**2.3. Optic chiasm**

**2. Optic structures**

The entire eye ball is to be contoured as a single structure. The entire retina is to be included. For contouring of substructures of the eye the European particle therapy network (EPTN) has

The cornea is located anterior to the vitreous humor, iris, lens and ciliary body [2]. It can be

RT can injure the cornea by damaging the deeper layers of stroma, but in most cases the acute

*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%

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

*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

Biconvex structure in the aqueous humor, it is clearly visible in CT [2]. The structure is about

*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%

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.

put forward a consensus based atlas based on CT and MRI [2].

delineated in MRI or CT and is contoured with a 2-3 mm brush.

5/6th of the globe. The optic nerve is not contoured along with it [4].

toxicity is as a result of loss of tear film [3].

which increases further if chemotherapy is added.

**2.1. Eye ball**

2 Cancer Survivorship

*2.1.1. Cornea*

*2.1.2. Retina*

*2.1.3. Lens*

latent period is 19 months [5].

risk of progressive cataract.

**2.2. Optic nerve**

10 mm in diameter seen in the coronal plane.

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

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

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

## **2.4. Lacrimal gland**

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 and inferiorly.

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

*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].

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

**Figure 1.** Optic apparatus.

**Figure 2.** Optic chiasm.

**Figure 3.** Eye balls and associated structures.

#### **3. Salivation related structures**

#### **3.1. Parotids**

Parotids are contoured based on the guidelines given by Water et al. [10] (**Table 1**). It is to be noted that in 20% of the cases, the parotid gland extends anteriorly over the surface of the masseter muscle following the parotid duct and in the anterior direction the deep lobe of the parotid gland may extend alongside the medial border of the mandible gland may extend

**OAR**

**Anatomic boundaries**

Cranial

> Parotid gland

> External auditory

Posterior part of

Masseter muscle,

Anterior belly of

Subcutaneous place,

Posterior belly of

digastric muscle, styloid,

parapharyngeal space

platysma muscle

SCM muscle,

posterior mandible,

medial and lateral

pterygoid muscle

submandibular

canal, mastoid

process

Submandibular

Medial pterygoid

Subcutaneous fat

Mylohyoid muscle,

Parapharyngeal

Mandible (medial border),

Superior and middle PCM,

anterior belly of digastric

muscle, mylohyoid muscle

(lateral surface), hyoglossus

muscle

Genioglossus muscle

platysma, medial surface of

medial pterygoid

space, SCM

lateral surface,

hyoglossus muscle

muscle, mylohyoid

muscle

gland

Sublingual

Mucus membrane

mylohyoid muscle,

Mandible, mylohyoid

Hyoglossus muscle

Medial surface of

mandibular bone,

mylohyoid muscle

geniohyoid muscle

muscle

covering FOM

gland

Soft palate

Hard palate,

BOT, Tonsils,

HP, BOT/tongue, oral

Pharynx (mucosal

Pterygoid process, medial

pterygoid plate, superior

PCM, medial pterygoid

m., PPS, palatine tonsil,

pharyngeal lumen

surface/airspace),

superior PCM

cavity (airspace)

oropharynx (air

space)

nasopharynx (air

space)

Upper lip

Hard palate, nasal

lower end of upper

Orbicularis oris

Teeth, maxillary

Depressor anguli oris

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

muscle, buccinator muscle,

levator anguli oris muscle/

risorius muscle

bone, HP, tongue

muscle, subcutaneous

tissue/fat

glands

Lower lip

Upper edge of

lower edge of

Orbicularis oris

Teeth, mandible,

Depressor anguli oris

muscle, buccinator muscle,

tongue/air

muscle, subcutaneous

teeth sockets/

mandibular body

Alveolar process of

Orbicularis oris

Posterior edge of

Buccinator muscle,

Mandible, teeth, tongue

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

5

subcutaneous fat

mandibular body

and maxilla

muscle

SCM—sternocleidomastoid, PCM—pharyngeal constrictor muscle, BOT—base of tongue, HP—hard palate, FOM—floor of mouth.

mandible

tissue/fat

lower lip

glands

Buccal mucosa

Line between

maxillary process

and alveolar

process of maxilla

**Table 1.**

Salivation related structures.

glands

spine

lip

space

Caudal

Anterior

Posterior

Lateral

Medial


**3. Salivation related structures**

**Figure 3.** Eye balls and associated structures.

Parotids are contoured based on the guidelines given by Water et al. [10] (**Table 1**). It is to be noted that in 20% of the cases, the parotid gland extends anteriorly over the surface of the masseter muscle following the parotid duct and in the anterior direction the deep lobe of the parotid gland may extend alongside the medial border of the mandible gland may extend

**3.1. Parotids**

**Figure 2.** Optic chiasm.

4 Cancer Survivorship

Overview of Important "Organs at Risk" (OAR) in Modern Radiotherapy for Head and Neck Cancer (HNC) http://dx.doi.org/10.5772/intechopen.80606 5 alongside the medial border of the mandible. The external carotid artery, the retromandibular vein and the extracranial facial nerve are enclosed in the parotid gland.

*3.5.1. Lower lip glands*

*3.5.2. Upper lip glands*

edentulate mandible) [10].

*3.5.3. Glands of buccal mucosa*

plane. Cranially it extends till the nasal spine [10].

ders are better visualized in the coronal plane.

**4. Swallowing related structures**

**4.1. Pharyngeal constrictor muscles**

*4.1.1. Superior PCM*

of second cervical vertebra.

Orbicularis oris muscle can be used to delineate the glands anteriorly.

ally hard to differentiate from PCM and are not contoured differently.

The upper and posterior limit of the lip is better identified in sagittal sections. The lower limit corresponds to the caudal limit of teeth sockets or the cranial mandibular body (in case of

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

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

7

As of the lower lip, the lower and posterior extend is more easily made out in the sagittal

*Dose recommendations:* seek for a lip dose less than the oral cavity dose. A mean dose of 30Gy and 50 Gy for oral cavity and non-oral cavity cancers respectively would be preferable.

The glands of buccal mucosa are difficult to distinguish. The cranial, caudal and medial bor-

Pharyngeal wall has two layers of muscle (**Table 2**). The outer circular layer which are the pharyngeal constrictor muscles (PCM) and inner longitudinal muscles which are levators (stylopharyngeus and palatopharyngeus). PCM has three parts—superior, middle and inferior constrictor. The caudal ends of the levators blend with the PCM. These muscles are usu-

They originate from sphenoid bone from its pterygoid hamulus and insert to the median raphe. Different authors have put forward different levels in regards to its cranial border. Generally, cranial border of the superior PCM is taken as the caudal tip of the pterygoid plate, i.e., the pterygoid hamulus. The lowest fibers of the superior PCM are separated by the muddle PCM by stylopharyngeus and glossopharyngeal nerve. These fibers also overlap onto the upper fibers of middle PCM. As these changes are hardly made out in CT, most authors define the lowest limit of the muscle as the cranial border of hyoid bone. But this can lead to missing of half of middle PCM. Thus, the cranial border can be considered at the lower border

The abovementioned have been summarized in **Table 1**: salivation-related structures.

The external carotid artery (ECA), retromandibular vein and the extra-cranial part of facial nerve are enclosed in the gland. If contrast agents are used, the vessels can be clearly demarcated and can be avoided from the gland contour. But as contrast administration is not routinely practiced, and to make the contouring practice uniform, it is recommended to include the vessels within the gland contour.

*Dose recommendations*: severe xerostomia or salivary output <25% of baseline can be avoided if at least one parotid is restricted to a mean dose <20 Gy or both parotids restricted to <25 Gy [11].

## **3.2. Submandibular gland**

Situated in the floor of the mouth, it is a predominantly serous gland having a large superficial lobe and a small deep process separated by the fibers of mylohyoid. In most cases the gland is hypo dense on CT and can be easily demarcated.

*Dose recommendations:* if deemed oncologically safe, the mean dose to the submandibular gland, restricted to 35Gy may reduce xerostomia symptoms [11].

#### **3.3. Sublingual gland**

Sublingual gland is the smallest of the three major salivary glands. It is a predominantly mucus gland, situated in the anterior part of oral cavity in the sublingual space.

#### **3.4. Extended oral cavity**

The extended oral cavity is contoured partly based on the work by Hoebers et al. [12] excluding the lips and buccal mucosa [4]. It includes the space posterior to the arch of mandible and maxilla. Posteriorly it is limited by the uvula, soft palate and the base of tongue.

#### *3.4.1. Soft palate*

Soft palate contains numerous minor salivary glands. It is better seen in the sagittal sections by a thin air line separating it from the tongue inferiorly. As the salivary glands are distributed along the length of the soft palate, the entire soft palate is contoured including the uvula.

*Dose recommendations*: oral cavity dose should be kept as low as possible. Seeking for V45 < 40% and V50 < 20% limits mucositis and improves QoL [12].

#### **3.5. Other minor salivary glands**

These glands are distributed along the inner aspects of lips and buccal mucosa between the mucous membrane and the muscle layer. Maximum depth from the mucosal surface is about 4 mm with lower lip glands deeper than the upper ones.

#### *3.5.1. Lower lip glands*

alongside the medial border of the mandible. The external carotid artery, the retromandibular

The external carotid artery (ECA), retromandibular vein and the extra-cranial part of facial nerve are enclosed in the gland. If contrast agents are used, the vessels can be clearly demarcated and can be avoided from the gland contour. But as contrast administration is not routinely practiced, and to make the contouring practice uniform, it is recommended to include

*Dose recommendations*: severe xerostomia or salivary output <25% of baseline can be avoided if at least one parotid is restricted to a mean dose <20 Gy or both parotids restricted to <25 Gy [11].

Situated in the floor of the mouth, it is a predominantly serous gland having a large superficial lobe and a small deep process separated by the fibers of mylohyoid. In most cases the

*Dose recommendations:* if deemed oncologically safe, the mean dose to the submandibular

Sublingual gland is the smallest of the three major salivary glands. It is a predominantly

The extended oral cavity is contoured partly based on the work by Hoebers et al. [12] excluding the lips and buccal mucosa [4]. It includes the space posterior to the arch of mandible and

Soft palate contains numerous minor salivary glands. It is better seen in the sagittal sections by a thin air line separating it from the tongue inferiorly. As the salivary glands are distributed along the length of the soft palate, the entire soft palate is contoured including the uvula. *Dose recommendations*: oral cavity dose should be kept as low as possible. Seeking for V45 < 40%

These glands are distributed along the inner aspects of lips and buccal mucosa between the mucous membrane and the muscle layer. Maximum depth from the mucosal surface is about

mucus gland, situated in the anterior part of oral cavity in the sublingual space.

maxilla. Posteriorly it is limited by the uvula, soft palate and the base of tongue.

vein and the extracranial facial nerve are enclosed in the parotid gland.

gland is hypo dense on CT and can be easily demarcated.

and V50 < 20% limits mucositis and improves QoL [12].

4 mm with lower lip glands deeper than the upper ones.

**3.5. Other minor salivary glands**

gland, restricted to 35Gy may reduce xerostomia symptoms [11].

the vessels within the gland contour.

**3.2. Submandibular gland**

6 Cancer Survivorship

**3.3. Sublingual gland**

**3.4. Extended oral cavity**

*3.4.1. Soft palate*

The upper and posterior limit of the lip is better identified in sagittal sections. The lower limit corresponds to the caudal limit of teeth sockets or the cranial mandibular body (in case of edentulate mandible) [10].

#### *3.5.2. Upper lip glands*

As of the lower lip, the lower and posterior extend is more easily made out in the sagittal plane. Cranially it extends till the nasal spine [10].

*Dose recommendations:* seek for a lip dose less than the oral cavity dose. A mean dose of 30Gy and 50 Gy for oral cavity and non-oral cavity cancers respectively would be preferable.

Orbicularis oris muscle can be used to delineate the glands anteriorly.

#### *3.5.3. Glands of buccal mucosa*

The glands of buccal mucosa are difficult to distinguish. The cranial, caudal and medial borders are better visualized in the coronal plane.

The abovementioned have been summarized in **Table 1**: salivation-related structures.
