Given a greater probability of lymph node metastasis in early-stage breast cancer with lower tumor-infiltrating lymphocyte (TIL) density, researchers examined the link between TILs and lymph node metastasis in cT1 breast cancer patients receiving surgery. They also determined whether TILs are beneficial in predicting sentinel lymph node metastasis (SLNM) in cT1N0M0 breast cancer. They examined 332 cases of breast cancer managed with surgery as the first-line treatment following preoperative diagnosis of cT1. A median of 2 (range, 1–8) SLNs (sentinel lymph nodes) were pathologically assessed for SLN biopsy. Experts observed 60 cases (19.4%) of SLNM (macrometastasis: 46, micrometastasis: 16). In breast cancers with tumor diameter > 10 mm, a significantly greater metastasis was reported compared with those with diameter ≤ 10 mm. Overall, in cT1 breast cancer, lymph node metastasis was found to be correlated with tumor immune-microenvironment. Also, a possible role of TIL density was suggested as a predictor of SLNM in breast cancer without lymph node metastasis on preoperative imaging.

Breast cancer frequently metastasizes to the axillary lymph nodes, and the status of axillary lymph nodes metastasis is a prognostic factor in early breast cancer. Sentinel lymph node (SLN) biopsy (SLNB) is commonly used for pathological evaluation even if axillary lymph node metastasis is not detected on imaging. SLNB is considered a minimally invasive method based on the results of previously reported randomized controlled trials [1, 2]. However, in recent years, SLNB is being considered excessively invasive for breast cancer patients with a small primary tumor because it is unlikely to have metastasized [3]. Therefore, clinical trials that omit SLNB for cN0 breast cancer patients diagnosed by ultrasonography (US) are underway [4, 5]. One of the prospective randomized trials targeted cT1 breast cancer patients and the other trial targeted small primary tumor that could be resected with breast-conserving surgery. However, to summarize the previous reports, the SLN metastasis (SLNM) rate in T1 breast cancer was 18.8–29.6%, which is substantial [6,7,8,9,10]. These studies have additionally reported various predictors of SLNM.

The tumor microenvironment, comprising cancer-associated fibroblastic cells, angiogenic vascular cells, and infiltrating immune cells, is strongly involved in cancer invasion and metastasis [11, 12]. Among these cells, lymphocytes around tumors, the so-called “tumor-infiltrating lymphocytes (TILs)”, are used as a simple indicator of tumor-related immune response. It has been suggested that TILs may also affect cancer invasion and metastasis [11]. However, in breast cancer, TILs are strongly affected by the subtype of breast cancer. Hormone receptor-negative breast cancers such as human epidermal growth factor receptor 2 (HER2)-enriched breast cancer (HER2-enriched BC) and triple-negative breast cancer (TNBC) are known to have higher TIL density than hormone receptor-positive breast cancers [13, 14].

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