The authors applied the sixth edition of the American Joint Committee on Cancer (AJCC) classification system to their previously published group of patients with breast carcinoma who had immunohistochemically detected lymph node metastases.
The original lymph node-negative slides from 218 patients, including 129 patients with infiltrating duct carcinoma (IDC) and 89 patients with infiltrating lobular carcinoma (ILC), were reviewed and then destained to perform immunohistochemistry. Each tumor cell deposit was measured. Single tumor cells could not counted or measured (because the distance separating the most distant cells could be > 2.0 mm), but they were separated into Class 1 (sparse) and Class 2 (numerous). According to the AJCC classification, isolated tumor cells (ITCs) should be classified as pN0(i+) when they measure ≤ 0.2 mm and pN1mi when they measure ≤ 2.0 mm but > 0.2 mm.
ITCs were found in 13 IDCs (10%) and in 37 ILCs (41%) and corresponded in IDCs to a single deposit of a few tumor cells that measured ≤ 0.2 mm (associated with a single tumor cell in 3 tumors) and were classified as pN0(i+). In ILCs, ITCs corresponded to single tumor cells with an irregular distribution throughout the entire lymph node section (24 ITCs and 13 ITCs in Class 1 and Class 2, respectively) and were difficult to classify.
The results suggest that there are 2 categories of pN0(i+): measurable tumor cell deposits ≤ 0.2 mm, which were found exclusively in IDCs; and nonmeasurable ITCs, which were found in ILCs and rarely in IDCs. The new classification is difficult to apply to ITCs that are detected by immunohistochemistry in ILCs. Cancer 2005. © 2005 American Cancer Society.