Pleomorphic adenoma (PA), also known as benign mixed tumor, is the most common neoplasm of the salivary glands. It accounts for approximately 60% of all salivary gland tumors and up to 70–80% of benign salivary neoplasms.1 PAs are benign but noteworthy for their diverse histopathology and clinical behavior. They typically present as slow-growing, painless masses and can occur in major or minor salivary glands.1 Despite their benign nature, PAs require careful management because inadequate excision can lead to recurrence, and a small subset may undergo malignant transformation to carcinoma ex pleomorphic adenoma (Ca ex PA).2, 3, 4

PA is the most frequent salivary gland tumor across all age groups, with a peak incidence in mid-adulthood (fourth to sixth decades).5 There is a slight female predilection, with roughly 60% of cases occurring in women.5 The annual incidence is estimated around 2–4 per 100,000 population.6 The parotid gland is the most common site: about 80–85% of PAs arise in the parotid (particularly the superficial lobe). Minor salivary glands (especially the palate) and the submandibular gland are less frequently involved.5 Known risk factors are not well-defined, though prior radiation exposure has been implicated in some studies.1 PAs are typically solitary; multiple or bilateral PAs are rare.7

Historically, recurrence rates varied widely depending on surgical technique. Simple enucleation of a PA is associated with high recurrence (up to 20–45% in older series) due to tumor spillage and residual microscopic nodules.7 With appropriate superficial or total parotidectomy (including a cuff of normal tissue), recurrence rates are much lower (on the order of 2–5%).6,7

Although malignant transformation is uncommon, it is a significant clinical concern. Ca ex PA accounts for approximately 3.6% of all salivary gland neoplasms and about 11–12% of salivary malignancies. It is estimated to develop in approximately 5–12% of long-standing PAs.4,8 In a Danish series, the risk of malignant transformation among recurrent PAs was 3.3%.6 Patients with PAs require long-term follow-up because recurrence or malignant change can occur decades after initial treatment.7

Clinically, PA usually presents as a solitary, mobile, slow-growing, and painless mass in the region of the affected gland.3 In the parotid, the tumor classically appears as a firm, well-circumscribed nodule in the preauricular area or angle of the mandible. The overlying skin or mucosa is typically normal. Symptoms are generally absent aside from the presence of the lump; facial nerve dysfunction is not a feature of benign PA and, if present, suggests malignant transformation or an alternative diagnosis.9

PAs often grow insidiously over years. Tumors in the superficial lobe of the parotid are usually readily palpable and mobile. In contrast, deep lobe parotid PAs may present as an oropharyngeal mass displacing the tonsil (parapharyngeal space tumor).10 PAs of minor salivary glands present as submucosal nodules, most commonly on the hard palate; they are typically firm and may cause a painless swelling of the palate that is covered by intact mucosa. Large lesions in the palate or nasopharynx can cause symptoms related to mass effect (difficulty in swallowing or nasal obstruction) in rare cases.11,12

Ca ex PA typically occurs in older patients (peak in the sixth to eighth decades), often with a history of a long-standing parotid mass.4 Many patients will report a tumor that was present for years and recently grew rapidly or became painful. Clinically, signs such as rapid enlargement, pain, fixation, or facial nerve palsy in a patient with known PA are red flags for malignant transformation. The most common site is the parotid gland, followed by the submandibular gland; Ca ex PA is rare in minor glands (where de novo carcinomas are more common).4