AbstractBackgroundThe axillary nodal status may influence the prognosis and the choice of adjuvant treatment of individual breast cancer patients. The variation in number of reported axillary lymph nodes and its effect on the axillary nodal stage were studied and the implications are discussed.MethodsBetween 1994 and 1997, a total of 4,806 axillary dissections for invasive breast cancers in 4,715 patients were performed in hospitals in the North‐Netherlands. The factors associated with the number of reported nodes and the relation of this number with the nodal status and the number of positive nodes were studied.ResultsThe number of reported nodes varied significantly between pathology laboratories, the median number of nodes ranged from 9 to 15, respectively. The individual hospitals explained even more variability in the number of nodes than pathology laboratories (range in median number 8–15, P < 0.0001). The number of reported nodes increased gradually during the study period. A decreasing trend was observed with older patient age. A higher number of reported nodes was associated with a markedly increased chance of finding tumor positive nodes, especially more than three nodes. The frequency of node positivity increased from 28% if less than six nodes to 54% if ≥20 nodes were examined, the percentage of tumors with ≥4 positive nodes increased from 4 to 31%. Multivariate analysis confirmed these results.ConclusionsThis population‐based study showed a large variation in the number of reported lymph nodes between hospitals. A more extensive surgical dissection or histopathological examination of the specimen generally resulted in a higher number of positive nodes. Although the impact of misclassification on adjuvant treatment will have varied, the impact with regard to adjuvant regional radiotherapy may have been considerable. J. Surg. Oncol. 2004;87:4–12. © 2004 Wiley‐Liss, Inc.