(0735) The 10% Rule in Sentinel Node Biopsy: Do We Need to Remove All Threshold Nodes?

Introduction: Sentinel node (SN) biopsy procedures has been guided by the “10% rule” which stipulates that any lymph node (LN) with gamma probe activity (GPA) measured at 10% of the "hottest" SN should also be considered a SN, and therefore, excised. We hypothesized that it would be unusual for a SN with less than 20% GPA to harbor a metastasis. This study was designed to determine if the 10% rule represented an excessive intervention and if SN biopsy could be simplified.

Methods: Patient records were retrieved from 2013-2023 from our tertiary academic institution for patients presenting with head and neck cutaneous and oral malignancy (HNM) who underwent SN biopsy. All patients during this period had each node recorded with anatomic location and numerical GPA. Patients were evaluated for detection of metastatic disease in SN removed during neck surgery that met the “10% threshold rule.”

Results: A total of 84 patients presenting with HNM that underwent SN biopsy from 2013-2023 were included in the study. 394 SNs were evaluated. Histologic patient diagnoses included squamous cell carcinoma (n=41, 48.8%), melanoma (n=30, 35.7%), Merkel cell carcinoma (n=9, 10.7%), and other rare cutaneous malignancies (n=4, 4.8%). Only 1 of these 84 patients (1.2%) had a solitary positive SNB biopsy that was 10-20% of the hottest node and represented the only known SN metastasis. Two additional patients had positive SNs that were 9% and 15% of hottest SN, but both had foci of melanoma in nodes with higher GPA.

Conclusions: Three metastases in less radioactive SN only occurred in patients with melanoma, a finding of unclear significance. One out of 84 patients in our cohort (1.1%) presented with a single positive SN with between 10 and 20% of the most radioactive SN. We would suggest a flexible threshold between 10 and 20% of the “hottest” SN. If seeking adherence to a 10% threshold complicates the procedure excessively, or generates an excessive number of nodal excisions, a 20% threshold could be used. The risk of subsequently failing to diagnose lymphatic metastases would be acceptably low (1.1%).