Salivary Gland Diseases: Benign, Malignant, Management, and Role of MRI

ANATOMY OVERVIEW

Parotid Gland

• Anterior: masseter muscle
• Superior: zygomatic arch
• Posterior: tragal cartilage and sternocleidomastoid muscle
• Inferior tail: between the ramus of mandible and SCM, overlying the digastric muscle
• Deep border: pretyloid compartment of the parapharyngeal space

Submandibular Gland

Sublingual Gland

BENIGN SALIVARY GLAND DISEASES

1. Benign Tumors of the Parotid (in detail)

Pleomorphic Adenoma (Benign Mixed Tumor)

• Most common salivary gland tumor overall, accounting for ~60-70% of all parotid tumors
• Typically presents as a slow-growing, painless, firm, mobile mass in the superficial lobe
• Pathology: True mixed tumor - epithelial and mesenchymal (myxoid/chondroid) components in a biphasic pattern; "pleomorphic" refers to morphology, not malignancy
• Risk of malignant transformation (carcinoma ex pleomorphic adenoma): ~5-10% lifetime risk if untreated; risk increases with time
• MRI features: Well-defined, lobulated; T2 hyperintense (due to myxoid/chondroid stroma); T1 hypointense; moderate gadolinium enhancement; high ADC on DWI (distinguishes from malignancy)
• Management: Superficial parotidectomy (with facial nerve preservation) is the standard. Enucleation alone is inadequate due to pseudopod extensions and risk of tumor seeding leading to recurrence. Extracapsular dissection (ECD) is gaining evidence support for small, mobile superficial tumors as a less morbid alternative

Warthin's Tumor (Papillary Cystadenoma Lymphomatosum)

• Second most common benign parotid tumor; almost exclusively occurs in the parotid (never in minor salivary glands)
• Strongly associated with smoking (cigarette smokers have 8x increased risk)
• 10% bilateral, 10% multifocal in the same gland - a distinctive feature
• Predominantly in males, typically in older patients (6th-7th decade)
• Pathology: Oncocytic bilayered epithelium lining papillary projections; abundant germinal-center-containing lymphoid stroma (likely arises from heterotopic salivary ductal epithelium within intraparotid lymph nodes)
• MRI features: T2 intermediate to low signal (unlike pleomorphic adenoma); cystic + solid components; often with bilateral nodularity. Can show high FDG uptake on PET-CT (pitfall for malignancy diagnosis)
• Management: Superficial parotidectomy or extracapsular dissection; formal monitoring acceptable in elderly poor surgical candidates as malignant transformation is extremely rare

Oncocytoma

• Rare; composed of oncocytes (mitochondria-rich oxyphilic cells); occurs predominantly in the parotid in older adults; treated with superficial parotidectomy

Basal Cell Adenoma

• Well-encapsulated; membranous type has higher recurrence risk; managed surgically

Lipoma of the Parotid

• Can mimic a parotid mass; homogeneous fat signal on MRI is diagnostic; surgical excision if symptomatic

2. Non-Neoplastic Benign Parotid Conditions

Sialadenitis (Acute Suppurative Parotitis)

• Most common organism: Staphylococcus aureus (post-operative dehydration is the classic trigger); also gram-negatives and anaerobes
• Predisposing factors: dehydration, elderly, post-operative, xerostomia, immunosuppression
• Presentation: Painful, tender parotid swelling; pus expressible from Stensen's duct orifice; fever
• Management: IV antibiotics (anti-staphylococcal coverage), hydration, sialagogues, oral hygiene; abscess requires surgical drainage

Viral Parotitis (Mumps)

• Caused by paramyxovirus; bilateral painful swelling; affects parotid predominantly
• Complications: orchitis, oophoritis, pancreatitis, meningoencephalitis
• Diagnosis clinical; serology confirmatory; management supportive

Chronic Recurrent Parotitis (Juvenile Recurrent Parotitis - JRP)

• Most common salivary gland disease in children after mumps
• Recurrent, unilateral or bilateral parotid swelling; sialography shows "sialectasia" (punctate collections); ductal ectasia on ultrasound
• Many resolve at puberty; acute episodes treated with amoxicillin; sialendoscopy (irrigation with saline/corticosteroids) is effective for recalcitrant cases

Sialolithiasis (Salivary Duct Stones)

• Most common in submandibular gland (85%) due to long uphill course of Wharton's duct, slower saliva flow, and alkaline mucous secretions that favor calcium phosphate precipitation; parotid stones are less common (10-15%)
• Presentation: postprandial swelling and pain ("mealtime syndrome"); may cause acute sialadenitis
• Investigations: Plain X-ray (most submandibular stones are radiopaque; parotid stones often radiolucent); ultrasound; CT without contrast; sialendoscopy (diagnostic + therapeutic)
• Management:
  • Distal duct stones: manual massage, increased fluid intake, sialagogues; transoral extraction under local anesthesia
  • Intraglandular stones: sialendoscopy ± lithotripsy; submandibular gland excision if stone cannot be retrieved and sialadenitis is recurrent
  • MR sialography: non-invasive assessment of ductal anatomy up to 2nd/3rd order branches; no radiation; can be performed even during acute inflammation - increasingly replacing conventional sialography

Sjögren's Syndrome

• Autoimmune exocrinopathy causing xerostomia and keratoconjunctivitis sicca; parotid enlargement is common
• Associated with anti-Ro (SS-A) and anti-La (SS-B) antibodies
• Parotid involvement: bilateral diffuse enlargement; MRI/US shows diffuse heterogeneous texture with multiple small cystic areas; salivary scintigraphy shows reduced uptake and excretion
• Salivary gland biopsy (labial minor salivary gland): shows focal lymphocytic sialadenitis (>1 focus/4mm²)
• Management: pilocarpine for xerostomia; artificial saliva; treat underlying autoimmune disease

Sarcoidosis

• Bilateral parotid enlargement; Heerfordt's syndrome = parotid enlargement + uveitis + facial nerve palsy + fever (uveoparotid fever)
• Elevated serum ACE; biopsy shows non-caseating granulomas; treat with corticosteroids

Benign Lymphoepithelial Cysts (BLEC) - HIV-Associated

• Virtually pathognomonic for HIV; bilateral cystic parotid masses with cervical lymphadenopathy; occur in 3-6% of adults with HIV
• FNAC diagnostic; CT/MRI shows bilateral cystic lesions ± lymphadenopathy
• Management: observation for asymptomatic lesions; aspiration/sclerotherapy; antiretroviral therapy; surgery if cosmetically bothersome

Ranula

• Sublingual gland extravasation pseudocyst; "plunging ranula" penetrates through mylohyoid into neck
• Management: marsupialization or complete excision including sublingual gland to prevent recurrence

MALIGNANT SALIVARY GLAND DISEASES

Epidemiology - The Inverse Rule

Malignant Tumors of the Parotid (in detail)

1. Mucoepidermoid Carcinoma (MEC)

• Most common malignant salivary tumor overall; most common malignancy of the parotid
• Histology: Mixed squamous, mucous-secreting, and intermediate cells; 15% of all salivary gland tumors
• Grading (Low/Intermediate/High):
  • Low-grade: predominantly cystic, mucous cells dominant; good prognosis; 5-year survival ~90%
  • High-grade: predominantly solid, squamous cells; aggressive; 5-year survival ~50%
• Treatment: Surgical excision (parotidectomy with facial nerve preservation); post-operative radiotherapy for high-grade, positive margins, perineural invasion, N+ disease

2. Adenoid Cystic Carcinoma (AdCC)

• Most common malignancy of submandibular and minor salivary glands
• Classic for perineural invasion (spreading along nerve sheaths, causing pain and facial palsy; may extend to skull base intracranially)
• Three growth patterns: cribriform (classic "Swiss cheese" pattern - best prognosis), tubular, and solid (worst prognosis)
• Behavior: Slow but relentless growth; late distant metastases (lung most common); high local recurrence
• MRI: Essential for perineural extension assessment - shows thickening and enhancement of facial nerve branches; "skip lesions" along nerves
• Treatment: Wide surgical excision (often requires facial nerve sacrifice if involved) + post-operative radiotherapy; neutron beam radiotherapy may be used for unresectable disease

3. Acinic Cell Carcinoma (AcCC)

• Predominantly parotid (90%); bilateral in ~3% of cases
• Low-grade malignancy; good prognosis; late recurrence (10-15 years later possible)
• Composed of serous acinar cells; basophilic granular cytoplasm on histology
• Treatment: Parotidectomy; radiotherapy for advanced cases

4. Carcinoma ex Pleomorphic Adenoma

• Malignant transformation within a pre-existing pleomorphic adenoma; risk ~1.5% at 5 years, ~10% at 15 years
• Suggested by rapid enlargement of a previously stable mass, pain, facial nerve involvement
• Treatment: Wide resection ± neck dissection + radiotherapy; prognosis depends on extent of invasion (intracapsular vs. widely invasive)

5. Salivary Duct Carcinoma (SDC)

• High-grade, aggressive; resembles ductal carcinoma of the breast histologically
• Predominantly in parotid; older males; poor prognosis
• Often overexpresses HER2/neu and androgen receptors (potential for targeted therapy)
• Treatment: Total parotidectomy ± neck dissection + adjuvant radiotherapy

6. Epithelial-Myoepithelial Carcinoma

• Low-grade; biphasic (inner ductal + outer myoepithelial cells)
• Predominantly parotid; local recurrence common; surgical resection

7. Polymorphous Adenocarcinoma

• Predominantly minor salivary glands (palate); low-grade

8. Lymphoma of the Parotid

• Non-Hodgkin lymphoma (NHL) is the most common parotid lymphoma; arises from intraparotid lymph nodes
• Associated with Sjögren's syndrome (50-fold increased risk of NHL)
• MALT lymphoma is the most common type

9. Metastases to the Parotid

• Most common primary: squamous cell carcinoma and melanoma of the face, scalp, and ear skin
• Via intraparotid lymph nodes

Clinical Features Suggesting Malignancy in a Parotid Mass

1. Rapid increase in size
2. Pain (present in 44% of parotid cancers)
3. Facial nerve paresis/paralysis (present in 25% - independent of tumor size; portends poor prognosis)
4. Fixation to skin or deep structures
5. Cervical lymphadenopathy
6. Skin invasion/ulceration

The incidence of parotid carcinoma has been estimated at up to 6% in patients presenting with lower motor neuron facial paralysis where no cause is found after initial MRI. This is more likely if palsy is slow-onset, progressive, shows no evidence of recovery, and affects only isolated VIIIN branches.

Malignant Tumors - Submandibular Gland (Brief)

• 50% are malignant (some series report 37-50%)
• Most commonly AdCC, then MEC
• Present as a slow-growing painless mass under the jaw; floor of mouth distortion; rarely skin invasion
• Minimally required surgery: Submandibular gland excision within a selective neck dissection (levels I-II-III)
• Post-operative radiotherapy indicated for: stage III/IV, positive margins, perineural invasion, N+ disease

Malignant Tumors - Sublingual Gland (Brief)

• Rarest site but 86% of sublingual tumors are malignant
• AdCC is most common (72%), followed by MEC
• Present at advanced stage; often floor of mouth involvement
• Surgery: Tumors <2 cm may be resected intraorally; >2 cm require en-bloc resection ("pull-through" or mandibulotomy approach); elective neck dissection (levels I-IIa) for N0 neck
• Post-operative radiotherapy for advanced/high-risk disease

TNM STAGING (AJCC 8th Edition - Major Salivary Glands)

MANAGEMENT: SURGERY IN DETAIL

Parotid Surgery

1. Tympano-mastoid suture line
2. Posterior belly of the digastric muscle (marks the depth of the nerve)
3. Tragal pointer (nerve lies 1 cm deep and anterior/inferior to the tip)
4. Retrograde dissection from a distal branch

Complications of Parotidectomy

1. Temporary facial weakness (neuropraxia): resolves within 4-6 weeks; most common complication
2. Permanent facial palsy: ~1-2% risk; more degeneration injuries take 6-12+ months
3. Frey's syndrome (gustatory sweating): auriculotemporal nerve aberrant reinnervation of sweat glands; subclinical in nearly all patients (positive Minor's starch-iodine test); clinically bothersome in a minority; treated with antiperspirant, botulinum toxin injection (most effective), or interposition of SMAS flap during surgery
4. Sensory loss: Greater auricular nerve distribution (angle of mandible, lower pinna); often permanent; preservation of posterior branch reduces deficit
5. Cosmetic defect: Loss of bulk behind mandibular ramus; mitigated with fat transfer

Neck Dissection

• cN+ neck: Comprehensive (modified radical) neck dissection, levels I-V
• cN0 neck: Elective neck dissection or radiotherapy when risk of subclinical nodal disease >15-20%; or when the neck is surgically entered for another reason

Adjuvant Radiotherapy

ROLE OF MRI IN PAROTID GLAND DISEASE

"Magnetic resonance imaging (MRI) is superior to computerized tomography (CT) in evaluating parotid tumours and allows better demonstration of the retromandibular parotid, the stylomastoid foramen area, VIIIN invasion and possible perineural extension."

Indications for Imaging (Parotid Mass)

• Tumor mobility is impaired
• Tumor diameter >4 cm
• Facial nerve is clinically involved
• Palpable cervical nodes present

MRI Sequences and Their Roles

Specific MRI Characteristics of Key Tumors

• Well-defined, lobulated capsule
• Characteristically T2 hyperintense (bright) due to myxoid/chondroid stroma
• T1 hypointense
• Moderate, delayed gadolinium enhancement
• High ADC on DWI (distinguishes from malignancy)

• Often bilateral, multifocal (bilateral nodularity on MRI helps distinguish from HIV-BLEC)
• T2 intermediate to low signal (unlike PA)
• Cystic-solid components; may be hyperintense in areas due to protein-rich fluid
• High FDG on PET-CT - false positive for malignancy (important pitfall)

• Irregular, infiltrative margins
• Low-intermediate T2 signal
• Low ADC values on DWI (restricted diffusion due to high cellularity)
• Loss of tissue planes; skin/deep structure invasion
• Perineural spread: thickening, enhancement of cranial nerve VII branches toward the stylomastoid foramen; "skip lesions" along nerve; enlargement of skull base foramina in AdCC

• MRI best delineates extension into the parapharyngeal space (medial displacement of parapharyngeal fat)
• CT and MRI help distinguish parapharyngeal space tumors from deep lobe parotid tumors (the fat plane between them is key)

Limitations of MRI/Imaging

• PET-CT: High false-positive rate in differentiating benign from malignant salivary disease - both Warthin's tumors and pleomorphic adenomas demonstrate increased FDG uptake
• Conventional MRI alone cannot always distinguish low-grade malignancy from benign tumors - complemented by FNAC
• DWI has limitations in Warthin's tumors (low ADC mimics malignancy)

FNAC and MRI: Complementary Roles

INVESTIGATIONS SUMMARY