Pleomorphic adenoma is a common benign salivary gland neoplasm characterised by neoplastic proliferation of parenchymatous glandular cells along with myoepithelial components, having a malignant potentiality.
It is the most common type of salivary gland tumor and the most common tumor of the parotid gland. It derives its name from the architectural Pleomorphism (variable appearance) seen by light microscopy. It is also known as "Mixed tumor, salivary gland type" (BMT's), which describes its pleomorphic appearance as opposed to its dual origin from epithelial and myoepithelial elements.
Pleomorphic adenomas account for 70-80% of benign salivary gland tumours and are especially common in the parotid gland (see below) 1,2. Patients are typically middle aged1, and prior head and neck irradiation is a risk factor for the development of these tumours.
The sonographic features are:
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 transmission3.
- It can be very hypoechogenic that confuse with a cyst (adjust gain). Use color Doppler to demonstrate solid nature1.
- Larger size adenomas (greater than 30mm) can suffer from cystic or hemorrhagic degeneration1.
- Dystrophic calcifications in long-evolving adenomas (overlap with findings of a malignant lesion), but without associated adenopathies3.
- Characterization of the adenoma by ultrasound: sensitivity 82%, specificity 86% and accuracy of 84%2.
- The Doppler pattern is variable, generally low vascularization is observed in its interior and peripheral flow with small branches that tend to radiate towards the center of the tumor (not pathognomonic). IR <0.8 and IP <1.8, suggest benignity3.
- 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 damage4,5.
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.
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 tumours2,6.