A 52-year-old man from Delhi presents with a 6-month history of a painless, slow-growing swelling in the right parotid region. On examination, the mass is firm, fixed to the underlying tissues, and the overlying skin shows a slight dimple. There is mild facial nerve involvement with drooping of the right angle of mouth. Fine needle aspiration cytology (FNAC) shows epithelial cells arranged in nests with myoepithelial cells. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis The patient presents with a **firm, fixed parotid mass** with **facial nerve involvement** (drooping of right angle of mouth) and **skin dimpling** — classic red flags for malignancy with perineural invasion. ### Key Distinguishing Features **High-Yield:** The combination of: - Fixed mass (not mobile) — capsular invasion - Skin dimpling (skin tethering) — local infiltration - **Facial nerve paresis** — perineural invasion (hallmark of adenoid cystic carcinoma) - Slow, relentless growth over 6 months - **FNAC: epithelial cells in nests with myoepithelial cells** — characteristic of adenoid cystic carcinoma ...points to **Adenoid Cystic Carcinoma (ACC)**. ### Why Adenoid Cystic Carcinoma? 1. **Perineural invasion is the hallmark** of ACC — facial nerve involvement is a classic and early feature, unlike other salivary gland malignancies where nerve involvement is a late finding. 2. **FNAC findings**: ACC shows epithelial cells arranged in nests/clusters with **myoepithelial cells** — the myoepithelial component is a defining cytological feature of ACC. The cribriform ("Swiss cheese") pattern is seen on histology, but FNAC may show epithelial-myoepithelial cell clusters. 3. **Slow but relentless growth** — ACC is notorious for slow progression with high rates of local recurrence and late distant metastases. 4. **Second most common malignant salivary gland tumor** and the most common in the submandibular and minor salivary glands; also occurs in the parotid. **Clinical Pearl (Harrison's / Cummings Otolaryngology):** Adenoid cystic carcinoma has the highest propensity for perineural invasion among all salivary gland tumors. Facial nerve palsy in a parotid mass with slow growth should raise ACC as the top differential. The myoepithelial cell component on FNAC is a key cytological clue. ### Differential Diagnosis of Parotid Malignancies | Feature | Mucoepidermoid Ca | Adenoid Cystic Ca | Acinic Cell Ca | |---------|-------------------|-------------------|----------------| | **Facial nerve involvement** | Advanced cases | **Classic/early (perineural)** | Rare | | **FNAC** | Mucoid material, epidermoid + mucous cells | Epithelial + **myoepithelial cells**, cribriform | Granular acinar cells | | **Growth** | Variable | Slow but relentless | Slow | | **Prognosis** | Grade-dependent | Poor (late mets) | Excellent | ### Why Not the Others? - **Pleomorphic adenoma (A):** Benign; mobile, no facial nerve involvement, no skin fixation. FNAC shows epithelial + myoepithelial cells in a chondromyxoid stroma — but the clinical malignant features (fixation, nerve palsy, skin dimpling) exclude this. - **Mucoepidermoid carcinoma (B):** Most common malignant parotid tumor, but FNAC characteristically shows **mucoid material with epidermoid and intermediate cells**, not a prominent myoepithelial component. Facial nerve involvement occurs in high-grade disease. - **Acinic cell carcinoma (D):** Low-grade, usually mobile, facial nerve involvement is rare; FNAC shows granular cytoplasm with acinar pattern. **Key Point:** The FNAC description of **epithelial cells in nests WITH myoepithelial cells** combined with **perineural facial nerve involvement** is the classic cytological-clinical combination for Adenoid Cystic Carcinoma (Cummings Otolaryngology, 7th ed.; KD Tripathi Pharmacology; Dhingra ENT).