**6. Intra cranial structures**

#### **6.1. Ear structures**

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

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 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

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

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

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

*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].

Guidelines for contouring brachial plexus has been put forward by Hall et al. They have put forward step-by-step technique. The contour starts form the neural foramina of C5-T1.

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

limit of BOT.

10 Cancer Survivorship

**4.5. Contouring of larynx structures**

The pyriform sinus is not to be included in the contour.

from lower end of glottis to lower edge of cricoid.

responsible for the movement of the vocal cords.

inferiorly till the upper edge of arytenoid cartilage.

and the thyroid cartilage should be excluded.

*4.5.1. Supraglottic larynx*

**5. Brachial plexus**

*4.5.2. Glottis*

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

#### *6.1.1. Middle ear*

The eustachian tube (ET), tympanic cavity and the mastoid air cells (M) may be contoured separately based on the CT/MRI anatomy.

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

## *6.1.2. Cochlea*

It is a small spiral structure of about 0.6cm3 volume located in the petrous part of temporal 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 canals should not be contoured.

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

## *6.1.3. Vestibular and semicircular canal*

Arranged in 3 planes, the canals are contoured in bone window (120/1500). They are located lateral and superior to the cochlea.

## **6.2. Brain stem**

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.

#### *6.2.1. Midbrain*

It starts from the nigral substance of the cerebral peduncle and ends upper border of pons.

aware about the importance of accurate delineation of these structures. Only an accurate delineation can lead to effective sparing and thus a desirable outcome in terms of QOL. There were several isolated guidelines available. In this chapter we tried to summarize all the available guidelines. For certain organs like temporal lobe, multiple guidelines are available in the literature. We have tried to incorporate them together to put forward a single uniform consensus. Having said that the delineation and the attempted dose constraints should also be evaluated based on the target volume and tumor control. In case of parallel structures, the target volume coverage should be made priority and the risks and side effects of the same should be communicated to the patients. In such cases the volume of OARs outside the planning target volume (PTV) may be delineated separately and similar dose constrains may be aimed for. It should also be kept in mind that even with the most sophisticated of technologies, not all of the dose constraints might not be achieved due the basic physics of the photon beam. In such situations a trade-off should be agreed upon. However, such liberties, should not be attempted with serial structures like spinal cord and brain stem. Organs such as these should always be given hard constraints. If the PTV is overlapping such structures,

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

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

13

The chapter summarizes basic day to day information for a radiation oncologist to delineate OAR's in HNC radiotherapy. There are several updates and the readers are encouraged to go through them at a regular interval. We will be publishing a detail clinical end point based

The second opportunity to work with IntechOpen publishers has been excellent. I must thank my colleague Dr. Nithin in crafting the manuscript and it was his enthusiasm which led to a detail manuscript. We both would like to thank our respective parents for all the support and blessings. My wife (Reshmi) and Nithin's wife (Dr. Nikhila) respectively have been the greatest source of support. We also would like to thank our organization (HCG Apex Cancer Centre in Mumbai, India) for encouraging academic engagements in all pos-

under dose the area is accepted.

acute and late toxicities of these OAR's at a later date.

**8. Conclusion**

**Acknowledgements**

sible aspects.

**Author details**

Trinanjan Basu\* and Nithin Bhaskar

\*Address all correspondence to: trinanjan.doctor@gmail.com

Department of Radiation Oncology, HCG Apex Cancer Centre, Mumbai, India

#### *6.2.2. Pons*

Better visualized as an oval structure in sagittal sections, it is easily delineated.

#### *6.2.3. Medulla*

Medulla starts from the lower end of pons to the lever of tip of dense of axis.

*Dose recommendations*: Dmax—54 Gy. The entire brain stem can be treated to a dose of 54 Gy with little risk of serious side effects. [21]. Mean time of onset of symptom is 17 months (range 4.5–19 months). Smaller volumes (1-10 cc) may be irradiated to 59Gy at fractionations ≤2Gy [22].

#### **6.3. Pituitary gland**

A small gland, it is difficult to visualize in CT, but sella turcica can be used as a surrogate marked and the inner boundary of the same can be contoured for its delineation. The gland lies immediately below the brain and is connected to the hypothalamus by its stalk.

CT density of pituitary gland is similar to brain. Upon contrast administration, the gland may become more hyperintense than brain due to the rich vascular supply.

*Dose recommendations*: DMax 45Gy (for pan hypopituitarism, lower for Growth hormone (GH) deficiency). The anterior pituitary has 5 different types of cells, each with different radiosensitivity. Most sensitive is the GH axis followed by the gonadotropin, ACTH and TSH axis. GH deficiency has been noted in relatively lower doses, and has been reported for TBI for doses as low as 10 Gy [23], but the incidence increases substantially after 30 Gy where the incidence can be as high as 50–100%.

#### **6.4. Temporal lobe**

Contouring of temporal lobe should include the hippocampus, parahippocampal gyrus and the uncus. The basal ganglia and insula are excluded from the contour. Cranially it starts for the superior end of sylvian fissure and ends inferiorly at the base of middle cranial fossa. Medial boundary is marked by cavernous sinus, sphenoid sinus and the sylvian fissure and laterally by the temporal bone.
