A 48-year-old woman undergoes staging investigations for newly diagnosed invasive lobular carcinoma (ILC) of the right breast. Regarding the TNM staging and surgical management principles in breast cancer, all of the following are correct EXCEPT:

Explanation

## TNM Staging and Prognostic Factors in Breast Cancer ### Tumour Size Assessment (T Stage) **Key Point:** T stage is based on the **largest diameter of the invasive component only**. DCIS, if present, is not included in the T measurement. This is a critical distinction in staging accuracy. ### Lymph Node Staging (N Stage) **High-Yield:** Micrometastases are defined as: - **Size:** 0.2–2 mm (larger than isolated tumour cells [ITCs], which are <0.2 mm) - **Classification:** N1mi (micrometastatic involvement) - **Stage impact:** N1mi does **NOT** upstage to stage III; it remains in stage II if T1–T2 **Mnemonic:** ITC < 0.2 mm (isolated), Micro 0.2–2 mm (N1mi), Macro >2 mm (N1a/b/c) ### Prognostic vs. Staging Factors **Key Point:** ER, PR, and HER2 status are **prognostic factors** (predict outcome) but are **NOT part of the TNM staging system**. They influence treatment decisions and prognosis but do not change T, N, or M classification. **Clinical Pearl:** The TNM system is anatomic-based (size, nodes, metastases). Molecular markers inform prognosis and therapy selection but are separate from TNM staging. ### HER2 and TNM Staging **Warning:** HER2 status is **NOT a component of the TNM staging system**. It is a prognostic and predictive factor that guides HER2-targeted therapy (trastuzumab, pertuzumab) but does not determine TNM stage. This is a common misconception. | Factor | TNM Component? | Prognostic? | Therapeutic Impact? | | --- | --- | --- | --- | | Tumour size | Yes (T) | Yes | Yes | | Axillary nodes | Yes (N) | Yes | Yes | | Distant metastases | Yes (M) | Yes | Yes | | ER/PR status | No | Yes | Yes (endocrine therapy) | | HER2 status | No | Yes | Yes (HER2-targeted therapy) | | Ki-67 | No | Yes | Yes (therapy intensity) | ### Why Option 3 is Incorrect HER2 status is **not** a component of the TNM staging system and does **not** directly determine stage grouping. It is a prognostic and predictive factor that guides treatment selection (HER2-targeted agents) but remains separate from anatomic TNM classification. [cite:AJCC Cancer Staging Manual 8e, NCCN Breast Cancer Guidelines 2023]