Elective Neck Dissection Strategies Guided by AJCC-8 Depth-of-Invasion (DOI) in cT1–T2N0 Oral Cavity Cancer—A Systematic Review

Background/Objectives: Clinically node-negative (cN0) neck management in cT1–T2 oral cavity squamous cell carcinoma continues to be a subject of controversy. The eighth edition AJCC has incorporated depth of invasion (DOI) as a significant factor in staging and consideration for possible neck dissection. Establishment of accurate DOI thresholds and their clinical relevance is crucial to maximize oncological outcomes with reduced unnecessary morbidity. Methods: A comprehensive analysis of clinical research assessing elective neck dissection (END) techniques in oral cavity cancers classified by DOI in cT1–T2N0 patients was carried out. The included studies reported occult nodal metastasis rates, overall survival, disease-specific survival, disease-free survival, and regional control. Results: With hazard ratios favoring END for overall survival (HR 0.64; 95% CI 0.45–0.92) and disease-free survival (HR 0.45; 95% CI 0.34–0.59), elective neck dissection provided advantages in both survival and regional control. In a national registry, DOI ≥ 5 mm independently raised the risk of nodal failure (HR 2.099; 95% CI 1.346–3.271), while END enhanced neck control in comparison to observation (HR 1.749; 95% CI 1.141–2.680). With ROC-derived cut-offs like 4.59 mm producing positive predictive values for nodal metastasis up to 41.7%, diagnostic thresholds clustered around 4 mm. Conclusions: Under DOI guidance, elective neck dissection consistently showed oncologic benefit, with practical thresholds convergent around 4 mm for sites in the mixed oral cavity and 3 mm for high-risk subsites. The synthesized results confirmed that DOI is the primary determinant of END when combined with histopathologic and subsite-specific risk factors.