The Relationship Between Depth of Invasion and Cervical Lymph Node Metastasis in Tongue Squamous Cell Carcinoma: A Retrospective Study

Depth of invasion (DOI) is a significant predictor of lymph node metastasis (LNM) in tongue cancer. However, the ideal DOI threshold for guiding prophylactic neck dissection in early-stage disease remains controversial. This study aimed to investigate the relationship between DOI and LNM and to determine a potential optimal DOI cutoff for tongue cancer. This retrospective study included 68 patients with histopathologically confirmed squamous cell carcinoma of the tongue who underwent primary tumor resection and levels I-III neck dissection at a tertiary referral hospital between 2023 and 2025. DOI was measured pathologically. The association between DOI and LNM was assessed using logistic regression, incorporating restricted cubic splines to account for potential nonlinear effects of DOI. Thirty-eight patients (55.9%) presented with stage III or IV tongue cancer. Cervical LNM was identified in 24 patients (35.3%), with 15 (22.1%) cases classified as occult LNM. The relationship between DOI and LNM was nonlinear, demonstrating a rapid increase in LNM probability between DOI values of 3 and 10 mm, followed by a plateau. The predictive performance of DOI for LNM was satisfactory, with an area under the curve of 0.77. A DOI of 3 to 6 mm exhibited a sensitivity of 96% for LNM prediction, with higher DOI improved specificity (from 23 to 53%, respectively). DOI is strongly associated with cervical LNM in tongue cancer, particularly within the 3-10 mm range. A DOI cutoff of greater than 3 mm may be considered for elective lymph node dissection in tongue cancer