**4. Swallowing related structures**

## **4.1. Pharyngeal constrictor muscles**

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

#### *4.1.1. Superior PCM*

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 of second cervical vertebra.


**Table 2.** Swallowing related structures. *4.1.2. Middle PCM*

border of hyoid bone.

contoured differently.

*4.1.4. Cricopharyngeus*

just below the level of arytenoid.

*Dose recommendations:* Dmean < 50 Gy.

**4.2. Esophageal inlet muscles (EIM)**

**4.3. Cervical esophagus (CE)**

**4.4. Base of tongue (BOT)**

notch.

*4.1.3. Inferior PCM*

The fibers originate from the greater and lesser horns of hyoid bone and insert along the median raphe. The upper fibers overlap, therefore these boundaries are arbitrary. The upper border is taken as the lower border of superior PCM. Lower border corresponds to the lower

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The thickest of the three constrictors, the inferior PCM has two parts—the thyropharengeal part which originate from the oblique line of thyroid cartilage and the cricopharyngeal part which originate from the lateral part of thyroid cartilage. Most authors delineate these two structures separately. The thyropharyngeus is referred to as inferior PCM and the latter is referred to as cricopharyngeal muscle. As functionally these muscles are different, they are

The inferior PCM is starts cranially from the caudal end of middle PCM, usually one slice below the caudal end of hyoid. The caudal border corresponds to the upper border of cricoid,

Anteriorly, the inferior PCM attaches to the posterior edge of thyroid cartilage, which can be recognized easily on CT, while the posterior border is defined by the prevertebral muscles.

Delineation starts cranially one slice below the level of arytenoids which also corresponds to the lower limit of inferior PCM. The contour continues till the lower end of cricoid cartilage.

Levendag et al. [10] recommends contouring the proximal 1 cm of esophagus as a separate

The contouring of CE varies from authors [13, 14]. For the purpose of consistency the upper border of CE starts 1 cm below the esophageal inlet muscles and end at the level of sternal

The delineation of BOT has been provided by three authors. All of them are consistent with the cranial extend defined as just below the soft palate. As this boundary is difficult to iden

tify in CT, the lower end of anterior tubercle of C1 vertebra (which corresponds to the same level) may be taken for the demarcation. The three authors vary in the definition of the lower


structure. The cranial border starts from the caudal end of cricopharyngeus.

#### *4.1.2. Middle PCM*

The fibers originate from the greater and lesser horns of hyoid bone and insert along the median raphe. The upper fibers overlap, therefore these boundaries are arbitrary. The upper border is taken as the lower border of superior PCM. Lower border corresponds to the lower border of hyoid bone.

#### *4.1.3. Inferior PCM*

The thickest of the three constrictors, the inferior PCM has two parts—the thyropharengeal part which originate from the oblique line of thyroid cartilage and the cricopharyngeal part which originate from the lateral part of thyroid cartilage. Most authors delineate these two structures separately. The thyropharyngeus is referred to as inferior PCM and the latter is referred to as cricopharyngeal muscle. As functionally these muscles are different, they are contoured differently.

The inferior PCM is starts cranially from the caudal end of middle PCM, usually one slice below the caudal end of hyoid. The caudal border corresponds to the upper border of cricoid, just below the level of arytenoid.

Anteriorly, the inferior PCM attaches to the posterior edge of thyroid cartilage, which can be recognized easily on CT, while the posterior border is defined by the prevertebral muscles.

*Dose recommendations:* Dmean < 50 Gy.

#### *4.1.4. Cricopharyngeus*

Delineation starts cranially one slice below the level of arytenoids which also corresponds to the lower limit of inferior PCM. The contour continues till the lower end of cricoid cartilage.

#### **4.2. Esophageal inlet muscles (EIM)**

Levendag et al. [10] recommends contouring the proximal 1 cm of esophagus as a separate structure. The cranial border starts from the caudal end of cricopharyngeus.

#### **4.3. Cervical esophagus (CE)**

The contouring of CE varies from authors [13, 14]. For the purpose of consistency the upper border of CE starts 1 cm below the esophageal inlet muscles and end at the level of sternal notch.

#### **4.4. Base of tongue (BOT)**

**Organ at risk**

**Anatomic boundaries**

Cranial

> Superior PCM

Middle PCM Inferior PCM

First slice inferior

Inferior edge of

Soft tissue of supraglottis/

Prevertebral

muscles

arytenoid cartilage

glottis

to inferior edge of

hyoid

Cricopharyngeal

First slice inferior

Inferior edge of cricoid

Posterior edge of cricoid

Prevertebral

Thyroid gland/

cartilage, fatty tissue

muscles

to arytenoid

cartilage

First slice inferior

1

cm inferior to the

Tracheal lumen

Prevertebral

Thyroid gland, fatty

tissue

muscles

upper extend

to cricoid

muscle

EIM Cervical

1 cricoid

cm inferior to the

Sternal notch

esophagus

BOT Supraglottic larynx

Glottic larynx

**Table 2.**

Swallowing related structures.

Superior edge of

Inferior edge of cricoid

Thyroid cartilage

Inferior PCM,

Thyroid cartilage

Pharyngeal lumen

(lumen to be

excluded)

pharangeal lumen/

cricoid

cartilage (only the soft

tissue element)

arytenoid cartilage

Tip of epiglottis

First slice superior to the

Hyoid, thyroid cartilage,

Pharyngeal lumen,

Thyroid cartilage

Pharyngeal lumen

(lumen to be

excluded)

inferior PCM

preepiglottic space

arytenoid cartilage

Inferior edge of C1

Superior edge of hyoid

Posterior 1/3 from

Pharyngeal lumen

pharynx

Width of the lumen of

mandibular bone to

pharyngeal lumen

Superior edge

Inferior edge of hyoid

BOT, hyoid

of C3

Inferior tip of

Inferior edge of C2

Pterygoid hamulus, BOT,

Prevertebral

Medial pterygoid

Pharyngeal lumen

muscles

muscles

Prevertebral

Hyoid—greater horn.

Pharyngeal lumen

Thyroid cartilage—

superior horn

Superior horn of

thyroid cartilage

muscles

pharyngeal lumen

pterygoid hamulus

Caudal

Anterior

Posterior

Lateral

Medial

8 Cancer Survivorship

The delineation of BOT has been provided by three authors. All of them are consistent with the cranial extend defined as just below the soft palate. As this boundary is difficult to identify in CT, the lower end of anterior tubercle of C1 vertebra (which corresponds to the same level) may be taken for the demarcation. The three authors vary in the definition of the lower extend. The lower limit of hyoid [13], the vallecular and the first slice of epiglottis [14] are mentioned, but for consistency, we follow the upper end of body of hyoid as the lower caudal limit of BOT.

It extends from outer edge of spinal canal to the space between anterior and middle scalene. Where no spinal foramina was present, only the space between anterior and middle scalene is contoured. The middle scalene will end in the region of the subclavian neurovascular bundle. In the lower part, the brachial plexus is contoured in the posterior aspect of neurovascular

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*Dose recommendations:* Dmax <60Gy. Emami et al. [17] has suggested TD 5/5 for the entire brachial plexus to be 60Gy. More recent studies with longer follow-up (upto 20 years) have shown that the risk of plexopathy keeps rising even after 5 yrs. and may not be apparent until

Ear structures (both the middle ear and inner ear) should be contoured using the bone

The eustachian tube (ET), tympanic cavity and the mastoid air cells (M) may be contoured

*Dose recommendations*: ET D30 < 52Gy; M D0.05cc < 41 Gy. Based on the study by Yao et al. [19], dose to 30% of ET and 0.5 cc of mastoid volume were the main predictors of severe ear disor-

bone. The small bony cavity can be visualized better with a setting of 120/1500 on CT. The structures of inner ear are visualized more in T2 weighted MRI images. The semicircular

*Dose recommendations*: Dmean <45 Gy. In children it is advisable to keep it below 35 Gy. [20]

Arranged in 3 planes, the canals are contoured in bone window (120/1500). They are located

Brain stem comprises of midbrain, pons and medulla. The cranial extend starts from the level of inferior section of lateral ventricle. The organ is better visualized better in MRI. The contour

extends till the level of the tip of dense of C2 vertebra or foramen magnum.

volume located in the petrous part of temporal

ders. Doses above these are associated with increase in grade 2 ear disorders post RT.

bundle in the inferior and lateral aspect one to two slices below the clavicle.

20 years after radiation [18].

**6. Intra cranial structures**

separately based on the CT/MRI anatomy.

It is a small spiral structure of about 0.6cm3

canals should not be contoured.

*6.1.3. Vestibular and semicircular canal*

lateral and superior to the cochlea.

**6.1. Ear structures**

*6.1.1. Middle ear*

*6.1.2. Cochlea*

**6.2. Brain stem**

window.

#### **4.5. Contouring of larynx structures**

Freedman et al. [15] has provided a 3 step method to delineate larynx. Step 1 and 3 identifies the cranial and caudal limit of larynx. The contouring starts form the slice just below the caudal edge of hyoid and ends where the cricoid cartilage is seen as a complete ring. Step 2 mentions the circumference limits of the larynx. The anterior border corresponds to the inner surface of the thyroid cartilage. The posterior border in the upper part corresponds to the lateral surfaces of the aryepiglottic folds and the posterior surface of the mucosa covering the arytenoids. In the lower part it corresponds to the posterior surface of the cricoid cartilage. The pyriform sinus is not to be included in the contour.

The above guidelines contour the larynx as a single structure. As the larynx consists of subglottic, glottic and the supraglottic area, several authors have delineated these sub-sites separately. The supraglottis includes the epiglottis, the aryepiglottic folds, the arytenoids, and the false vocal cords. The glottis is composed of the true vocal cords and the subglottis extends from lower end of glottis to lower edge of cricoid.

The delineation of supraglottis and glottis is based on the function of the two subsites. While the supraglottis includes the muscles responsible for the closure of larynx, the glottis part is responsible for the movement of the vocal cords.

#### *4.5.1. Supraglottic larynx*

The contour includes the supraglottic adductors (oblique arytenoids and aryepiglottic muscles) and epiglottis. The cranial border is the tip of the epiglottis and the contour continues inferiorly till the upper edge of arytenoid cartilage.

#### *4.5.2. Glottis*

The contour starts from the upper end of arytenoid cartilage and ends caudally at the lower edge of the cricoid. Only the soft tissue is contoured (except for the arytenoids). The cricoid and the thyroid cartilage should be excluded.

*Dose recommendations*: To minimize laryngeal edema, the volume of larynx receiving 50 Gy and mean dose should be kept as low as possible, ideally ≤20% and 40 Gy respectively [16].

The above has been summarized in **Table 2**: Swallowing related structures.
