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HNC Critical Review|Articles in Press, 101042

MRI-based delineation of organs at risk in the head and neck region

Open AccessPublished:July 30, 2022DOI:https://doi.org/10.1016/j.adro.2022.101042
MRI-based delineation of organs at risk in the head and neck region
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      Abstract

      Background

      The aim of this paper is to establish a comprehensive contouring guideline for treatment planning using only MR images through an up-to-date set of organs at risk (OAR), recommended organ boundaries and relevant suggestions for the MRI based delineation of OARs in the head and neck (H&N) region.

      Materials and Methods

      After a detailed review of literature MRI data were collected from the H&N region of healthy volunteers. OARs were delineated in the axial, coronal, and sagittal planes on T2 weighted sequences. Every contour defined has been revised by four radiation oncologists and subsequently, by two independent senior experts, a H&N radiation oncologist and a radiologist. After revision, the final structures were presented to the consortium partners.

      Results

      Definitive consensus was reached after multi-institutional review. On that basis we have provided detailed anatomical and functional description and specific MRI characteristics of the OARs

      Conclusions

      In the era of precision radiotherapy, the need for well-built, straightforward contouring guidelines is on the rise. Precise, uniform delineation based automatized OAR segmentation on MR imaging may lead to increased accuracy in terms of organ boundaries, and analysis of dose dependent sequelae for adequate definition of the normal tissue complication probability.

      Introduction

      Accurate organ at risk (OAR) delineation in the era of precision radiotherapy (RT) is essential for the irradiation of head and neck (H&N) cancer. Both postoperative and definitive organ-preserving radiotherapy with or without chemotherapy are widely applied in the complex management of this disease. While radiotherapy is highly effective in the treatment of cancer a substantial rate of serious acute side effects and often severe, treatment-related late toxicities occur. Acute mucositis during dose delivery as well as late functional damages of healthy organs may lead to xerostomia and swallowing difficulties, significantly deteriorating the quality of life of the patients. Consistent definition and delineation of OARs using highly selective, conformal plans is vital in order to spare normal structures.
      Both traditionally and at present, the generally used imaging method for contouring purposes is computed tomography (CT). A consensus guideline [1] was published in 2015 on CT-based OAR delineation for the H&N region. The hegemony of CT is, however, challenged by newly emerged, advanced methods that provide detailed images with far superior soft tissue resolution in comparison to computed tomography (magnetic resonance imaging; MRI, [2; 3]).
      MRI-only treatment planning has several clinical advantages, such as the avoidance of multiple imaging processes and exposure to ionizing radiation during imaging. The latter has a greater relevance if repeated imaging is applied for replanning purposes during adaptive radiotherapy.
      In the meantime, MRI-only based contouring has a number of drawbacks as well. Due to the more complex physical background, selecting and interpreting the adequate sequences require more experience. Moreover, getting to know the specific MRI characteristics of the given OARs may take a longer time and more practice. In the future, thanks to automated organ at risk delineation, this learning process will become needless and errors due to human mistakes will be less likely to occur.
      It is with the availability of such modalities, that the need for a large number of accurate OAR contours in the head and neck region, including subunits of larger organs and organelles, can be satisfied. Although magnetic resonance imaging has long been used for tumor delineation, its implication in organ at risk contouring remains a question of debate [4-6].
      Since 2015 many publications have appeared, focusing on new potential organs at risk in the anatomical region at hand. As a result of this process, numerous different sets of OARs coexist, in some cases accompanied by MRI based contouring atlases.
      The purpose of this paper is to establish a comprehensive contouring guideline for treatment planning using only MR images through an up-to-date set of OARs, recommended organ boundaries and relevant suggestions of the MRI based delineation of organs at risk.

      Materials and Methods

      The ongoing project: XXXX aims to develop a software that can provide automatic OAR contours on MR images [7]. The first phase of development required numerous precise, manually contoured expert cases. During the project, the need emerged for definition of MRI acquisition techniques and delineation of OARs in the head and neck region on MR images.
      To that aim, the technical details of MRI acquisition for treatment planning, reasonably required MRI sequences and an up-to-date set of OARs were defined, relying on the latest existing guidelines [1; 8-14]. The method comprised of a systematic review of the latest publications and discussions for consensus in the divergent points by the representatives of the participating institutions.
      The next step was MRI data collection from the head and neck region of 7 healthy volunteers in diagnostic setting (supine position, dedicated Head/Neck coil, as detailed in Table 1). The OARs were delineated using the ECLIPSE treatment planning system (Varian Medical Systems, version: 13.6) in the axial, coronal and sagittal planes on T2 weighted sequences. Every contour defined has been revised by four radiation oncologists and subsequently, by two independent senior experts, a head and neck radiation oncologist and a radiologist. Following this two-step revision, the final structures were presented to the consortium partners in Rotterdam. Definitive consensus was reached after multi-institutional review. In order to visualize our findings and suggestions, we have compiled an atlas containing an expert case, which can be found in the supplementary materials section.
      Table 1Scan parameters for the three T2-weighted sequences used for OAR delineation.
      2D T2 frFSE2D T2 PROPELLER3D T2 CUBE
      MRI scanner modelGE DISCOVERY MR750wGE SIGNA ArtistGE SIGNA Artist
      RF receive coil arrayAIR RT Head&NeckGEM Head&NeckGEM Head&Neck
      B0 [T]31.51.5
      BW [kHz]62.583.3125
      TR [ms]1186676242002
      TE [ms]99.387.376.6
      ETL2226100
      Scan time [s]332.6229.1259.0
      Scan orientationaxialaxialsagittal
      FOV [mm^2]300 × 300300 × 300340 × 340
      AcqRes [mm^2]1.04 × 1.041.04 × 1.041.18 × 1.18
      InterpRes [mm^2]0.59 × 0.590.59 × 0.590.66x.066
      Dz [mm]331
      Nz10080340
      With B0 the main magnetic field strength, BW the bandwidth (pixel bandwith * rows), TR the repetition time, TE the echo time, ETL the echo train length, AcqRes the acquires resolution (reconstruction diameter/acquisition matrix), InterpRes the interpolated resolution (reconstruction diameter/rows), Dz the slice thickness, and Nz the number of slices.
      MR imaging for OAR delineation was based on 2D and 3D T2-weighted fast spin echo (FSE) sequences with sufficient geometric coverage to include all relevant OARs (i.e., top of the head to the middle of the neck).  More specifically, the T2 FSE sequences included 2D fast-recovery FSE (frFSE), 2D PROPELLERTM, and 3D CUBETM.  Detailed sequence parameters are outlined in Table 1.
      From the point of view of tissue contrast, no significant differences were found between the tested sequences. However 2D sequences are more sensitive to patient motion (swallowing, eye movements) and thus imaging artifacts are more likely to occur. This is the main reason why the volunteer scan included in the atlas is a 3D Sagittal T2 CUBE sequence, though our contouring atlas is applicable to any diagnostic T2 weighted MRI sequence. The reconstruction diameter was 350 mm, the pixel size 0.664 mm and the spacing-between-slices 0.5 mm. The selection criteria for the atlas case included high resolution, good contrast and lack of imaging artifacts. Prior to manual contouring, the original scan was reformatted to axial orientation and isotropic voxel size (0.664 mm) using Volume Viewer application of Advantage Workstation 4.7 to make it compatible with all radiation therapy planning software.
      Our choice of MRI sequence relied on a scoring table (Table 6.), in which the visibility of the organs at risk was evaluated on T1 and T2 weighted MRI sequences, as well as on CT. The most important aspects of evaluation were sharpness of margins and demarcation from the surrounding tissues. A scale ranging from 1 to 3 was utilized, where 3 stood for excellent, 2 for average and 1 for poor visibility. Based on the results of this evaluation, we have found that T2 weighted MRI sequences are more suitable for OAR delineation than T1 ones.
      In addition to organ contouring, the MR-only radiation therapy planning requires synthetic CT to enable dose calculation. The technical feasibility of the MR-based synthetic CT was already demonstrated by several studies [15; 16].

      Results

      We summarized here our findings during the contouring session as well as the recommended OAR boundaries. Table 6. also contains useful information on the MRI characteristics of the different OARs by the end of this section.

      Parotid glands

      The parotid gland is the largest of the major salivary glands, approximately 55-60 mm in craniocaudal and 30-35 mm in anteroposterior dimension [8]. It is composed of an inverted triangle-shaped superficial and a deep lobe, located behind the ramus of the mandible. A number of important vessels and nerves pass through the gland. The facial nerve pierces its substance in postero-anterior direction; this can be regarded as the border between the superficial and deep lobes. Deep to the facial nerve passes the external carotid artery to terminate as two branches inside the parotid gland: maxillary and superficial temporal artery. Superficial to the arteries are the superficial temporal and maxillary veins, joining to form the retromandibular vein.

      Contouring suggestions

      • •
        The delineation of the external carotid artery and retromandibular vein was carried out in a few cases in order to precisely define the medial border of the organ and to distinguish between the vessels and the styloid process, as well as the muscles arising from the latter.
      • •
        An accessory parotid gland is sometimes present alongside the parotid duct, on the outer surface of the masseteric muscle that has to be included in the contour.
      • •
        Fatty infiltration/replacement of the secretory tissue may be present at older ages, which may make the outline of the organ more difficult to define [17]. This may also arise as a therapeutic side effect of previous irradiation.
      • •
        See Table 2. for the recommended anatomical boundaries.
        Table 2Anatomical borders of the salivary glands.
        Organ boundariesParotid glandsSubmandibular glands
        AnteriorMasseteric muscle, man-dibular ramus, pterygoid musclesPosterior margin of mylohyoid muscle, with the deep process spreading above the mylohyoid muscle
        PosteriorSternocleidomastoid mus-cle and the posterior belly of the digastric muscleParapharyngeal space, great vessels of the neck
        MedialStyloid process, styloglos-sus, stylohyoid and stylo-pharyngeal musclesSuperior: lateral surface of hyoglossus and partly mylohyoid muscles Middle: lateral surface of styloglossus and stylohyoid muscles, digastric muscle Inferior: lateral surface of the body of hyoid bone, pharyngeal constrictor muscles
        LateralPlatysma, subcutaneous tissueSuperior: medial surface of medial pterygoid muscleMiddle: medial surface of the body of the mandibular bone Inferior: platysma, investing layer of deep cervical fascia, fat tissue
        CranialSuperior wall of of the external auditory canal, mastoid processMedial pterygoid muscle
        CaudalNo distinct border, the organ gradually disappears in the fat tissue of the neckNo distinct border, the organ gradually disappears in the fat tissue of the neck

      Submandibular glands

      The largest portion of the walnut-sized (approximately 27 mm in length and 13-14 mm in width) submandibular gland lies in the submandibular triangle, and usually exceeds the level of the inferior border of the mandibular corpus (i.e. the cranial border of the submandibular triangle) in the cranial direction. Inferiorly, it reaches the insertion of the stylohyoid muscle (body and greater horn of hyoid bone) and the intermediate tendon of the digastricus. A tongue like extension of the gland, often referred to as the deep process, arises from the medial surface of the gland and spreads above the mylohyoid muscle. The facial artery leads a tortuous path, which passes through the medial part of the organ, while the facial vein is situated on the superficial surface.

      Contouring suggestions

      • •
        See Table 2. for the recommended anatomical boundaries.

      Mandible

      The mandible, or jawbone consists of a parabolic-shaped body that is connected by a right and left ramus to the rest of the cranium. The two rami terminate in the condyles, which form the articular head of the temporomandibular joints.

      Contouring suggestions

      • •
        The alveoli or teeth sockets of the mandibular corpus are still included in the contour, but not the teeth.
      • •
        We have omitted the coronoid process from the contour since its cranial border is hard to define univocally on axial MRI slices and radiation necrosis affects primarily the body [18].

      Supraglottic larynx

      The upper part of the larynx is composed of the epiglottis, the aryepiglottic folds, the false vocal or vestibular cords, the arytenoid cartilages and the mucosa coating them. Charlotte L. Brouwer et al. [1] defined the inferior border of the supraglottis as the cranial edge of arytenoid cartilages. This, however, contradicts the fact that the vestibular cords form an integral part of the supraglottic larynx, since their origin and insertion (anter-olateral surface of the arytenoid cartilage, just above the vocal process – angle of thyroid cartilage, below the attachment of the epiglottis) is situated below the apex of the arytenoids. The pyriform sinus shall be excluded from the contour, as it belongs to the hypopharynx.

      Glottic larynx/glottic area

      The glottic larynx is an anatomical subsite of the larynx, below the supraglottis, and above the subglottis. The name of the area is derived from the glottis, the gap between the true vocal cords. According to the 7th edition of AJCC Cancer Staging Manual [19], the glottis contains the true vocal cords, including its anterior and posterior commissures. The overall thickness of the structure is approximately 10 mm in the horizontal plane.

      Contouring suggestions

      • •
        See Table 3. for the recommended anatomical boundaries of the laryngeal structures.
        Table 3Anatomical borders of the laryngeal structures and the oral cavity.
        Organ boundariesSupraglottic larynxGlottic larynxOral cavity
        AnteriorHyoid bone, pre-epiglottic space, thyroid cartilage [1]Thyroid angleInner surface of superior and inferior dental arches
        PosteriorPosterior pharyngeal wallInner surface of cricoid and arytenoid cartilagesPosterior border of soft palate and uvula, root of the tongue [1]
        MedialNA (lumen of the larynx)NA
        LateralInner surface of thyroid cartilageInner surface of dental arches, maxilla and mandible
        CranialTip of epiglottis [1]Caudal boundary of supraglottic larynx, i.e. the arytenoidsMucosa of hard and soft palate
        Caudal1-2 slices below the appearance of arytenoid cartilages, individually. Thus, the false vocal cords fall within the borders of the structure.Clinically, it varies from 0 to 1 cm below the free level of the true vocal cord, extending inferiorly from the lateral margin of the ventricle [19; 23]. From practical point of view, the disappearance of the thyroid angle is a good landmark.Anterior: mylohyoid muscle + anterior belly of the digastric muscle Posterior: root of the tongue and hyoid bone [1]

      Oral cavity

      Opposite to the previously enumerated structures, the oral cavity is not an organ on its own, but an anatomical area within the mouth that is located anterior to the oropharynx [20]. Its contour includes the hard and soft palate as well as the lingual tonsils, since their mucosa must be spared from an excessive dose of ionising radiation. This consideration led us to an OAR contour that is somewhat larger than the oral cavity proper (i.e., the cavity behind the dental arches, excluding the oral vestibule between the lips and teeth).

      Contouring suggestions

      • •
        See Table 3. for the recommended anatomical boundaries.

      Pharyngeal constrictor muscles (PCMs)

      The muscles of the pharynx can be divided into an outer circular and an inner longitudinal layer. The former includes the superior (superior constrictor; SC), middle (middle constrictor; MC) and inferior (inferior constrictor; IC) pharyngeal constrictor muscles, responsible for propelling the bolus into the esophagus.

      Contouring suggestions

      • •
        Apart from the PCMs, various other structures play an indispensable role in the process of swallowing. These structures are the muscles of the floor of the mouth (the anterior belly of digastric muscle, mylohyoid and geniohyoid muscle), the thyrohyoid muscle, the posterior digastric/stylohyoid muscle complex, the longitudinal pair of the pharyngeal constrictors i.e. the longitudinal pharyngeal muscles (LCMs), the hyoglossus/styloglossus complex, the genioglossus muscle and, finally, the muscles responsible for tongue motion; the intrinsic tongue muscles, which are referred to collectively as functional swallowing units (abbreviated as FSUs [9; 21]). The three major physiological roles of these structures are tongue motion, tongue base retraction and hyolaryngeal elevation. The usefulness of FSU delineation in daily routine is a source of debate, due to the extreme workload demand and eventual overlaps with other sensitive areas, such as the oral cavity and certain laryngeal structures. The contouring process might be facilitated in some cases by the usage of automatic OAR segmentation, as stated in the article of the MD Anderson Head and Neck Cancer Symptom Working Group [22]. To the best of our knowledge, neither dose constraints, nor exact fields of implication have been defined for these structures.
      • •
        See Table 4. for the recommended anatomical boundaries.
        Table 4Anatomical borders of the pharyngeal constrictor muscles.
        Organ boundariesSuperior pharyngeal constrictor muscle (SC)Middle pharyngeal constrictor muscle (MC)Inferior pharyngeal constrictor muscle (IC)
        AnteriorCaudal tip of the pterygoid plates [13]Hyoid bone, root of the tongue [13]Soft tissue of the larynx [13]
        PosteriorLongus capitis and colli muscles
        The former muscle stretches between the base of the skull (insertion: basilar part of occipital bone) and the upper cervical vertebrae (origin: transverse processes of third to sixth cervical vertebrae). The latter lies beneath the longus capitis muscle, on the anterior surface of vertebral bodies and can be followed all the way down to the level of upper thoracic vertebrae (origin: bodies of C5 – Th3 vertebrae, insertion: anterior arch of the atlas)
        , i.e. the prevertebral muscles [13; 24]
        MedialNA/pharyngeal lumen [13]
        LateralMedial pterygoid muscle [13], parapharyngeal spaceGreater horn of hyoid bone [13]Superior horn of thyroid cartilage [1; 13]
        CranialCaudal tip of pterygoid plates
        The pterygopharyngeal part of the superior pharyngeal constrictor muscle originates from the lower third of the medial pterygoid plate and its hamulus. Finding the pterygoid process on MRI images may be challenging, therefore CT correlation is advisable.
        [13; 25]        		Cranial edge of C3 vertebra [13]First slice caudal to the caudal edge of hyoid bone [13]
        CaudalCaudal edge of C2 vertebra [13]Caudal edge of hyoid bone [13; 25]Caudal edge of arytenoid cartilages [13]
        1 The former muscle stretches between the base of the skull (insertion: basilar part of occipital bone) and the upper cervical vertebrae (origin: transverse processes of third to sixth cervical vertebrae). The latter lies beneath the longus capitis muscle, on the anterior surface of vertebral bodies and can be followed all the way down to the level of upper thoracic vertebrae (origin: bodies of C5 – Th3 vertebrae, insertion: anterior arch of the atlas)
        2 The pterygopharyngeal part of the superior pharyngeal constrictor muscle originates from the lower third of the medial pterygoid plate and its hamulus. Finding the pterygoid process on MRI images may be challenging, therefore CT correlation is advisable.

      Inner ear

      The inner ear is comprised of two main functional parts, the cochlea, and the vestibular system. They consist of an outer bony labyrinth, a network of passages with bony walls within the petrous part of the temporal bone and an inner fluid-filled membranous labyrinth. The cochlea is a spiraled tunnel that makes 2¾ turns about its axis, the modiolus, which is perpendicular to the longitudinal axis of the petrous bone. The semicircular canals are situated posterolaterally to the cochlea and are made up of three tubes according to the three main planes of space, interconnected by a central vestibule [26].

      Contouring suggestions

      • The cochlea and vestibular system have been delineated separately in some cases. However, due to the proximity of the two structures, we have defined the inner ear as one single OAR, following the practice of Sun et al. [27].

      Eye (eyeball)

      The eyeball consists of three outer tunics (sclera, choroid, and retina) encompassing the core of the eye itself. This can be subdivided into four further anatomical structures, the anterior & posterior chambers, lens, and vitreous body. Finer microscopic anatomical details, however, cannot be observed on MRI scans.

      Contouring suggestions

      • •
        A meticulous delineation of the fluid filling the anterior chamber and the vitreous body can be carried out. The extension of this contour by 1 mm (corresponding to the outer layers of the eye) in all dimensions may also lead us to an adequate OAR contour.

      Lens

      The lens of the eye is a biconvex, lentiform structure suspended between the iris and vitreous body. Its overall diameter typically ranges between 9 and 10 mm, with a thickness of approximately 4,5 mm, though this varies with age.

      Optic nerve

      Also known as cranial nerve II, the optic nerve is composed of ganglion cell axons that carry the excitation of retinal photoreceptors to the vision centers of the cortex. The orbital portion of the nerve is usually between 20 and 30 mm in length and 2-5 mm in thickness. It travels in the axis of the orbit, above the rectus inferior muscle and below the rectus superior. It enters the cranial cavity via the optic canal to terminate in the optic chiasm. The intracranial segment of the optic nerve is approximately 10 mm long.

      Contouring suggestions

      • •
        The optic nerve can be confused for the rectus superior and inferior muscles. The muscles have a flat, shorter appearance, while the nerve is slimmer and longer.
      • •
        The meningeal layers ensheathing the nerve have also been included in the contour.

      Optic chiasm

      The optic chiasm is the location of the partial decussation of optic fibers. It rests upon the tuberculum sellae, in the suprasellar cistern. The crossing fibers form an X-shape, posteriorly bordered by the pituitary stalk, laterally by the internal carotid arteries and inferiorly by the third ventricle. The circle of Willis encircles the pituitary stalk and optic chiasm. The overall size of the structure is usually 14 × 8 × 5 mm [10].

      Contouring suggestions

      • •
        The X-shape is not always visible on one single section, especially when operating with small slice thickness (1 mm or below). In such cases, the fibers of the optic nerve entering the chiasma and the axons forming the optic tract can be delineated on consecutive MRI slices.
      • •
        The pituitary stalk and internal carotid arteries may additionally be delineated to help distinguish between the chiasm and the surrounding structures.

      Lacrimal gland

      The lacrimal gland is a small exocrine gland that is situated in a shallow depression of the superolateral corner of the orbit.

      Contouring suggestions

      • •
        The easiest way to find the gland is to look for an approximately 15 × 20 × 5 mm area with low signal intensity, above the lateral rectus muscle and laterally to the superior rectus muscle [28].
      • •
        The volume of the lacrimal gland is usually around 0.6 cm3 with a slight right-sided dominance [29; 30].

      Brainstem

      The rostral continuation of the spinal cord can be subdivided into three levels in rostro-caudal order. The lowermost one-third, the medulla oblongata has no well-determined inferior border since transition from the spinal cord to the brainstem is continuous. To overcome this uncertainty, we have commenced the contouring of the medulla at the level where the tip of the odontoid process first appears, in concordance with CT based guidelines [1]. The rostral limit of the mesencephalon, or midbrain (the uppermost third of the brainstem) is similarly ill-defined. A recent study carried out on OAR contouring in the central nervous system using MRI technique suggests to delineate the midbrain till the disappearance of the nigral substance [10]. The previously mentioned CT based consensus guideline defines the cranial beginning of the midbrain as the bottom section of the lateral ventricles. We do not entirely agree with this approach, since the temporal horns of the lateral ventricles appear already at the level of the ponto-mesencephalic junction, therefore more caudally than the expected organ margin. We have found that the central part of the lateral ventricle is a more reliable landmark for the upper border of the mesencephalon. Another study by Beddok et al. [11] suggests to place the brainstem between the uppermost and lowermost endpoints of the Sylvian aqueduct, a CSF filled narrow cavity that is well visible in the sagittal plane.

      Contouring suggestions

      • •
        The average volume of the brainstem is expected to fall between 27 and 43 cm3 [31].

      Spinal cord

      The spinal cord is the caudal continuation of the brainstem, extending from the lowermost section of the medulla to the intervertebral disc between the first and second lumbar vertebrae.

      Brain

      Our brain contour includes the brainstem, diencephalon, cerebellum, hemispheria of the telencephalon, smaller cerebral vessels, and the cerebrospinal fluid. Since our approach treats the brainstem as a subunit of the OAR brain, the lowermost section of these two is located in an identical plane.

      Contouring suggestions

      • •
        The contouring of this organ mainly involves following the outline cerebrospinal fluid in the subarachnoid space.

      Pituitary gland

      The pituitary gland, or hypophysis, is a cherry-sized endocrine organ located within the cranial cavity. It can be regarded as a caudal protrusion of the hypothalamus, connected by the pituitary stalk to the latter.

      Contouring suggestions

      • •
        The hypophysis rests in a small, saddle-shaped bony nest of the sphenoid bone, the sella turcica. The organ itself is usually well visible on any diagnostic T2 weighted MRI sequences, though the sella itself is difficult to find. On CT scans with appropriate bone window, we can localize the clinoid processes, the dorsum and the tuberculum sellae, important anatomical landmarks bordering the hypophyseal fossa, and thus, the hypophysis.

      Thyroid gland

      The thyroid gland is an endocrine organ that lies against and around the thyroid and cricoid cartilages of the larynx. It is made up of two elongated lobes interconnected by a narrow isthmus, giving the thyroids the shape of a butterfly. The size of the lobes may vary on a wide range, but the average antero-posterior diameter of the organ falls usually between 13 and 28 mms, with a length of 40-60 mm. The volume of the OAR is 12-18 ml in the male and 10-15 ml in the female population [32; 33].

      Brachial plexus (BP)

      The brachial plexus is a network of nerves that provides sensory and motor innervation of the upper limb and shoulder girth. The course of the plexus can be divided into four distinct portions, each related to characteristic anatomical landmarks. The first portion, i.e. the radices or roots of the brachial plexus correspond to the anterior rami of C5-T1 spinal nerves. These roots later merge to form the superior, middle and inferior trunks of the plexus, which are situated above the clavicle, in the scalene hiatus. Behind the clavicle, each trunk splits in two, forming a total of six divisions. The last portion of the brachial plexus are the cords, located below the clavicle, in the axillary fossa. They are named by their position with respect to the axillary artery. The recommended anatomical boundaries for the brachial plexus are included in Table 5.
      Table 5Anatomical borders of the brachial plexus.
      RootsTrunksDivisionsCords
      First, the intervertebral foramina between C4-C5 and T1-T2 should be identified [14; 34]. Keep in mind that the cervical spinal nerves emerge above their corresponding vertebrae
      Unlike the rest of the spinal nerves that leave the spinal canal below their corresponding vertebrae
      . This is the reason why the fifth cervical spinal nerve is to be found above the fourth cervical vertebra.      		The next step is delineating the trunks of the brachial plexus in the scalene hiatus [14; 34]. The anterior and middle scalene muscles may also be contoured for better understanding anatomical relations.The last two portions of the brachial plexus are defined as the posterior part of the subclavian and axillary neurovascular bundle, below the insertion of the middle scalene muscle and the sternal extremity of the clavicle [14; 35]. A 5 mm paint tool thickness is recommended for the delineation of the OAR [34; 35].Furthermore, a study by Van der Velde et al. suggests to add a margin of 4.7 mm around this BP contour in order to achieve full coverage of OAR and anatomical variants [36].
      1 Unlike the rest of the spinal nerves that leave the spinal canal below their corresponding vertebrae
      Table 6Comparison between T1 and T2 weighted MRI sequences and CT. The visibility of the organs is graded from 1 to 3, where 3 stands for excellent, 2 for average and 1 for poor visibility.
      OrganT1wT2wCTRemarks
      Parotid glands232Any diagnostic T2w MRI sequence is eligible for delineation purposes, since the saliva content of the glands creates a well-visible contrast with the surrounding tissues.
      Submandibular glands232Similar MRI morphology to the parotid glands.
      Mandible223Contouring the mandible on CT is easier, due to the sharp contrast between the hyperdense bone and the surrounding soft tissues. On T2w MRI the cortical bone appears as a low signal intensity layer enveloping the inhomogeneous spongious bone.
      Supraglottic larynx222
      Glottic larynx/glottic area222Nonossified cartilages appear with intermediate signal intensity on T2w images. Ossified cartilages are similar to bone, i.e. high signal central marrow and low signal cortical rim [39].
      Oral cavity221The visibility of the muscles of the floor of the mouth and palate is poor on CT, therefore the cranial and caudal border of the region is hard to define. The usage of coronal and sagittal MRI slices beside the axial plane is crucial to correctly define the cranio-caudal and latero-lateral extent of the oral cavity [40].
      Pharyngeal constrictor muscles231The constrictor muscles are virtually not distinguishable on CT.
      Inner ear122The fluid content of both the cochlea and semicircular canals is discernible on T2w MRI images. It is surrounded by a narrow, hypointense zone, corresponding to the compact substance of the bony labyrinth.
      Eye (eyeball)232Greater contrast on T2 between the tunics of the eyeball (i.e. the outer border of the organ; hypointense), fluid (hyperintense) and tissues of the orbit.
      Lens333Well visible on T1 and T2 as well.
      Optic nerve332
      Optic chiasm331
      Lacrimal gland121Similar MRI morphology to the parotid and submandibular glands.
      Brainstem331The demarcation of the organ from the liquor is clearly visible on T2w MR images. On these scans, the nigral substance (SN) appears as a longitudinal stripe of higher signal intensity, compared to the neighboring red nucleus and pes pedunculi [41].
      Spinal cord231We have included only spinal cord proper in the contour, not the entire spinal canal. This practice (i.e. contouring the spinal canal) was performed mainly due to the poor image quality and low contrast on native topometric CT scans [42].
      Brain332
      Pituitary gland222Thin MRI slices (slice thickness: 1 mm) with CT correlation is recommended for delineation purposes [43].
      Thyroid gland222Similar MRI morphology to the parotid and submandibular glands.
      Brachial plexus221
      Esophagus222Hyperintense to muscle on T2w images [43].
      Sum435035

      Esophagus

      Starting at the level of the sixth cervical vertebra, the thumb-thick foodpipe interconnects the pharynx with the cardia of the stomach. It rests on the vertebral bodies, just behind the larynx and the trachea.

      Contouring suggestions

      • •
        Between the ventral trachea and the dorsal esophagus runs the shallow trache-oesophageal groove, which contains the recurrent laryngeal nerve. The sparing of this nerve may be desirable in order to prevent late-onset radiation induced neuropathy [37; 38].

      Discussion

      The role of novel anatomical structures and subsites as potential organs at risk emerges, just as we have seen in the case of the masticatory muscles [45] or the freshly described tubarial salivary glands [46]. With the growing number of clinical trials requiring MRI based radiation therapy planning, the need for well-built, straightforward contouring guidelines [47; 48]. is on the rise. Automatized OAR segmentation (not only in the head and neck region) combined with MRI based imaging may lead to increased accuracy in terms of organ boundaries and decreased interobserver variability [49; 50]. Generally speaking, the rapid evolution of artificial intelligence (AI) based contouring software may take a huge burden off the shoulders of radiographists and radiation oncologists and may result in the expansion of this proposed organ set. OAR delineation should always be governed by clinical rationality that takes into account the stage of the disease, tumor volume and involvement/infiltration of different anatomical structures, functional units, as well as the curative or palliative intent of the radiotherapy itself.

      Conclusions

      Within the framework of cooperation between several European clinical centers, a consensus guideline was established on OAR delineation in the head and neck region, using exclusively MR images. Such uniform guidelines may increase treatment accuracy and facilitate the comparison of results between different centers, collaborations and multi-institutional clinical trials.

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      Appendix. Supplementary materials

      Article Info

      Publication History

      Accepted: July 24, 2022
      Received: April 20, 2022

      Publication stage

      In Press Journal Pre-Proof

      Footnotes

      Author Responsible for Statistical Analysis: N/A

      Conflict of Interest: None

      Declaration of interests

      The authors declare the following financial interests/personal relationships which may be considered as potential competing interests

      Funding: This research is part of the Deep MR-only Radiation Therapy activity (project numbers: 19037, 20648, 210995) that has received funding from EIT Health. EIT Health is supported by the European Institute of Innovation and Technology (EIT), a body of the European Union receives support from the European Union´s Horizon 2020 Research and innovation program.

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      DOI: https://doi.org/10.1016/j.adro.2022.101042

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