Pleomorphic adenoma

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Overview
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Pleomorphic adenoma from other Diseases
Epidemiology and Demographics
Risk Factors
Natural History, Complications and Prognosis
Diagnosis
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History and Symptoms
Physical Examination
Laboratory Findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Maneesha Nandimandalam, M.B.B.S.[2] Maria Fernanda Villarreal, M.D. [3]

Synonyms and keywords: Pleomorphic adenoma of the salivary glands; BMT; Benign mixed tumors of the salivary gland

Overview

Pleomorphic adenoma (also known as "Benign mixed tumor of the salivary gland") is a benign neoplastic tumor of the salivary glands. Pleomorphic adenoma is the most common type of salivary gland tumor and the most common tumor of the parotid gland. Pleomorphic adenoma was first described by Beahrs in 1957. Pleomorphic adenoma may be classified according to anatomical distribution into 4 groups: parotid gland pleomorphic adenoma, submandibular gland pleomorphic adenoma, minor salivary gland pleomorphic adenoma, and sublingual gland pleomorphic adenoma. The pathogenesis of pleomorphic adenoma is characterized by an admixture of polygonal epithelial and spindle-shaped myoepithelial elements. A mutation on the PLAG1 gene is associated with the development of pleomorphic adenoma. Pleomorphic adenoma is common and it accounts for 60% of all parotid gland tumors (represents 45-75% of all salivary gland neoplasms). The incidence of pleomorphic adenoma is approximately 2-3.5 per 100,000 individuals worldwide. The most characteristic clinical feature of pleomorphic adenoma is a painless single-nodular parotid mass. Pleomorphic adenoma may be initially asymptomatic. Common symptoms of pleomorphic adenoma may include xerostomia, dysphagia, and painful chewing. MRI is the imaging modality of choice for pleomorphic adenoma. Surgical excision in conjunction with pre-surgical biopsy is the most common approach to the management of pleomorphic adenoma. The recurrence rate after surgery will depend on the histological subtype (mixed pleomorphic adenomas have a higher recurrence rate). In some cases, patients with pleomorphic adenoma may receive medications to stimulate saliva production such as pilocarpin and cevimeline for symptomatic relief.

Historical Perspective

Pleomorphic adenoma was first described by Beahrs in 1957.

Classification

Pleomorphic adenoma may be classified according to anatomical distribution into 4 groups:

• Parotid gland pleomorphic adenoma (80%)

• Superficial lobe pleomorphic adenoma (most common)

• Submandibular gland pleomorphic adenoma
• Minor salivary gland pleomorphic adenoma
• Sublingual gland pleomorphic adenoma

Pleomorphic adenoma may also be classified according to histological type into 3 subtypes:

• Mixed (hypocellular)
• Cellular
• Classic

Lacrimal gland pleomorphic adenoma is a variant of pleomorphic adenoma which accounts for approximately 50% of lacrimal gland tumors.

Pathophysiology

Pathogenesis

The pathogenesis of pleomorphic adenoma is characterized by an admixture of polygonal epithelial and spindle-shaped myoepithelial elements.

Genetics

The mutation on PLAG1 gene has been associated with the development of pleomorphic adenoma.

Gross Pathology

On gross pathology, characteristic findings of pleomorphic adenoma include:

• May have cartilaginous appearance
• Typically well-circumscribed
• Tumor is usually enveloped by a fibrous capsule

Microscopic Pathology

On microscopic histopathological analysis, characteristic findings of pleomorphic adenoma include:

• Mixture of epithelial and myoepithelial elements
• Background stroma that may be mucoid, mixed, cartilaginous, or hyaline
• Epithelial elements may be arranged in duct-like structures, sheets, clumps and/or interlacing strands
• Epithelial elements consist of polygonal, spindle or stellate-shaped cells (hence pleomorphism)

On immunohistochemical analysis, characteristic findings of pleomorphic adenoma include:

• Positive S-100 protein
• Positive SMA
• Positive GFAP

Causes

There are no established causes for pleomorphic adenoma.

Differentiating Pleomorphic Adenoma from Other Diseases

Pleomorphic adenoma must be differentiated from other diseases that cause xerostomia, single-nodular parotid mass, and dysphagia such as:

• Warthin's tumor
• Mucoepidermoid carcinoma
• Myoepithelioma
• Adenoid cystic carcinoma

Epidemiology and Demographics

Epidemiology

• Pleomorphic adenoma accounts for 60% of all parotid gland tumors and represents 45-75% of all salivary gland neoplasms.
• The incidence of pleomorphic adenoma is approximately 2-3.5 per 100,000 individuals worldwide.

Age

• Pleomorphic adenoma is more commonly observed among patients aged 60 to 70 years old.
• Pleomorphic adenoma is less commonly observed in children.

Gender

• Pleomorphic adenoma affects men and women equally.

Race

• There is no racial predilection for pleomorphic adenoma.

Risk Factors

Common risk factors in the development of pleomorphic adenoma include:

• Ionizing radiation
• Occupations associated with an increased risk such as:
  • Rubber products manufacturing
  • Asbestos mining
  • Plumbing
  • Some types of woodworking

Natural History, Complications and Prognosis

Natural history

The majority of patients with pleomorphic adenoma remain asymptomatic for years. Early clinical features include dry mouth, difficulty swallowing, and a palpable single nodular mass. If left untreated, patients with pleomorphic adenoma may progress to develop malignant transformation.

Complications

Common complications of pleomorphic adenoma include:

• Nerve injury
• Cosmetic deformity
• Hemorrhage or hematoma
• Frey's syndrome

Prognosis

If malignant transformation occurs, prognosis is generally poor. The 5-year survival rate of patients with malignant pleomorphic adenoma is approximately 30%.

Diagnosis

Symptoms

Pleomorphic adenoma may be initially asymptomatic. Symptoms of pleomorphic adenoma may include the following:

• Dry mouth
• Dysphagia
• Painful chewing

Physical Examination

Patients with pleomorphic adenoma may present with a facial deformity. Physical examination of the head and neck may be remarkable for:

• Submandibular single, mobile, painless, and firm nodular mass
• Eversion of the ear lobe
• Limited mandiblular motion

Laboratory Findings

There are no specific laboratory findings associated with pleomorphic adenoma.

Imaging Findings

MRI Findings

MRI is the imaging modality of choice for pleomorphic adenoma. On MRI, characteristic findings of pleomorphic adenoma include:

• Pleomorphic adenomas are usually well-circumscribed
• Homogeneous masses with low intensity on T1-weighted images and high intensity on T2-weighted images
• Commonly, a rim of decreased signal intensity on T2-weighted images is observed
• After gadolinium enhancement, the tumors are homogeneously enhancing unless they are large.

CT Findings

• On CT scan, characteristic findings of pleomorphic adenoma include:
• Smooth and well-marginated tumors
• The attenuation values of the mass are usually homogeneous and higher than that of the surrounding gland
• Tumor enhancement is variable and can result in a missed diagnosis if delayed images are not acquired
  • Pleomorphic adenomas are poorly enhancing in the early phase of contrast enhancement, though the amount of enhancement increases over time

Ultrasound

On Ultrasound, characteristic findings of pleomorphic adenomas include:

• Pleomorphic adenomas are hypoechoic, well-defined, lobulated tumors with posterior acoustic enhancement and may contain calcifications.
  • Ultrasound is frequently used to guide FNA or core needle biopsy

Examples of Imaging Findings

• 
  MRI: A right parotid pleomorphic adenoma Images courtesy of RadsWiki
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  MRI: A right parotid pleomorphic adenoma Images courtesy of RadsWiki
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  MRI: A right parotid pleomorphic adenoma Images courtesy of RadsWiki
• 
  MRI: A right parotid pleomorphic adenoma Images courtesy of RadsWiki
• 
  MRI: A right parotid pleomorphic adenoma Images courtesy of RadsWiki
• 
  PET: A right parotid pleomorphic adenoma Images courtesy of RadsWiki

Other Diagnostic Studies

• Pleomorphic adenoma may also be diagnosed using fine needle aspiration (FNA) and core needle biopsy.
• FNA can determine whether the tumor is malignant in nature with a sensitivity approximately 90%
• FNA can also distinguish primary salivary tumor from metastatic disease
• Core needle biopsy is more accurate compared to FNA with diagnostic accuracy greater than 97%
• Findings on fine needle aspiration (FNA) and core needle biopsy, include:

• Admixture of polygonal epithelial and spindle-shaped myoepithelial elements
• Mesenchymal stroma (important feature)
• Proliferation of myoepithelium and epithelium

Treatment

Medical Therapy

• There is no treatment for pleomorphic adenoma; the mainstay of therapy is surgery.
• In some cases, patients with pleomorphic adenoma can receive medications to stimulate saliva production, such as:

• Pilocarpine
• Cevimeline

Surgery

• Surgery is the mainstay of therapy for pleomorphic adenoma.
• Surgical excision in conjunction with pre-surgical biopsy is the most common approach to the treatment of pleomorphic adenoma.
• The recurrence rate after surgery will depend on the histological subtype.
• Mixed pleomorphic adenomas have a higher recurrence rate.

Prevention

Primary Prevention

There are no primary preventive measures available for pleomorphic adenoma.

Secondary Prevention

Once diagnosed and successfully treated, patients with pleomorphic adenoma are followed-up every 6 or 12 months.

References

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