Pleomorphic adenoma of the salivary glands

Pleomorphic adenomas, also known as benign mixed tumors (BMTs), are the most common salivary gland tumors.

On imaging, they commonly present as well circumscribed rounded masses, most commonly located within the parotid gland, hypoechogenic on ultrasound and bright on T2WI with homogeneous enhancement on MRI. 

Pleomorphic adenomas account for 70-80% of benign salivary gland tumors and are especially common in the parotid gland (see below) 1,6. Patients are typically middle aged 1, and prior head and neck irradiation is a risk factor for the development of these tumors.

Patients typically present with a smooth, painless, enlarging mass.

Distribution among the salivary glands is as follows 1:

  • parotid gland: 84% 1
    • commoner in the superficial lobe
  • submandibular gland: 8%
  • minor salivary glands: 6.5%
    • widely distributed including the nasal cavity, pharynx, larynx, trachea 5
  • sublingual glands: 0.5%

They are less common in salivary glands other than the parotid but remain the most common benign tumor of each gland.

Pleomorphic adenomas are also commonly found in the lacrimal glands where they account for approximately 50% of lacrimal gland tumors 7.

As the name suggests, pleomorphic adenomas are composed of a mixture of variable histology. They contain both epithelial and myoepithelial (mesenchymal) tissues, with mixed histology. They appear encapsulated and well circumscribed however the pseudocapsule is delicate and incomplete with microscopic extensions reaching beyond it, accounting for the high risk of recurrence when these tumors are enucleated (see below) 5,6.

The gross appearance depends upon the relative proportion of epithelial elements and a stromal component which may range from myxoid to cartilage. Tumors with a prominent cartilaginous matrix have a bluish-grey opalescent appearance. 

Three histological types have been described:

  1. myxoid (hypocellular): most common, highest rate of recurrence
  2. cellular
  3. classic

On all modalities, these tumors typically appear as rounded masses with well-defined, "bosselated" or "polylobulated" borders (many small undulations, not truly lobulated). They are most commonly located within the parotid gland, particularly the superficial lobe.

When they arise from the deep lobe of the parotid they can appear entirely extraparotid, seen in the prestyloid parapharyngeal space, without a fat plane between it and the parotid, and widen the stylomandibular tunnel. Pleomorphic adenomas can also arise from salivary rest cells in the parapharyngeal space itself without connection to the parotid gland.

They are typically hypoechoic and may show posterior acoustic enhancement.

Ultrasound is also useful in guiding a biopsy (both FNAC and core biopsies) but needs to be carried out with care to avoid facial nerve damage 8,9.

When small, they have homogeneous attenuation and prominent enhancement. When larger, they can be heterogeneous with less prominent enhancement, foci of necrosis, and possible delayed enhancement. Small regions of calcification are common 1,10.

The signal characteristics are homogeneous when the tumor is small. Larger tumors may be heterogeneous.

  • T1: usually of low intensity
  • T2
    • characteristically of very high intensity (especially myxoid type) 6
    • often have a rim of decreased signal intensity on T2-weighted images representing the surrounding fibrous capsule
  • T1 C+ (Gd): usually demonstrates homogeneous enhancement
  • typically hypovascular

FDG-PET may show some uptake; SUV more than 3 in 25% of cases 11.

Surgical excision is curative, however as the tumor is poorly encapsulated (despite imaging suggesting otherwise) there is a significant rate of recurrence in the tumor bed. Exact rates of recurrence vary widely depending on series and surgical technique (1-50%) 1.

Historically these tumors were removed by enucleation, resulting in recurrence rates of 20-45% 6.

To minimise this occurrence, no open surgical biopsy should be performed. Rather, a partial (superficial) or total parotidectomy ensures a wide margin. The facial nerve should be spared 4,6. Using this approach, recurrence rate has reduced dramatically to 1-4% 6.

Percutaneous ultrasound biopsy (both FNAC and core biopsy) can be performed safely and is associated with very low tumor seeding rates and without facial nerve injury provided meticulous technique is used 8,9.

When in the minor salivary glands, a 5 mm margin should be obtained. These tumors do not invade into periosteum, thus bone need not be resected 4.

When tumor bed recurrences occur, they can be extremely difficult to control, with management options including monitoring only, surgery, or radiotherapy.

There is a small risk of malignant transformation into a carcinoma ex-pleomorphic adenoma which is proportional to the time the lesion is in situ (1.5% in first 5 years, 9.5% after 15 years), thus excision is recommended in essentially all cases. Additional risk factors for malignancy include advanced age, large size, radiation therapy and recurrent tumors 2,6. In addition to carcinoma ex-pleomorphic adenoma, true malignant mixed tumors of the salivary glands usually arise from pre-existing pleomorphic adenomas 1,3. Metastasising pleomorphic adenoma is the third type of malignant mixed tumor of salivary glands and is the rarest. It presents with metastases to lung, bone and soft tissues despite having 'benign' histology 1.

When in the parotid gland consider:

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Article information

rID: 1893
Tag: parotid
Synonyms or Alternate Spellings:
  • Benign mixed tumour of parotid
  • Benign mixed tumour (BMT)
  • Benign mixed tumour of the parotid
  • Pleomorphic adenomas
  • Pleomorphic adenoma of salivary glands
  • Pleomorphic adenoma

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Cases and figures

  • Figure 1: frequency of salivary gland tumors
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  • Case 1: parotid
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  • Pleomorphic adeno...
    Figure 2: gross pathology
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  • Case 2: parotid - with tiny calcific foci
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  • Figure 3: FNA cytology: pap stain
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  • Case 3: parotid
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  • Figure 4: FNA cytology: diff quick stain
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  • Case 4: parotid
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  • Case 5: submandibular
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  • Case 6
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  • Case 7: in right stylomandibular tunnel
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  • Case 8: large with little enhancement
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  • Case 9
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  • Case 10: recurrent
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  • Case 11
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  • Coronal reformat
    Case 12
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  • Case 13
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  • Case 14: MRI - T1
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  • Case 15: recurrent
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  • Case 16
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  • Case 17
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  • Case 18
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  • Case 19: parapharyngeal (minor salivary gland)
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  • Case 20
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  • Case 21: myxoid type
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  • Case 22
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