Pleomorphic adenomas, also known as benign mixed tumours (BMT's), are the most common salivary gland tumours.
On imaging, they commonly present as well circumscribed rounded masses, most commonly located within the parotid gland, showing to be hypoechogenic on ultrasound and with bright T2 signal and homogeneous enhancement on MRI.
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.
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 tumour of each gland.
Pleomorphic adenomas are also commonly found in the lacrimal glands where they account for approximately 50% of lacrimal gland tumours 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 tumours 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. Tumours with a prominent cartilaginous matrix have a bluish-gray opalescent appearance.
Three histological types have been described:
- myxoid (hypocellular): most common, has the highest rate of recurrence
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 extra parotid, 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.
Typically hypoechoic. May show a lobulated distinct border +/- posterior acoustic enhancement with through transmission.
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.
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 1,10.
When the tumour is small, the enhancement tends to be prominent. In larger tumours enhancement is less marked but can demonstrate delayed enhancement 10.
They are commonly seen as well-circumscribed and homogeneous when small. Larger tumours may be heterogeneous.
- T1: usually of low intensity
- 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
- typically hypovascular
FDG-PET may show some uptake; SUV more than 3 in 25% of cases 11.
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, 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 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.
When in the parotid gland consider:
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- 3. Kim Hun-Soo, Cho Hyang-Jung, Chung Yeun-Tai, Park Soon-Ah, Cho Hae-Joong, Kim Jin-Man. Carcinosarcoma (True Malignant Mixed Tumor) of the Parotid Gland - A Report of a Case with Small Cell Carcinoma as the Carcinoma Component Vol.42, No.3:175-180, June 2008 Korean journal of pathology link
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- 6. Moonis G, Patel P, Koshkareva Y et-al. Imaging characteristics of recurrent pleomorphic adenoma of the parotid gland. AJNR Am J Neuroradiol. 2007;28 (8): 1532-6. doi:10.3174/ajnr.A0598 - Pubmed citation
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