**2. Optic structures**

#### **2.1. Eye ball**

The entire eye ball is to be contoured as a single structure. The entire retina is to be included.

*Dose recommendations*: Dmax <54 Gy. The incidence of radiation induced optic neuropathy (RION) is unusual for doses less than 55 Gy. At 55–60 Gy the risk becomes 3–7% and for doses

Overview of Important "Organs at Risk" (OAR) in Modern Radiotherapy for Head and Neck Cancer (HNC)

http://dx.doi.org/10.5772/intechopen.80606

3

A small structure is usually confined to 2 or 3 slices in the superior-inferior direction. Better demarcated in MRI, the chiasm is situated about 1 cm superior to the pituitary gland. Laterally it is bounded by the carotid arteries. It is better visible in MRI with a high signal on T1. A good landmark to look for is the pituitary stalk. It lies just posterior to the chiasm and appears hyperintense even on plain CT [7]. On average it measures 8 x 14 mm (APxTrans) and is about

It should be kept in mind that, the chiasm should be contoured in continuity with the optic

Freedman et al. [8] has given a step-by-step instruction to contour the lacrimal gland. The contour starts by identifying the mid portion of the gland and thereafter tracking it superiorly

Superior extend corresponds to the super-lateral corner of orbit, just below the orbital rim.

*Dose recommendations*: Dmean <30 Gy. Doses above 40 Gy have shown to steeply increase the incidence of dry eye while doses above 57–60Gy can cause permanent loss of tearing [9].

Inferiorly it does not extend below the level of insertion of lateral rectus.

The gland is better delineated in brain (120/40) or soft tissue (350/50) window.

>60 Gy the risk is quite significant at 7–20% [6].

2–5 mm thickness in the super-inferior dimension [7].

*Dose recommendations*: D max <54 Gy [6].

The details are given in **Figures 1**–**3**.

**2.3. Optic chiasm**

nerves.

**2.4. Lacrimal gland**

**Figure 1.** Optic apparatus.

and inferiorly.

For contouring of substructures of the eye the European particle therapy network (EPTN) has put forward a consensus based atlas based on CT and MRI [2].

#### *2.1.1. Cornea*

The cornea is located anterior to the vitreous humor, iris, lens and ciliary body [2]. It can be delineated in MRI or CT and is contoured with a 2-3 mm brush.

RT can injure the cornea by damaging the deeper layers of stroma, but in most cases the acute toxicity is as a result of loss of tear film [3].

*Dose recommendation*: < 40 Gy. Edema of the corneal stoma appears at a dose of 40-50Gy, but is usually transient. With doses of 60 Gy the chance of corneal ulceration is increased to 17–20% which increases further if chemotherapy is added.

#### *2.1.2. Retina*

It is the innermost layer of the globe and is about 0.25 mm in thickness and is not usually visualized in a standard MRI [2]. Contoured using a 3 mm brush, the retina covers the posterior 5/6th of the globe. The optic nerve is not contoured along with it [4].

*Dose recommendations*: Dmax—45Gy. Acute retinal toxicity is not reported. Being a part of the central nervous system, the retina behaves as a late reacting tissue [3]. Usually there is a latent period of 6 months to 3 years before the onset of clinically significant retinopathy. The mean latent period is 19 months [5].

#### *2.1.3. Lens*

Biconvex structure in the aqueous humor, it is clearly visible in CT [2]. The structure is about 10 mm in diameter seen in the coronal plane.

*Dose recommendations*: Dmax—5 to 10 Gy. Acute lens toxicity is not reported. A single dose of 2 Gy can cause cataract, but is usually visually insignificant. [3] The time of onset is dose related. For doses in the range of 2.5–6.5Gy, the latency is 8 yrs. with the possibility of 33% progressive cataract, whereas doses of 6.5–11.5Gy, the latency reduces to 4 years with the 66% risk of progressive cataract.

#### **2.2. Optic nerve**

While moving craniocaudally, optic nerve is seen below the superior rectus. The nerve is 2-5 mm thick and is delineated from the posterior margin of retina and continued along its course posteriorly till it merges with the optic chiasm after passing through the superior orbital fissure.

*Dose recommendations*: Dmax <54 Gy. The incidence of radiation induced optic neuropathy (RION) is unusual for doses less than 55 Gy. At 55–60 Gy the risk becomes 3–7% and for doses >60 Gy the risk is quite significant at 7–20% [6].
