Lobe-Specific Risk of Malignancy in Parotid Gland Lesions: A Milan System-Based Analysis

To evaluate the diagnostic accuracy of fine-needle aspiration cytology (FNAC) for parotid gland lesions using the Milan System while eliminating verification bias, and to systematically compare risk of malignancy (ROM) between superficial and deep lobe lesions across all Milan categories.

A retrospective review was conducted of 332 patients who underwent ultrasound-guided FNAC and subsequent surgical excision between 2019 and 2025. Cytology results were classified according to the Milan System. ROM values and diagnostic performance metrics were calculated for each category, with subgroup analysis according to lobe location. Final pathology results were available for all cases, ensuring complete diagnostic verification.

Histopathology confirmed malignancy in 37 cases (11.1%), with significantly higher frequency in the deep lobe compared to the superficial lobe (24.6% vs. 7.9%, p < 0.001), representing a 3.1-fold increased malignancy risk. Deep lobe lesions demonstrated mostly higher ROM across multiple Milan categories. Most notably, within the "benign neoplasm" category, deep lobe lesions exhibited a significantly higher ROM of 7.3% compared to 1.0% for superficial lobe lesions (p = 0.009), representing a 7.3-fold increased malignancy risk even within this traditionally low-risk cytological category. In contrast, high-risk categories (suspicious for malignancy and malignant) showed similar ROM values regardless of anatomical location.

FNAC with Milan System classification provides reliable diagnostic performance for parotid lesions. However, anatomical location significantly influences malignancy risk, with deep lobe lesions carrying substantially higher ROM across multiple diagnostic categories.

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