Discussion
Minor salivary gland pleomorphic adenoma (PA) - usually seen in the palate - is uncommon, making up 40% of minor salivary glands tumours [1-5]. ~50% are malignant, adenoid cystic carcinoma being the most common [1, 5]. The smaller the salivary gland, the more likely the malignancy [5].
PA is composed of mixed elements surrounded by a pseudocapsule [1-5]. Unilateral, slow growing, painless and well-circumscribed submucosal lumps without overlying mucosal changes often presents in adult females in their 4-6th decade. Associated mechanical symptoms can coexist [1, 2, 4, 5].
Malignant transformation, most commonly in carcinoma ex pleomorphic adenoma (arising from primary or recurrent PA), is much more common in the parotid than elsewhere. Increased preoperative duration increases the risk, with a mean lead time of 9 years. Exposure to radiation is also a factor. Malignant change occurs at ~1.5% in the first year and ~9.5% after 15 years (M Sherif Said) and 40-50% metastasise to lymph nodes [11].
Imaging is central in the diagnosis and characterisation of such lesions.
Ultrasound (US) shows a well-defined lobulated solid hypoechoic lesion with posterior enhancement. Vascularity is variable, often poor, but suggestive of the diagnosis when peripheral [8, 9].
On cross-sectional imaging small tumours are well-defined, homogeneous, and show intense enhancement. Larger tumours may be pedunculated and heterogeneous, with necrotic/haemorrhagic areas. CT may show calcifications. Well-defined multi-lobulated margins, strong enhancement and a high signal intensity (SI) on T2W imaging are typical of benign lesions [7].
On CT, the hard palate is usually intact, minor erosion is unusually and full thickness erosion is rare. Deep infiltration, bone involvement and perineural spread suggest malignancy. New MR techniques (DCE-MRI, DWI and spectroscopy) have shown promising results in this differentiation [6, 7]. Malignant lesions take less time for peak enhancement (at 120s time-SI curves), have rapid wash-out with a >30% ratio, and lower ADC-values [7, 8]. FDG PET/CT SUV(max), metabolic tumour volume and total glycolytic activity have also been suggested as useful parameters, with malignant lesions showing higher values, although no cut-off standardised values have yet been set [10].
Cysts, abscesses, soft tissue lesions, other salivary gland benign or malignant tumours, and lymphoma, are differential diagnoses. History, physical examination and imaging can aid in making diagnosis, however confirmation depends on histopathology. FNA can infer the origin and nature, however differentiation from malignant tumours may not be achieved [2-5].
Complete excision is necessary to avoid recurrence [1, 2, 3, 5].
