Pathology and Molecular Pathology of Salivary Tumors: General Considerations

Prognostic Categories of Salivary Tumors

The histologic classification of salivary tumors is difficult in a significant proportion of tumors. The main reasons are that they are rare compared with tumors of other organs, and they comprise an unusual large spectrum of benign ( n = 11) and malignant ( n = 20) tumor entities. Moreover, most entities show a heterogenous and overlapping spectrum of cytologic and structural differentiation. As in other organs, tumorous lesions of salivary glands comprise four prognostically relevant categories: metaplastic lesions, benign tumors, and low-grade and high-grade cancer ( Table 6.1 ). While the diagnosis of metaplastic lesions and of high-grade cancer is mostly straightforward, the distinction between benign tumors and low-grade cancer is more frequently difficult than in other organ systems.

An important reason for the difficulty is the multitude of low-grade malignant entities, which often comprise no or only a minor degree of cellular atypia, no increase of proliferation, and/or absence of clear invasion (e.g., acinic cell carcinoma, mucoepidermoid carcinoma, polymorphous adeno carcinoma, clear cell carcinoma). On the other hand, benign tumors may exhibit some degree of cellular atypia and may show pseudo-infiltration, mimicking malignancy.

Therefore in difficult situations, the primary aim of the pathologist should not be to enforce a distinction between the categories: benign vs low-grade malignant. Rather, the pathologist should concentrate on the exact identification of the tumor entity on the basis of cellular, structural, immunohistological, and, with increasing relevance, molecular criteria. The primary identification of the tumor entity establishes its status (benign vs low-grade malignant). Clinicians should appreciate that histologic classification (and hence establishment of status: benign versus low grade malignant) in many cases is not easy, particularly in incisional biopsies, cytology, and frozen sections.