A 52-year-old woman presents with a 2-year history of a painless, slow-growing mass in the superficial lobe of the parotid gland. On examination, the overlying skin is intact, facial nerve function is normal, and the mass is mobile. Which histopathological feature best distinguishes this likely pleomorphic adenoma from an acinic cell carcinoma?

Explanation

## Histopathological Distinction: Pleomorphic Adenoma vs. Acinic Cell Carcinoma ### Microscopic Features Comparison | Feature | Pleomorphic Adenoma | Acinic Cell Carcinoma | |---------|-------------------|---------------------| | **Myoepithelial cells** | Prominent, well-preserved | Reduced or absent | | **Basement membrane** | Intact, well-defined | Disrupted, incomplete | | **Architecture** | Mixed epithelial and mesenchymal | Predominantly acinar | | **Atypia** | Minimal | Present (low to intermediate grade) | | **Mitotic activity** | Low | Low to moderate | | **Malignant potential** | Yes (if untreated, ~5% transformation) | Yes (inherent malignancy) | | **Growth rate** | Slow | Slow to intermediate | ### Key Point: **The presence of prominent myoepithelial cells with an intact basement membrane is the defining histological feature of pleomorphic adenoma. Acinic cell carcinoma, despite being low-grade and slow-growing, lacks this myoepithelial layer and shows basement membrane disruption.** ### Clinical Pearl: Acinic cell carcinoma is a **low-grade malignancy** that clinically mimics benign tumors (slow growth, painless, no nerve involvement). However, it is inherently malignant and will metastasize if untreated. Pleomorphic adenoma, while benign, has a small risk of malignant transformation if left in situ for decades. The myoepithelial cell layer is the histological "guardian" that distinguishes benign from malignant in this context. ### High-Yield: Acinic cell carcinoma accounts for ~10% of parotid malignancies and is the **most common low-grade malignant parotid tumor**. It arises from serous acinar cells. Despite its indolent clinical behavior, it is a true carcinoma and requires surgical excision with neck dissection if lymph node involvement is present. Pleomorphic adenoma's myoepithelial differentiation is a hallmark of its benign nature. ### Mnemonic: **MYOEP** = MYoepithelial cells, Epithelial-mesenchymal mix, Pleomorphic adenoma's defining feature. [cite:Robbins 10e Ch 16]