The protocol was approved by the Ethics Committee of Nanjing First Hospital, Nanjing Medical University (approval number: KY20230615-02-KS-01) on June 26th, 2023. Initially, a total of 150 early breast cancer patients hospitalized for surgery herein between January 2020 and September 2022 were identified, including 80 patients in the observation group (breast-conserving surgery + sentinel lymph node biopsy) and 70 patients in the control group (breast-conserving surgery + axillary lymph node dissection). After propensity score matching using the nearest-neighbor method with a caliper width of 0.2 to ensure robust comparability, 55 matched patients were included in each group for the final analysis. The key baseline characteristics (age, tumor size, etc.) were evaluated to confirm that the two groups were well-matched, without statistically significant differences. To protect patient confidentiality, the following measures were implemented: data anonymization, confidential storage, minimization of identifiable information, and secure communication.

In the control group, the diameter of tumor was 2 cm in 23 cases and $\ge$2 cm in 32 cases. The lesions were located in the inferior medial breast in 4 cases, superior medial breast in 7 cases, inferior lateral breast in 10 cases, and superior lateral breast in 19 cases. Histologically, there were 32 and 8 cases of invasive ductal carcinoma and invasive lobular carcinoma, respectively. Among them, American Joint Committee on Cancer (AJCC) stage Ⅰ in 29 cases and stage II in 11 cases were detected. In the observation group, the diameter of tumor was 2 cm in 24 cases and $\ge$2 cm in 31 cases. The lesions were located in the inferior medial breast in 3 cases, superior medial breast in 6 cases, inferior lateral breast in 11 cases, and superior lateral breast in 20 cases. Histologically, invasive ductal carcinoma and invasive lobular carcinoma were recorded as 29 and 11 cases, respectively. Among them, 30 cases were in AJCC stage Ⅰ and 10 cases were in stage II. All enrolled patients were classified according to the classification criteria for BC in the Diagnosis and Treatment Guidelines for Breast Cancer (2011 Edition) [6].

The under-mentioned inclusion criteria were adopted: (1) patients eligible for the criteria for BC diagnosis, (2) those with lesions less than 3.0 cm in size, (3) those with single lesions $\ge$2 cm away from the areola margin, and (4) those who could tolerate the operation and had no contraindication to intraoperative anesthetics.

The exclusion criteria involved: (1) patients with non-primary BC, (2) those with severe damage to the heart, cerebrovascular system, liver, kidney or other organs, or (3) those with local lymph node metastasis or distant organ metastasis. All patients were informed of details of the treatment and signed surgical protocols.

Axillary lymph node dissection + breast-conserving surgery were implemented for the control group. Under general anesthesia and intubation, the patient lay flat, with the upper limb abducted by 90°. The breast shape and weight and the location and size of tumors were detected in advance. The tumor was excised. With the nipple as a plane, the breast skin surface was made with an arc incision if the tumor was above the plane, while the breast skin surface was made with a radial incision if the tumor was below the plane. The excision extent was at least 2 cm away from the tumor boundary to ensure that the tumor tissue was completely removed. The incisal margin was sent for pathological biopsy, and tissue excision was repeated if it was tested positive. Routine axillary lymph node dissection: An oblique incision parallel to the axillary fold line was made for ipsilateral axillary lymph node dissection, with the anterior end inside the lateral edge of the pectoralis major and the posterior end not exceeding the lateral edge of the latissimus dorsi. If the armpit was narrow, a U-shaped incision was made. Group Ⅰ lymph nodes (i.e., subaxillary group: lateral to pectoralis minor, including lateral mammary, central, subscapular and axillary venous lymph nodes, and lymph nodes between pectoralis major and pectoralis minor) and group Ⅱ lymph nodes (i.e., midaxillary group, including axillary venous lymph nodes deep to pectoralis minor) were dissected during operation.

The observation group was treated with breast-conserving surgery + sentinel lymph node biopsy. Under general anesthesia, breast-conserving surgery was conducted by the same surgeons in the same way as that for the control group. Meanwhile, axillary lymph node biopsy with nano-carbon suspension was performed. Specifically, 0.4–0.6 mL of nano-carbon suspension was injected into the breast tissue at the direction of 3, 6, 9 and 12 o’clock. The injection site under the skin at the direction of 12 o’clock or near the intraoperative biopsy site was gently massaged to make the staining solution move faster. About 10 min after the staining solution was injected, the pectoralis major muscle layer was carefully incised at the level of the third rib to find the stained lymphatic vessels from bottom to top and from inside to outside. After the sentinel lymph nodes were identified, all the stained lymphatic vessels were removed, and intraoperative tissue sections and postoperative paraffin sections were prepared. Sentinel lymph nodes were evaluated on the basis of rapid pathological examination, and axillary lymph nodes were dissected if they were tested positive.

Comparisons and analyses were conducted on the extubation time, intraoperative blood loss, mean operation time, and postoperative drainage volume.

The World Health Organization Quality of Life Scale was employed to assess patients’ quality of life, mainly covering cognitive function, social function, emotional function, physical function and general health, with 100 points for each item. The higher the score was, the better the quality of life would be [11].

The Functional Assessment of Cancer Therapy-Breast (FACT-B) was applied for evaluation of patients’ social adaptability. The range of FACT-B scores was 0–144 points, and a higher score indicated stronger social adaptability.

Elbow venous blood (10 mL) was collected from all patients before and at 72 h after operation, and centrifuged at 4000 r/min to obtain the supernatant. Then enzyme-linked immunosorbent assay was carried out to determine the levels of PCT and TNF-$\alpha$, and those of serum CA125 and CEA were measured by chemiluminescence assay and radioimmunoassay, respectively.

The postoperative cosmetic effect (excellent, good and poor) was evaluated according to the Joint Center for Radiation Therapy criteria [12].

Human epidermal growth factor receptor-2 (HER-2) expression was determined by staining of cancer cell membrane: (+++), (++), (+) and (–).

Statistical analysis was completed by means of SPSS 19.0 software (IBM Inc., Armonk, NY, USA). The expression format of mean $\pm$ standard deviation ($\overline{x}$ $\pm$ SD) was selected for continuous variables, and the independent samples t-test for normally distributed data was adopted for between-group comparisons. The Shapiro-Wilk test was performed to appraise the normality of distribution. In case of non-normally distributed data, the Mann-Whitney U test was used as a non-parametric alternative. Relative frequencies (percentages) were applied to describe categorical variables, which were subjected to the Chi-square ($\chi$²) test or Fisher’s exact test for comparison. The rank-sum test was used to compare the rank data. Multiple comparisons were conducted across multiple variables, and potential type I errors were adjusted through the Bonferroni correction. p 0.05 signified a difference of statistical significance.

The observation group had significantly shortened extubation time and operation time, as well as decreased intraoperative blood loss and postoperative drainage volume contrasted with the control group (p 0.05) (Table 1).

Before operation, TNF-$\alpha$ and PCT levels showed no significant between-group differences (p $>$ 0.05). After operation, TNF-$\alpha$ and PCT levels significantly declined in the two groups, with significant inter-group differences (p 0.05) (Table 2).

The levels of CEA and CA125 in the two groups were not significantly different before operation. Both levels significantly declined after operation, and by contrast to those in the control group, such levels presented significant decreases in the observation group (p 0.05) (Table 3).

Compared to the control group, the observation group displayed significantly raised scores of cognitive function, social function, emotional function, physical function and general health (p 0.05) (Table 4).

The two groups were not significantly different in terms of FACT-B score before operation. However, the FACT-B score rose significantly in both groups at 6 months following operation in comparison with that before operation, and the observation group, contrasted with the control group, had a significantly higher score (p 0.05) (Table 5).

The observation group had 32 excellent cases, 17 good cases and 6 poor cases. One patient died, one patient relapsed, and one patient underwent bone metastasis. Totally 23 excellent cases, 10 good cases and 22 poor cases were recorded in the control group. Two patients died, two patients relapsed, and two patients underwent bone metastasis. The good appearance rate had a statistically significant between-group difference (89.09% vs. 60.0%) (p 0.05). No significant differences were observed between the control group and the observation group regarding the postoperative mortality rate, recurrence rate and metastasis rate (1.82% vs. 3.64%, 1.82% vs. 3.64%, 1.82% vs. 3.64%) (p $>$ 0.05) (Table 6).

In the observation group, 18 cases were HER-2 (+), 2 cases were HER-2 (++), 14 cases were HER-2 (+++), and 21 cases were HER-2 (–). The control group was detected with 6, 5, 26 and 18 cases of HER-2 (+), HER-2 (++), HER-2 (+++) and HER-2 (–), respectively. The difference in the expression of HER-2 between the two groups was of no statistical significance (Z = 0.881, p = 0.379).