Is the "10% rule" equally valid for all subsets of sentinel-node-positive breast cancer patients?

In breast cancer, a combination of radioisotope and blue dye mapping maximizes the success and accuracy of sentinel node (SLN) biopsy. When multiple radioactive nodes are present, there is no single definition of isotope success, but the popular "10% rule" dictates removal of all SLN with counts >10% of the most radioactive node. Here we determine how frequently a positive SLN would be missed by the 10% rule.

Between 9/96 and 12/04, we performed 6,369 successful SLN biopsies using (99m)Tc sulfur colloid and isosulfan blue dye, removing as SLN all radioactive and/or blue nodes, and taking counts from each node ex vivo. Standard processing of all SLNs with a benign frozen section included hematoxylin and eosin (H&E) staining, serial sectioning, and immunohistochemistry (IHC).

33% of patients (2,130/6,369) had positive SLNs. Of these patients, 1,387/2,130 (65%) had >1 SLN identified. The most radioactive SLN was benign in 29% (398/1,387), and 107/1,387 (8%) had a positive SLN that was neither blue nor the hottest. From this group 1.7% (24/1387) of patients had positive SLN with counts <10% radioactive counts of the hottest node. The 10% rule captured 98.3% of positive nodes in patients with multiple SLNs. No patient characteristics were predictive of failure of the 10% rule.

With combined isotope and blue dye mapping, the 10% rule is a robust guideline and fails to identify only 1.7% (24/1387) of all SLN-positive patients with multiple SLNs. This guideline appears to be equally valid for all subsets of patients.