Breast cancer is currently the most frequent malignancy and one of the leading causes of cancer death in the United States (estimated 279,100 new cases and 42,690 death) (1) and China mainland (estimated 304,000 new cases and 70,000 deaths) (2). Although the long-term survival of patients with breast cancer has been significantly increased in the past years with the application of targeted therapy (3), endocrine therapy (4), and even immunotherapy (5, 6), distant metastasis (DM), as the most common form of recurrence and the main cause (approximately 90%) of death, could reverse this favorable outcome (7, 8). Historically, the “Halsted” hypothesis indicated that the processing steps of breast cancer metastasis were mechanized and orderly, including primary focus enlargement, invasion to the regional lymph nodes, and further metastasis to distant organs via the bloodstream. However, subsequent studies on the biological characteristics of breast cancer metastasis have shown that the DM in breast cancer was a non-random process as it allowed circulating tumor cells (CTC) to seed at specific distant tissues, which suggested the metastasis did not require circulation through the lymph system but directly invade the distant organs via the bloodstream. Consequently, the CTC analysis technique has become a novel utility tool for predicting the prognosis of breast cancer patients, which could provide better treatment guidance for clinicians (9, 10).

Indeed, as a key component of tumor stage classification, the status of the regional lymph nodes plays an important role in predicting the biological aggressiveness and propensity to spread in patients with breast cancer (11, 12). Some scholars believe that regional nodal disease may precede metastatic dissemination (11). Therefore, after surgery, patients with negative lymph node status could remain a favorable outcome, and only a small fraction of them need adjuvant therapy during the postoperative follow-up (11). Additionally, reviewing the recent literature, negative lymph node status was frequently referred to as the “control group” in the study when scholars aimed to explore the risk factors of DM (13–16). Patients with negative lymph node status were more likely to be assigned to the low-risk group. However, one thing that cannot be ignored was that there were still a considerable proportion of patients screened out having DM but negative lymph node status (17). The insufficient adjuvant therapy and management for this population might increase the risk of relapse in those lymph-node-negative (N0) patients with multiple risk factors. And clinicians may underestimate the risk of relapse and make insufficient treatment for N0 patients with breast cancer.

Therefore, it is equally important to identify the independent risk factors of DM in this particular subpopulation, which would not only help oncologists to begin tailoring treatment strategies to patients but also encourage researchers to investigate the underlying molecular mechanisms in breast cancer metastasis. Although some scholars have made efforts on evaluating the DM in lymph node negative primary breast cancer via evaluating the gene expression profiles and the integration of proliferation and immunity (17, 18), whether there was a different clinical pattern between DM and non-DM patients without lymph node involvement was still unclear.

In the present study, we aimed to extract the potential risk clinicopathological factors in promoting DM of N0 primary breast cancer, which would fill the gap in identifying high-risk subgroups. Besides, we also evaluated the cancer-specific survival (CSS) in this subpopulation and further developed a novel predictive model to provide quantitative predictions on the outcome for N0 patients with DM. More aggressive treatment modalities and active surveillance may be justified in high-risk subgroups of patients.

Nowadays, breast cancer has become the most frequent malignancy among women worldwide (1, 2, 20). While the overall survival (OS) rate in breast cancer patients has improved with the help of early-detection and multiple treatment modalities, patients who were diagnosed with DM at presentation still underwent a worse prognosis. In the last decades, great advances have been achieved in understanding and detecting breast cancer metastasis. Breast cancer was no longer regarded as a locoregional but systemic disease with an inherent feature of metastasis (8). There is no doubt that regional lymph node involvement is one of the important predictive factors in breast cancer DM. Even some scholars suggested and validated that regional node metastasis could precede metastatic dissemination (11). Notably, a considerable number of N0 patients were observed occurring de novo DM. With the wide application of circulating tumor cell (CTC) analysis, many scholars recognized that the DM was considered triggered by hematogenous spread of CTCs, rather than by lymphatic or direct intracavitary spread, which possibly occurred by a different mechanism. For this reason, breast cancer patients without regional lymph node metastasis but distant organ invasion would be the objects for exploring the underlying mechanisms.

However, only a few previous studies could be reviewed in predicting the risk factors and the prognosis of N0 patients (17, 18, 21). Herein, we provided a new insight in exploring whether there was a significant difference between N0 patients with DM or not, and the prognosis of those patients with DM was also evaluated. In this study, the incidence rate of DM in N0 patients was about 1.34% (1,069/79,746). Several clinicopathological factors including age at diagnosis, tumor size, race, tumor location, differentiation grade, histology, and subtype were significantly associated with DM. Younger patients (especially <40 years) have nearly twice the risk of DM than elderly patients, which was in accordance with Sabiani’s report (22). Consistent with previous studies on evaluating the risk factors of DM in patients with invasive breast cancer, patients with tumor size (>10 mm), ILC, estrogen receptor (ER), and progesterone receptor (PR) as well as Her-2-positive subtype, and Black race ( Supplementary Figure S2 ) presented a higher risk of DM (12, 14, 21). In terms of the tumor location, it has been determined that tumor location was significantly associated with the regional lymph node metastasis, especially when the tumor originated from the nipple and central location as well as overlapping of the breast (23–26). We took it a step further that the nipple and central tumor locations were identified had a higher risk of DM (p<0.0001) in N0 women. Despite that we have discovered seven independent risk factors associated with DM in N0 patients, further studies are needed to verify the underlying molecular mechanisms in promoting this complex process.

Notably, some researchers have conducted to explore the risk factors of DM and the prognosis of patients with DM at presentation (7, 13, 15, 16, 27). For instance, Rosa Mendoza determined that tumor stage, primary tumor size, and lymph node involvement were the major predictors of DM in adult breast cancer (14). Besides, the Black race and Her-2-enriched subtype were also identified as the risk factors of DM in a recent study (28–30). In the present study, we explored the prognostic factors of 1-, 3-, and 5-year CSS among 748 N0 patients with DM. Although the N0 women at a young age were more likely to have DM, compared with elderly women, the young population, however, had better long-term outcomes than the elderly population (HR= 1:1.58; 95% CI: 1.03–2.44). This result was consistent with one recent large population-based epidemiological study in Brazil that young women had a lower rate of modality (31). On the contrary, in another study by Sabiani and colleagues, they concluded that patients at a young age (<35 years) had the lower estimated disease-free survival (DFS) and OS rate (22). These discrepancies might be due to the differences in sample size and patient inclusion criteria. For example, all included patients in their study were under 50 years old, while the patients in ours were at the age between 20 and 79 years old combined with negative lymph node status. Additionally, we determined five independent risk factors in the poor CSS probability of N0 patients with DM.

Moreover, the role of surgical treatment for the primary focus is regarded as a palliative surgery for patients with DM, and whether patients with DM can benefit from it remains controversial (32–35). One meta-analysis derived from two randomized controlled trials presented that there was no final conclusion about the role of surgery performed in breast cancer patients with DM at presentation (35). With further exploration, some studies, including the present study, found that locoregional surgery would improve the CSS and OS outcomes of metastatic breast cancer (15, 32, 34, 36). Indeed, there were still many questions on the discussion of the timing, type, and extension of the surgical procedures, which needed to be addressed in future works (33). Noticeably, compared with the previous study on evaluating the prognostic factors for patients with DM, primary tumor size (p=0.123) and grade differentiation (p=0.101) were not significantly associated with the CSS in the N0 population. In a similar studied population, Yu and his colleagues (29) determined that the larger tumor size was non-linear with the DM in N0 patients. They consequently believed the primary tumor biological features rather than the accumulated metastatic ability during tumor evolution likely determined the potential of distant dissemination, which indicated the indolent biological characteristics of the tumor. Accordingly, our results support this hypothesis but need further evaluation.

To visualize and more intuitively present the prognostic factors we determined for clinical use, the nomogram model was subsequently plotted. Markedly, in the nomogram, the breast cancer subtype accounted for a major part of the scoring system. Referencing similar nomograms for evaluating the prognostic of breast cancer (37, 38), the TNBC subtype was determined to yield the highest score. Consequently, the clinicians could obtain the risk coefficient in 1-, 3-, and 5-year CSS probability. Compared with other recent works on evaluating the 3- and 5-year CSS in breast cancer women with bone metastasis, the C-index of the present nomogram was 0.694, which was higher than Liu’s (0.660) (16) and very close to the C-index of nomograms developed by Wang (0.705) (15) and Zhao (0.723) (37), confirming the promising discrimination of our model. To evaluate the accuracy of the nomogram, an independent cohort was subsequently used for validation. Expectedly, the AUC of the 1-, 3-, and 5-year CSS predicting ROC in the validation cohort reached 0.725, 0.695, and 0.699, respectively, which further proved the utility of our model to be applied to access the long-term CSS in this subpopulation. Besides, compared with the study of Wang and colleagues (17), the number of N0 patients in the training cohort of the present study was considerably large (748 vs 286). Different from previous studies of N0 patients with a focus on gene expression profiles (17, 18), we provide a new insight in accessing the individual risk of DM and long-term CSS probability in N0 patients based on the clinicopathological characteristics. For instance, a 65-year-old black woman was diagnosed with HER2+ tumor, with only bone involvement. This patient would have a total of 125 points and an estimated 1-year, 3-year, and 5-year CSS of 84, 62, and 41% probability after surgery.

Alternatively, this study has some limitations that have to be addressed in the future works. First, this is a retrospective study in which selection bias inevitably exists. Second, while the SEER database contains approximately 28% population-based cancer registration data, some significant confounding prognostic factors including but not limited to Ki-67 index (39), BRCA1- and BRCA2-related mutation (40, 41), as well as high 21-Gene Recurrence Score (21-GRS) (42), which have been proved to be related to worse survival in patients with breast cancer, are unavailable in the SEER database. Third, further information about adjuvant management of these patients was not reported in the present study, as these data were limited in the SEER database. Consequently, future works are supposed to fill this gap to get robust clinical evidence. Besides, with the technical advances in multidisciplinary management, the CSS in patients with breast cancer would increase in the future, which could influence the predictive ability of the model. Lastly, another weakness of this study is the lack of an external validation cohort, which limits further enforcing the reliability and clinical application of the nomogram. Thus, more external validation cohorts from multicenter and countries are urgently demanded to further evaluate the feasibility of our nomogram.

In summary, this study first identified the potential risk clinicopathological characteristics of DM in N0 patients and the prognostic factors in patients with DM at presentation. N0 patients with younger age at diagnosis, larger tumor size, central tumor location, Black race, poorer differentiated grade, ILC, and luminal B subtype have the highest risk of DM, which could help clinicians to avoid underestimating the risk of DM and subsequent undertreatment in N0 patients. However, DM patients with elderly age at diagnosis, TNBC subtype, and multiple metastasis sites have the worst prognosis. Besides, the novel validated nomogram could help clinicians to better stratify patients who are at high risk of cancer-specific death for receiving relatively aggressive treatment and management. Meanwhile, we propose more external validation to further strengthen our findings.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

Ethical approval was waived by the local Ethics Committee of the Chongqing Medical University in view of the retrospective nature of the study and all the procedures being performed were part of the routine care.

All authors contributed to conception and design of the study. YM, XL, and HC organized the database. YM, YF, JC, KX, GY, and YH performed the statistical analysis. All authors wrote the first draft of the manuscript. All authors wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported in part by the National Natural Science Foundation of China (NSFC No. 82072938) for HL.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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