A 48-year-old woman undergoes sentinel lymph node biopsy for a 1.8 cm invasive breast carcinoma. The SLN shows a 0.3 mm cluster of tumour cells detected on IHC (AE1/AE3+) in the subcapsular sinus. According to the AJCC 8th edition TNM staging, how is this SLN finding classified?

• A Isolated tumour cells (ITC) — pN0(i+): deposits ≤0.2 mm or <200 cells; these do not change nodal staging and are classified pN0 ✓
• B Micrometastasis — pN1mi: deposits >0.2 mm but ≤2 mm, upstaging to pN1mi
• C Macrometastasis — pN1a: deposits >2 mm in the SLN
• D In-transit metastasis — tumour cells in lymphatics between primary and regional nodes

Explanation

AJCC 8th edition classifies SLN findings by size: isolated tumour cells (ITC) = clusters ≤0.2 mm or <200 cells — classified pN0(i+) and do not alter nodal stage. Micrometastasis = >0.2 mm and ≤2 mm, classified pN1mi, which upstages to N1. Macrometastasis = >2 mm, classified pN1a. This 0.3 mm cluster is >0.2 mm, which makes it a micrometastasis (pN1mi), not an ITC. Therefore, the correct answer is B — 0.3 mm exceeds the 0.2 mm ITC threshold. ITC classification is pN0(i+); this deposit at 0.3 mm is pN1mi and changes the N stage.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.