A 38-year-old woman is found to have a painless, mobile mass in the submandibular gland on routine examination. Biopsy reveals adenoid cystic carcinoma. Which histologic feature most reliably distinguishes adenoid cystic carcinoma from pleomorphic adenoma?

## Discriminating Feature: Cribriform Pattern with Perineural Invasion **Key Point:** Adenoid cystic carcinoma (ACC) is distinguished by its characteristic cribriform (Swiss-cheese-like) or tubular pattern with prominent **perineural invasion**. Critically, it lacks the mesenchymal stromal component (chondroid, myxoid, osseous) that defines pleomorphic adenoma. **High-Yield:** Perineural invasion is the pathognomonic feature of ACC and is virtually absent in pleomorphic adenoma. This single finding — tumor cells tracking along nerve sheaths — is the most reliable discriminator. ### Comparative Histopathology | Feature | Adenoid Cystic Carcinoma | Pleomorphic Adenoma | |---------|--------------------------|--------------------| | **Growth pattern** | Cribriform, tubular, solid | Mixed epithelial-mesenchymal | | **Perineural invasion** | Prominent, pathognomonic | Absent | | **Mesenchymal stroma** | Absent or minimal fibrous | Abundant chondroid/myxoid/osseous | | **Myoepithelial component** | Present but not prominent | Prominent, characteristic | | **Capsule** | Infiltrative, ill-defined | Well-defined, pushing borders | | **Malignant behavior** | Intrinsically aggressive; late recurrence common | Benign; low malignant potential | | **Clinical course** | Often painful; rapid progression | Slow, painless, long indolent history | **Clinical Pearl:** Perineural invasion in ACC explains its aggressive behavior, high recurrence rate, and poor prognosis. Patients often present with pain or facial nerve involvement — features absent in pleomorphic adenoma. **Mnemonic:** **ACC = Aggressive, Cribriform, Carcinoma** — the cribriform pattern and perineural invasion are the hallmarks. **Warning:** Do not mistake the myoepithelial cells present in ACC for the abundant myoepithelial component of pleomorphic adenoma. In ACC, myoepithelial cells are present but not the dominant architectural feature; the cribriform pattern and perineural invasion are.