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Parotid gland – Anatomy & tumours

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Parotid gland• Paired unilobular glands divided non anatomically by the facial nerve into deep and superficial lobes• Accessory parotid tissue may extend along parotid duct into buccal space• Pyramidal in shape and lies in pre auricular area, inferomedial to external auditory meatus – Posterior: mastoid and tympanic processes of temporal bone, external auditory canal and styloid process – Lateral: Parotid fascia, SMAS – Medial: Masseter – Superior: Zygomatic arch – Inferior: Sternocleidomastoid, posterior belly of digastric• Apex may extend low into neck along sternocleidomastoid, and postero-medial gland extends into retromandibular area through stylomastoid tunnel into parapharyngeal space

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Supply of parotid gland•Blood supply is terminal branches of ECA,especially STA and internal maxillary artery•Venous drainage is to posterior facial vein•Sensation is via auriculotemporal branch ofV3 and greater auricular nerve (C2,3)•Sympathetic supply from inferior (IX to oticganglion, V3, auriculotemporal nerve) andsuperior salivatory nuclei (CN VII)•Parasympathetic supply from T1-2 viasympathetic chain and superior cervicalsympathetic ganglion and arterial plexus•Parotid and preauricular lymph nodesreceive drainage from upper half of face andscalp –Then onto jugulodigastric nodes of deep cervical chain

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Parotid (Stensen’s) duct•Arises at anterior border ofgland as confluence of severallarge ducts•Runs across lateral surface ofmasseter, closely accompaniedby buccal branch of facial nerve•At anterior border of masseter,the duct pierces buccinator andenters the oral cavity oppositethe second molar

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Histology•Salivary glands are composed of serous and mucous acini,the proportions of which determine the type of salivarysecretion from each duct –Parotid is mainly serous –Sublingual is mainly mucous –Submandibular is mixed•Minor salivary glands tend to be under local control,whereas major glands are parasympathetically controlled•Saliva is hypotonic, with low concentrations of NaCl, highKHCO3 –Antibodies (IgA), amylase, lysozyme. lactoperoxidase –Submandibular saliva has relatively high Ca•Main function is lubrication and cleansing oral cavity –Initiation of starch digestion –Immunological –HCO3 retards growth of acidophilic bacteria –Maintenance of dentition (Ca, Po4, Mg) –Normal outflow is 1-2L/day

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Anatomy of facial nerve – Extratemporal• Exits stylomastoid foramen 1cm superior to mastoid process and 1cm deep to lateral surface – Indicated by tragal pointer (junction of cartilaginous paortion of EAM with skull), which is 5-6mm from stylomastoid foramen• Other method of finding facial nerve trunk is to follow the tympanomastoid fissure (junction of posterior bony auditory canal and mastoid portion of temporal bone) – Nerve lies 6-8mm below inferior end of this line• 3 branches given off just below stylomastoid foramen – Posterior auricular nerve (to postauricular and occipital muscles) – Nerve to stylohyoid – Nerve to posterior belly of digastric• Nerve runs lateral to styloid process and enters parotid gland between stylohyoid and digastric, lateral to external carotid artery and posterior facial vein• Lateral to EJV, CNVII branches into zygomaticotemporal and cervicofacial trunks (pes anserinus) – Usually within 2cm from exit of stylomastoid foramen and within 1cm of entering the parotid gland• Runs between deep and superficial lobes of parotid gland• Innervates muscles of facial expression from their deep surface; except mentalis, buccinator and levator anguli oris which all lie deep to CN VII• Nerve to muscle fibre ratio is 1:8 (normal muscle is 1:50)

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Branches of facial nerve• Many variations – Temporal and marginal mandibular are most consistent• Pitanguy’s line – 6mm below EAM to 1-2cm above lateral brow is the extent of anterior branches – Usually 2-3 frontal branches in an area 1- 3cm lateral to lateral canthus along zygomatic arch – Anterior to superficial temporal vessels and deep to SMAS layer• Buccal branch – Lies along line joining tragus to midline upper lip – Many connections between buccal and zygomatic branches in buccal fat pad• Marginal mandibular branch – Always lies superficial to facial vessels – Runs within 2.5cm inferior to mandible. Anterior division of cervical branch usually runs parallel and 1cm inferior

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Tumours•WHO classification identifies 46 types of salivary gland tumours•2 theories of development –Dedifferentiation – mature elements respond to oncogenic stimulus –Bicellular (Eversole) – neoplasms arise from stem or reserve cells•Majority of tumours arise in parotid gland, but tumours of minorsalivary glands are more likely to be malignant (20% parotid, 50%submandibular, 70% minor)•Histological features and stage rather than site of origin are majordeterminants of outcome

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Relative frequency of salivary gland tumours% All glands Parotid Submandibular MinorPleomorphic adenoma 43 40-70 43-60 40-53Monomorphic adenoma 12 3-20 - 3Mucoepidermoid carcinoma 12 12-21 4-11 16-45Adenoid cystic carcinoma 6 2-8 15-19 20-24Adenocarcinoma 3 4-1 7 18Squamous cell carcinoma 2 3-5 7 -Acinous cell carcinoma 2 2-3 1 6Undifferentiated carcinoma 1 1 - 1Carcinoma ex pleomorphic 3 3 3-11 3adenoma

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Pleomorphic adenoma•Most common primary salivary tumour•Benign epithelial cells surrounded by myoepithelial cellsinterspersed with areas of myxoid or chondroid stroma•Proportions of cellular and myxoid stroma vary considerablyand do not predict malignancy•Cell of origin is the reserve cell of intercalated ducts, which maydifferentiate into epithelial or myoepithelial cells•Usually present as a solitary painless mass•Most grow in the parotid gland and 70% parotid tumours arepleomorphic adenoma•Most grow in tail, but 10% involve the deep lobe•Slight F>M, tumour usually presents in 5th decade•Slow growing and encapsulated early•Can extend as pseudo pods beyond the tumour mass, whichincreases risk of local recurrence•Risk of malignant change is <10%, but increased in recurrences

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Monomorphic adenoma•Benign neoplastic growths composed entirely ofa single epithelial cell type –Epithelium forms a regular (usually glandular) pattern without mesenchymal tissue characteristics of pleomorphic adenoma•Most common subtypes are papillarycystadenoma lymphomatosum (Warthin’stumour) and oxyphillic adenoma (Oncocytoma)•Arise from intercalated duct cell•Account for 4-8% salivary gland neoplasms•Propensity towards multicentricity•Have the capacity to transform into pleomorphicadenomas

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Warthin’s tumour (Papillary cystadenoma lymphomatosum)•Most common monomorphic adenoma•Neoplastic duct epithelium and prominentlymphoid element•Accounts for 6-10% parotid tumours –May also occur in heterotopic salivary tissue•Occurs when lymphoid encapsulated epithelialtissue is subjected to an oncogenic stimulus•High incidence of multicentricity and bilaterality(10%) leading to high local recurrence•5:1 M:F, > 90% are smokers•Usual presentation is asymptomatic mass in tailof parotid at angle of mandible

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Mucoepidermoid carcinoma•Most common primary parotidgland malignancy, and second toadenoid cystic carcinoma in theother salivary glands•Most commonly occurs in 5th decadeof life, but also most commonsalivary gland tumour in children•F:M = 2.4:1•Arise from basal cells of theexcretory duct system•Composed of mixed epidermoid andmucous secretory malignant cells

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Adenoid cystic carcinoma•Most common malignant tumour of thesubmandibular, sublingual and minor salivaryglands•Only 15% parotid gland cancers•Cell of origin is intercalated duct reserve cell•Also called cylindromas because of cribriformpattern, but histological appearance variesconsiderably•Tend to grow slowly and invade locally•Characterised by invasion beyond thepalpable and visible mass•Perineural invasion is common•More commonly than any other tumour willpresent with pain and facial nerve paralysis

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Metastatic carcinoma• May occur by lymphatic or haematogenous spread, contiguous extension – Contiguous spread is most commonly seen with locally invasive or advanced overlying tumours of the skin – Lymphatic spread can occur from anywhere in upper face/scalp, and occasionally deeper structures to deep lobe – SCC and melanoma are the most frequent tumours to metastasize to the parotid – Haematogenous metastatic carcinoma to the parotid most commonly arises from primary tumours in the lung, breast, kidney or GIT

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Other tumours• SCC – True SCC parotid is only diagnosed after exclusion of metastatic SCC – Arises from basal cell of excretory duct – Strongly associated with tobacco use – High grade malignancy with perineural invasion, regional and distant metastatic disease and local recurrence• Lymphoma – Primary malignant lymphoma of salivary glands is rare, constituting 4-5% extranodal lymphoma – Typically patients in 6th -7th decades – Usually NHL, then diffuse large cell lymphoma