Variations in sentinel node isolated tumour cells/micrometastasis and non-sentinel node involvement rates according to different interpretations of the TNM definitions

Breast cancers with nodal isolated tumour cells (ITC) and micrometastases are categorised as node-negative and node-positive, respectively, in the tumour node metastasis (TNM) classification. Two recently published interpretations of the TNM definitions were applied to cases of low-volume sentinel lymph node (SLN) involvement and their corresponding non-SLNs for reclassification as micrometastasis or ITC. Of the 517 cases reviewed, 82 had ITC and 435 had micrometastasis on the basis of one classification, and the number of ITC increased to 207 with 310 micrometastases on the basis of the other. Approximately 24% of the cases were discordantly categorised. The rates of non-SLN metastases associated with SLN ITCs were 8.5% and 13.5%, respectively. Although the second interpretation of low-volume nodal stage categories has better reproducibility, it may underestimate the rate of non-SLN involvement. The TNM definitions of low-volume nodal metastases need to be better formulated and supplemented with visual information in the form of multiple sample images.