Risk of Malignancy by Cytological Category in Oral Brush Liquid-Based Cytology: Baseline Data Toward a Structured Reporting System

Background/Objectives: Structured cytopathology reporting systems attach an evidence-based risk of malignancy (ROM) to every diagnostic category for the thyroid, salivary gland, urinary tract, lung and other sites. None exists for the oral cavity, and the ROM of an oral brush cytology category has not, to our knowledge, been reported. Methods: All oral liquid-based cytology specimens accessioned at one tertiary dental hospital between 2015 and 2024 were reviewed; those with a site-concordant histopathological diagnosis obtained within 90 days formed the analytic cohort (n = 137). Cytological reports were recovered from primary laboratory records and assigned to a five-tier ordinal scale. The target condition was malignancy or high-grade dysplasia, and diagnostic indices were calculated at every ordinal threshold with Wilson 95% confidence intervals. Results: ROM rose monotonically: 15.6% (95% CI 8.7–26.4) for negative, 53.3% (30.1–75.2) for atypical favoring reactive, 76.9% (61.7–87.4) for atypical cells of undetermined significance (ACUS), 87.5% (52.9–97.8) for atypical favoring neoplastic and 100% (74.1–100) for malignancy. The ACUS value far exceeded that of the similarly named tier in the thyroid (22%) and salivary gland (30.5%) systems and fell within the range spanned by their suspicious-for-malignancy tiers (74% and 83.8%). Malignancy was never reported in a histologically benign or low-grade lesions. Sensitivity at the ACUS threshold was 25.0% in keratotic against 79.3% in non-keratotic lesions (p = 0.004). Conclusions: These are, to our knowledge, the first category-specific ROM estimates for oral brush cytology, and they identify a nomenclature problem: a category named for the pathologist’s uncertainty carried a risk equivalent to suspicious for malignancy elsewhere; while a negative report was followed by disease in one lesion in six. Any oral reporting system should attach an explicit ROM to each category; our exploratory data further suggest that the surface keratinization of the sampled lesion should be recorded.