Chapter 31 Eye and orbit
Anatomy
The orbit is defined by bony margins and is conical in shape. It contains the eye, the optic nerve and the recti and oblique extraocular muscles. The optic nerve exits from just below the centre of the back of the eye and extends to the optic foramen at the apex of the orbit posteriorly. The globe is composed of three layers, an outer fibrous layer called the sclera, a middle layer which is vascular and is composed of the choroid, the ciliary body and the iris and an inner neural layer, the retina. The vascular choroid covers the inner surface of the sclera. The diameter of the eye is 2.4 cm and the lens is 0.5 cm from the anterior surface.
The movement of each eye is performed by six muscles, (four recti and two oblique muscles) and three cranial nerves, the IIIrd, IVth and VIth. The VIth cranial nerve supplies the lateral rectus muscle which moves the eyeball laterally. The medial, superior and inferior recti elevate, depress and move the eyeball inward, while the inferior oblique moves the eyeball upward and outward. All these four muscles are supplied by the IIIrd cranial nerve. The superior oblique which moves the eye downward and outward is supplied by the IVth cranial nerve.
The membrane lining the inner surface of the eyelids is the conjunctiva which courses over the anterior surface of the globe, extending to the corneoscleral junction. Tears are secreted by minor lacrimal glands situated on mainly the lower eyelid and can be supplemented by the lacrimal gland situated in the upper lateral part of the orbit. They drain into the nose through the nasolacrimal duct.
Pathology
Malignant
Tumours can be classified as arising from the skin and adnexia, orbit or intraocularly. Basal cell carcinomas (BCC) and squamous cell carcinomas (SCCs) are skin tumours commonly arising on the face, especially around the eye and lower eye lid. In contrast, lacrimal gland and nasolacrimal duct tumours are rare, as are orbital tumours. Rhabdomyosarcoma of the orbit is a tumour of childhood. It is usually embryonal and is discussed in further detail in Chapter 33. Intraocular tumours are rare with retinoblastoma occurring in the very young and melanoma in adults. The choroid is the commonest site in the eye for metastatic disease.
Primary tumours of the eye and orbit are listed below:
Radiation and ocular morbidity
The lens
The eye contains structures that are both very radiosensitive and radioresistant. One of the most sensitive tissues in the body is the lens. Cataracts have been reported at doses as low as 2 Gy. However, it should be remembered that they invariably occur over the age of 70 and are associated with various common diseases, such as diabetes, and medications, such as steroids. The appearance and region of the cataract can help in deciding their causation. Radiation cataracts are caused by damage to cells in the anterior central area of the lens, which then start to form a cataract at the back of the lens centrally. The total dose, energy of radiation, volume of the lens, health of the eye and concomitant disease are all factors that play a part in cataract formation. When high doses are given with β-irradiation eye plaques, usually for treatment of posterior choroidal melanomas, cataracts often will not occur because of the rapid fall off of dose (as opposed to the historical use of cobalt plaques). After external beam radiotherapy, fractionated doses of over 10 Gy are likely to cause detectable lens opacities, usually occurring within 2–3 years of radiotherapy. Figure 31.1 shows a radiation cataract. Treatment of a cataract is by surgical removal, although if the eye is dry (most commonly as a result of radiotherapy), this may adversely affect the success of surgery.
The sclera and retina
The sclera, on the other hand, is radioresistant because it is avascular and can tolerate doses of up to 100 Gy by radioactive eye plaques to a small area. Above this dose there is a risk of necrosis. The retina can tolerate doses of 50 Gy but, above this, retinal damage is manifest by haemorrhages, exudates and atrophy. Radiation atrophy and post-subcapsular cataract are shown in Figures 31.2 and 31.3. The macula is the most sensitive area of the retina and doses here should be minimized or avoided if possible when using radioactive eye plaques. Similarly, as the dose increases above 50 Gy, there is an increasing likelihood of optic atrophy.
The cornea and lacrimal apparatus
These structures usually tolerate doses of up to 50 Gy well, depending on radiotherapy technique, energy, fractionation and attention to good eye care. It will result in erythema of both skin and conjunctiva, local irritation and lacrimation. If megavoltage radiotherapy is used, these reactions will be reduced due to build-up and skin sparing. Tear production is from the minor lacrimal glands mainly located on the lower eyelid and supplemented by the major lacrimal gland in the upper lateral part of the orbit anteriorly. This should be shielded in radiotherapy planning in an attempt to preserve tear production as much as possible. It is reduced above 30 Gy and patients may require hypomellose eye drops (artificial tears) above this dose and lacrilube. Doses of 50 Gy and above result in more serious problems.
Stenosis or occlusion of the nasolacrimal duct due to a tumour adjacent to the inner canthus will result in a weeping eye (epiphora). There is evidence that this does not happen due to radiotherapy alone if it is carefully fractionated and the tumour has not comprised the function of the duct already. A dose of 45 Gy in 10 fractions on a superficial unit (100 kV) is the minimum fractionation recommended. Figures 31.4 (before radiotherapy) and 31.5 (after radiotherapy) illustrate a basal cell carcinoma in this region treated with superficial radiotherapy.
Developing a dry eye is not only very uncomfortable but may result in loss of vision. Corneal damage occurs, partly due to reduced sensation. Punctate keratitis and edema lead on to corneal ulceration, scarring, infection and impairment of vision. If high dose radiotherapy is given, the involvement of an ophthalmologist to give advice, eye protection and ensure good eye care is imperative during the course of radiotherapy. This will aid in achieving patient comfort, maximizing vision and minimizing late complications. Keratinization of the cornea is a late complication and occurs after doses in excess of 50 Gy. Rarely, it leads on to secondary revascularization.
Principles of irradiation
These are governed by whether radical or palliative treatment is being given, the type of tumour, benign or malignant, radiosensitivity, its extent and patient factors, such as fitness and co-morbidity. The patient should have a comfortable set-up for radiotherapy, which is likely to make the treatment more reproducible, and appropriate immobilization. For palliative treatment, a simple device such as an orfit can be used but, for radical treatment, a full beam directional shell is usually necessary. It is often difficult to protect part of the eye (especially as it is a small structure), particularly the lens and cornea, although internal eye shields will achieve this when treating skin tumours around the eye. The major lacrimal gland can be shielded in some external beam radiotherapy plans.
Technique
For radical radiotherapy, patients are planned with the aid of a computed tomography (CT) scan. Paired wedged fields are usually most appropriate. A combination of three fields planned conformally may achieve a better isodose distribution for extensive tumours and reduce the dose to normal tissues (Figure evolve 31.6 
). A prescription to a dose of 54–60 Gy with 2 Gy daily treatments is commonly employed. Great attention should be paid to eye care and regular reviews should be performed by clinicians during the course of radiotherapy.
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