A Diagnostically Challenging Parotid Gland Tumor With Hybrid Features

Salivary gland tumors are relatively uncommon with most being benign. When diagnosed the most common benign and malignant tumors are pleomorphic adenoma and mucoepidermoid carcinoma (MEC), respectively. However, not uncommonly, it is difficult to differentiate between the histopathological entities, leading to a diagnostic dilemma that can impact a patient's treatment and prognosis. A 24-year-old woman presented with a three-year history of asymptomatic left-sided facial swelling. She denied any prior history of head and neck radiation. There was no history of alcohol consumption or smoking exposure and there was no personal or family history of head and neck cancers. Additionally, she did not have any known occupational or environmental exposures. Due to the chronicity and painless nature of this facial mass, our patient did not pursue evaluation initially. Subsequently, she experienced an increase in size and pain for a few months exacerbated by swallowing. She had no other symptoms. On physical examination, a 3 x 3 cm left parotid gland mass was noted. There was no associated head or neck lymphadenopathy and compression of the left facial mass did not elicit secretions from the opening of Stensen's duct. Due to the rapid increase in size, she was sent for CT neck/soft tissue with contrast which confirmed a 3.56 x 2.67 cm solid nodule within the superficial portion of the left parotid gland. This was followed by an MRI orbit/face/neck with and without contrast, for further delineation, which demonstrated a 4 x 3.7 x 3 cm complex heterogeneous mass within the superficial left parotid gland. Thereafter the patient underwent an uncomplicated ultrasound-guided biopsy of the parotid mass. The histopathological appraisal concluded that this was a cellular pleomorphic adenoma, with mucinous and squamous metaplasia with reactive lymph nodes. Due to the new rapid increase in size and intense painful nature of this tumor, nerve-sparing left parotidectomy, fat grafting and reconstruction were completed. Cellular pleomorphic adenomas are benign low-grade neoplasms, typified as biphasic with both epithelial and myoepithelial components. However, they have increased cellularity and focally increased mitotic activity, not advanced enough to qualify as malignant. The presence of mucinous and squamous metaplasia is of diagnostic interest as it makes diagnosis on fine-needle aspiration (FNA) morphologically challenging. These findings are typical of MEC and on FNA can be misleading in the setting of a pleomorphic adenoma. However, on histopathological evaluation of the gross specimen along with immunohistochemical staining the diagnosis is made much easier. A diagnosis of MEC would have potentially required neck dissection and adjuvant therapy with a potential increased risk of morbidity and mortality. This case emphasises the importance of an adequate tissue biopsy in regards to parotid gland tumors to optimise a patient's care plan.