The eye contains several structures that can be damaged by radiation:
- The lacrimal apparatus
- The cornea
- The lens
- The retina
- The optic nerve and chiasm
The lacrimal gland is a tubulo-acinar gland that secretes the tears; it has a similar structure and function to the salivary glands. The cells are sensitive to radiotherapy and doses over 30 Gy have a risk of causing long term dry eye. Histopathological changes include loss of acinar cells, fibrosis, and typical vascular changes.
The cornea is most frequently affected by dry eye resulting from lacrimal gland damage. In itself, it is actually radiation resistant.
Cataract formation is seen with doses over 0.5 Gy (IRCP, 2012). The typical location is posterior subcapsular. The changes after radiation exposure are:
- Initial mitotic inhibition of the lens epithelium (immediate)
- Resumption of mitosis with abnormal mitotic activity seen (eg. anaphase bridges) (days-weeks)
- Disorganisation of epithelium (weeks)
- Proliferation of abnormal epithelial cells along posterior capsule
- Death and rupture of abnormal cells
Clinically, the later stages of cataract formation can be visualised with slit lamp examination. The severity of opacity is graded from 0 to 6 (none to complete).
The retina is a collection of rods, cones, supporting cells and neurones that detect and convey light signals from the eye to the optic nerve. Radiation changes appear to be mostly vascular related, with hyalinisation, loss of arterioles, venous dilatation, ischaemia and neovascularisiation (similar to other retinopathies).
Optic Nerve & Chiasm
Damage to the optic nerve and chiasm is less well understood. It is likely that an interplay of reactions (vascular, supporting cells) results in optic nerve damage. Of importance, the optic nerve is supplied by a single major artery (ophthalmic) which may contribute to its lower radiation resistance than the rest of the brain.