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Polymorphous adenocarcinoma (PAC), formerly known as polymorphous low-grade adenocarcinoma (PLGA), is a malignant tumor that arises almost exclusively in the minor salivary glands, especially in the palate. It is characterized by slow growth, histological diversity (“polymorphous” architecture), and indolent clinical behavior. Despite being low-grade, PAC can infiltrate surrounding tissues and recur locally if not adequately treated.
Definition and Classification
Polymorphous adenocarcinoma is a malignant epithelial tumor of salivary gland origin that exhibits cytologic uniformity but architectural diversity. The tumor shows multiple growth patterns, including:
- Tubular
- Cribriform
- Papillary
- Solid
- Trabecular
PAC is recognized by the WHO as a distinct entity separate from other minor salivary gland malignancies.
Epidemiology and Etiology
- Predominantly affects middle-aged and older adults, with a slight female predilection.- Accounts for 10–15% of minor salivary gland malignancies.
- Etiology is largely unknown; no strong associations with tobacco, alcohol, or radiation have been established.
Common Sites
PAC almost exclusively arises in minor salivary glands, with the most common sites being:
- Palate (hard and soft palate junction)- Buccal mucosa
- Upper lip
- Retromolar region
Major salivary gland involvement is rare.
Clinical Presentation
- Slow-growing, painless, firm swelling- Well-circumscribed or slightly nodular
- May present with ulceration if trauma occurs
- Usually asymptomatic, which may delay diagnosis
- Rarely, patients may report mild discomfort or bleeding
The tumor rarely causes nerve involvement, unlike adenoid cystic carcinoma.
Diagnostic Approach
Accurate diagnosis of PAC requires clinical, radiological, and histopathological correlation:
- Imaging (CT or MRI) is used to determine lesion extent and bone involvement.- Incisional biopsy is preferred; small superficial biopsies may be misleading.
- Histopathological evaluation is critical to distinguish PAC from other minor salivary gland tumors, especially adenoid cystic carcinoma and pleomorphic adenoma.
Immunohistochemistry can assist: PAC cells are typically positive for cytokeratins, S100, and SOX10, and negative for high-grade markers such as Ki-67 in low-grade tumors.
Histopathological Features
- Cytologic uniformity: Cells are small, cuboidal or polygonal, with bland nuclei- Polymorphic growth patterns: Tubular, cribriform, papillary, trabecular, or solid arrangements
- Perineural invasion may be present but usually limited
- Well-circumscribed but infiltrative margins
- Minimal mitotic activity and absence of significant atypia
These features distinguish PAC from other salivary malignancies like adenoid cystic carcinoma (more aggressive, more perineural invasion) and pleomorphic adenoma (benign with myxoid stroma).
Management and Treatment
- Wide surgical excision with clear margins is the treatment of choice.- Neck dissection is generally not required unless there is evidence of nodal metastasis (rare).
- Radiotherapy is reserved for recurrent or incompletely excised tumors.
- Long-term follow-up is important due to the potential for late local recurrence, even decades after initial treatment.
Prognosis
- PAC is considered low-grade, with excellent long-term survival.- Local recurrence rates range from 10–20%, often due to incomplete excision.
- Distant metastasis is extremely rare.
- Prognosis is significantly better than adenoid cystic carcinoma.
Role of Dental and Medical Professionals
Dental professionals are pivotal in early detection:
- Careful examination of palatal and minor salivary gland regions- Recognition of firm, slow-growing nodules as potentially malignant
- Prompt referral for biopsy and surgical management
- Educating patients on the need for long-term follow-up
For students, PAC highlights the importance of correlating cytology and architecture, recognizing low-grade malignancies, and differentiating them from benign lesions and other salivary gland tumors.
Conclusion
Polymorphous adenocarcinoma is a low-grade malignant tumor of minor salivary glands with a favorable prognosis if surgically excised. Despite its indolent growth, adequate margins and long-term follow-up are essential to prevent local recurrence. Awareness of its clinicopathologic features allows dental and medical professionals to identify and manage this tumor effectively, improving patient outcomes.