Pleomorphic adenoma
Pleomorphic adenomas (also known as benign mixed tumour (BMT)) are the most common salivary gland tumours.
Epidemiology
Pleomorphic adenomas account for 70-80% of benign salivary gland tumours and are especially common in the parotid gland (see below) 1,6. Patients are typically in their middle age 1, and prior head and neck irradiation is a risk factor for the development of these tumours.
Clinical presentation
Patients typically present with a smooth painless enlarging mass.
Distribution
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%
Although they are less commonly seen in salivary glands other than the parotid, even then they remain the most common benign tumour in those locations.
Pleomorphic adenomas are also commonly found in the lacrimal glands where they account for approximately 50% of lacrimal gland tumours 7.
Pathology
As the name suggests pleomorphic adenomas are composed of a mixture of variable histology. They contain both epithelial and myoepithelial (mesenchymal) tissues, with varied 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 tumours are enucleated (see below) 5-6.
Three histological types have been described :
- myxoid (hypocellular) : most common, has highest rate of recurrence
- cellular
- classic
Radiographic features
Although findings do depend on tumour size, in general they are well circumscribed rounded masses, most commonly located within the parotid gland.
When they arise from the deep lobe of the parotid they can appear entirely extraparotid, seen in the 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.
CT
Smoothly marginated or lobulated homogeneous small spherical mass is the most common appearance. When larger they can be heterogeneous with foci of necrosis. Small regions of calcification are common 1.
When small enhancement tends to be prominent. In larger tumours enhancement is less marked, but can demonstrate delayed enhancement.
MRI
They are commonly seen as well-circumscribed and homogeneous when small. Larger tumours may be heterogeneous.
- T1 - usually of low intensity
-
T2
- usually 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
Ultrasound
Typically hypoechoic. May show a lobulated distinct border +/- posterior acoustic enhancement.
Ultrasound is also useful in guiding biopsy (both FNAC and core biopsies) but needs to be carried out with care to avoid facial nerve damage 8-9.
Angiography (DSA)
Typically hypovascular.
Treatment and prognosis
Surgical excision is curative, however as the tumour is poorly encapsulated (despite imaging suggesting otherwise), there is a significant rate of recurrence in the tumour bed. Exact rates of recurrence vary widely depending on series and surgical technique (1-50% )1.
Historically these tumours 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 tumour 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 tumours do not invade into periosteum, thus bone need not be resected 4.
When tumour bed recurrences occur they can be extremely difficult to control, with management options including monitoring only, surgery, radiotherapy.
Complications
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 tumours 2,6. In addition to carcinoma ex pleomorphic adenoma, true malignant mixed tumours of the salivary glands usually arise from pre-existing pleomorphic adenomas 1,3. Metastasising pleomorphic adenoma is the third type of malignant mixed tumour of salivary glands, and is the rarest. It presents with metastases to lung bone and soft tissues despite having 'benign' histology 1.
Differential diagnosis
When in the parotid gland consider
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