Salivary gland tumours are rare, constituting only 2–6.5% of all head and neck tumours. The tumours of minor salivary glands are uncommon and constitute only 10–25% of all salivary gland tumours. In the minor salivary glands, malignant transformation is more common[1,2] Of the benign tumours of the salivary glands, pleomorphic adenoma tops the list. It is also known as a benign mixed tumour. The parotid gland accounts for 85% of cases of pleomorphic adenomas. However, these tumours can also present unusual locations, for example, minor salivary glands of the lip, para-pharyngeal space, the cheek, the floor of the mouth, heterotopic salivary gland tissues, etc.[3,4] The heterotopic rests of salivary gland tissue can be found in various places, including the pituitary, thyroglossal duct, mandible, neck, etc.
Here, we discuss two cases of pleomorphic adenoma occurring at heterotopic rests of salivary tissue in the anterior triangle of the neck corresponding to level II lymph nodes and the minor salivary gland of the lower lip.
The first patient is a 28-year-old with slow-growing swelling in the anterior triangle of the neck. The swelling is along the anterior border of the sternocleidomastoid muscle at level II of cervical lymph nodes [Figure 1]. It was a small-circumscribed, mobile, well-circumscribed swelling. She gave a history of exactly swelling at the exact location 10 years back, which was excised surgically; however, no histopathology testing was done then. The clinical diagnosis of the current swelling was lymphadenopathy or keloid, and the patient was sent for fine-needle aspiration cytology (FNAC) testing. Fine-needle aspiration (FNA) yielded thick mucoid material in the needle’s hub. The smears were prepared from the mucoid material. Microscopic examination showed cellular smears with a biphasic tumour. The stromal component was seen as abundant chondromyxoid stroma, and the epithelial component was seen as monolayered sheets of benign-looking epithelial cells with bland round to oval nuclei and a moderate amount of cytoplasm [Figure 2]. Myoepithelial cells were also present in the background exhibiting spindle-shaped morphology. Based on these features, the diagnosis of pleomorphic adenoma was made. The swelling was excised successfully by the surgeon. Histopathologically, the features were consistent with pleomorphic adenoma [Figure 3]. The patient was followed up and is doing well without any recurrence.
The second patient is a 46-year-old female. She also presented with a slowly progressive asymptomatic swelling on the lower lip from the last ten months. It was around 1 × 1 cm in size, non-tender and had soft swelling. No other notable lesion was identified in or around the oral cavity. There was no history of trauma, and the dental history was unremarkable. The provisional diagnosis made by the clinician was mucocele. FNAC was done, and it yielded blood admixed aspirate. The smears were prepared, showing high cellularity comprising groups and clusters of benign ductal epithelial cells and myoepithelial cells. There was abundant chondromyxoid stroma in the background. Therefore, a diagnosis of pleomorphic adenoma was made. The patient was lost to follow-up.
The most common benign tumour in the salivary glands is a pleomorphic adenoma, yet the exact aetiology is unknown. It is thought that these tumours originate from the reserve cells of intercalated ducts. The molecular studies show chromosomal alteration at 8q12 and 12q15. It can occur at any age and sex, yet females in their fourth to sixth decade are more commonly affected. The female/male ratio is 1.9:1. It occurs more typically between 7 and 16 years of age in children.
Pleomorphic adenoma can also occur at heterotopic salivary gland tissue (HGST) as seen in our case. HGST refers to salivary gland tissue present outside the major/minor/accessory salivary gland tissue. These ectopic islands can be present in various places like the pituitary, temporal bone, mandible, thyroglossal duct, and rarely in the neck.[4,7] In the neck region, although rare, the heterotopic rests of salivary tissue can be found along the anterior border of the sternocleidomastoid muscle. Various theories have been put forward to explain the basis of HGST, for example, epithelial remnants from the brachial apparatus. Pleomorphic adenoma is the second most common tumour of HGST, the first being Warthin’s tumour. Very few cases of pleomorphic adenoma at ectopic salivary tissue in the upper cervical region have been described so far.
In the case of minor salivary glands of lips, the propensity of pleomorphic adenoma is more on the upper lip than the lower lip. The upper lip tumours are more commonly benign, whereas lower lip tumours are more commonly malignant due to the difference in their embryonic development. In our case, this is an exception, as we reported a benign pleomorphic adenoma on the lower lip. The incidence of pleomorphic adenoma on lips is seen a decade earlier and peaks in the third or fourth decade. Fine-needle aspiration cytology is a safe and reliable pre-operative test for diagnosing pleomorphic adenoma. However, FNAC may pose some diagnostic difficulties due to variations in the cellular composition of pleomorphic adenoma. According to a study by Vaidya et al., FNAC has a sensitivity of 95.83% and a specificity of 97.05%. The study also showed that the diagnostic accuracy of FNAC in diagnosing pleomorphic adenoma is 96.55%.
Pleomorphic adenoma at heterotopic sites, like swelling at the anterior triangle of the neck and lower lip, can be kept as a differential diagnosis. The clinician should keep a high index of suspicion and adequately clear all the margins with a cuff of surrounding dispensable normal tissues, which is essential for preventing recurrence. Therefore, the complete wide local resection of the tumour with negative margins is the treatment of choice. Many have used post-operative radiation to prevent a recurrence. The patients should be kept on the long-term follow-up to see for any recurrence or malignant transformation of the swelling.
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