was diagnosed in 7 of 77 patients who received irradiation to a relatively large retinal surface, but no variables were found to correlate with this complication. Eight patients developed ipsilateral blindness due to optic neuropathy and 11 patients had bilateral visual impairment secondary to chiasm injury. The total radiation dose was identified as the predominant determinant. None of patients receiving a dose of <50 Gy developed optic neuropathy or chiasm injury. In contrast, the 10-year actuarial incidences of optic nerve-chiasm injury were similar to 5% and similar to 30% for patients receiving 50-60 Gy and 61-78 Gy, respectively. A fit of the LQ model yielded an alpha/beta estimate of 1.6 Gy for optic neuropathy. Actuarial dose-incidence curves were generated for different types of visual complications. This information would be useful in guiding rational selection of radiation dose in various subgroups of patients based on the extent of disease and risk of recurrence.