Evaluation of the 10 %-rule in sentinel lymph node biopsy for clinically node-negative oral squamous cell carcinoma

•The 10%-rule effectively identified metastatic sentinel nodes intraoperatively.•Applying the 10%-rule per neck side must be considered.•Raising the threshold is not advised, as this leads to missed metastatic nodes.•More research on radioactivity in false-negative sentinel node procedures is needed. Sentinel lymph node biopsy (SLNB) has proven to reliably stage the clinically negative neck in early-stage oral squamous cell carcinoma (OSCC). The 10%-rule, used to define sentinel lymph nodes (SLN) intraoperatively, states that an SLN is defined by gamma counts that are at least 10% of the hottest harvested lymph node (LN). However, this intraoperative rule has not yet been adequately evaluated for early-stage OSCC. This study aims to evaluate the 10%-rule intraoperatively and explore possibilities for redefining this criterion. A single center retrospective study was performed between 2014 and 2023. Patients (n = 66) with clinically node-negative OSCC (cT1-3N0) and positive SLNB were included in this study. Radioactivity of all harvested LNs were measured ex-vivo. Metastatic LNs were assessed for complying with the 10 %-rule. The accuracy of alternative thresholds was evaluated. If multiple positive SLNs on one side of the neck were found, the hottest positive SLN on that side was considered the LN accountable for upstaging the corresponding neck side from N0 to N+. A total of 233 LNs were harvested, of which 98 contained metastases and 190 met the 10%-rule. Of these metastatic LNs, 70 were accountable for upstaging the side of the neck. The 10%-rule correctly staged 69 of 70 metastatic sides of the neck. By increasing the intraoperative cut-off point to 20%, 68 of 70 metastatic sides of the neck were correctly staged and 18 LNs (172 vs. 190) were not defined as SLN. Applying the 10%-rule to each side of the neck increased detection of one metastatic LN. However, this LN was not accountable for upstaging that side of the neck. The 10%-rule in SLNB applies to patients with OSCC and effectively identifies metastatic SLNs intraoperatively. Applying the 10%-rule per neck side must be considered. Increasing the threshold percentage is not advisable.