Parotid gland

Parotid gland – Anatomy & tumours

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

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 of V3 and greater auricular nerve (C2,3)•Sympathetic supply from inferior (IX to otic ganglion, V3, auriculotemporal nerve) and superior salivatory nuclei (CN VII)•Parasympathetic supply from T1-2 via sympathetic chain and superior cervical sympathetic ganglion and arterial plexus•Parotid and preauricular lymph nodes receive drainage from upper half of face and scalp

–Then onto jugulodigastric nodes of deep cervical chain

Parotid (Stensen’s) duct

•Arises at anterior border of gland as confluence of several large ducts•Runs across lateral surface of masseter, closely accompanied by buccal branch of facial nerve•At anterior border of masseter, the duct pierces buccinator and enters the oral cavity opposite the second molar

Histology•Salivary glands are composed of serous and mucous acini, the proportions of which determine the type of salivary secretion 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, high KHCO3

–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

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)

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

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 minor salivary glands are more likely to be malignant (20% parotid, 50% submandibular, 70% minor)•Histological features and stage rather than site of origin are major determinants of outcome

Relative frequency of salivary gland tumours% All glands Paroti

dSubmandibula

rMinor

Pleomorphic adenoma 43 40-70 43-60 40-53

Monomorphic adenoma 12 3-20 - 3

Mucoepidermoid carcinoma 12 12-21 4-11 16-45

Adenoid cystic carcinoma 6 2-8 15-19 20-24

Adenocarcinoma 3 4-1 7 18

Squamous cell carcinoma 2 3-5 7 -

Acinous cell carcinoma 2 2-3 1 6

Undifferentiated carcinoma 1 1 - 1

Carcinoma ex pleomorphic adenoma

3 3 3-11 3

Pleomorphic adenoma

•Most common primary salivary tumour•Benign epithelial cells surrounded by myoepithelial cells interspersed with areas of myxoid or chondroid stroma•Proportions of cellular and myxoid stroma vary considerably and do not predict malignancy•Cell of origin is the reserve cell of intercalated ducts, which may differentiate into epithelial or myoepithelial cells•Usually present as a solitary painless mass•Most grow in the parotid gland and 70% parotid tumours are pleomorphic 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, which increases risk of local recurrence•Risk of malignant change is <10%, but increased in recurrences

Monomorphic adenoma

•Benign neoplastic growths composed entirely of a single epithelial cell type

–Epithelium forms a regular (usually glandular) pattern without mesenchymal tissue characteristics of pleomorphic adenoma

•Most common subtypes are papillary cystadenoma lymphomatosum (Warthin’s tumour) 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 pleomorphic adenomas

Warthin’s tumour (Papillary cystadenoma lymphomatosum)

•Most common monomorphic adenoma•Neoplastic duct epithelium and prominent lymphoid element•Accounts for 6-10% parotid tumours

–May also occur in heterotopic salivary tissue•Occurs when lymphoid encapsulated epithelial tissue 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 tail of parotid at angle of mandible

Mucoepidermoid carcinoma

•Most common primary parotid gland malignancy, and second to adenoid cystic carcinoma in the other salivary glands•Most commonly occurs in 5th decade of life, but also most common salivary gland tumour in children•F:M = 2.4:1•Arise from basal cells of the excretory duct system•Composed of mixed epidermoid and mucous secretory malignant cells

Adenoid cystic carcinoma

•Most common malignant tumour of the submandibular, sublingual and minor salivary glands•Only 15% parotid gland cancers•Cell of origin is intercalated duct reserve cell•Also called cylindromas because of cribriform pattern, but histological appearance varies considerably•Tend to grow slowly and invade locally•Characterised by invasion beyond the palpable and visible mass•Perineural invasion is common•More commonly than any other tumour will present with pain and facial nerve paralysis

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

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