Incidence and Survival Outcomes in Patients with Synchronous Lung Metastases Upon Initial Metastatic Breast Cancer Diagnosis in China

Background: The incidence and survival of breast cancer lung metastases (BCLM) patients at initial diagnosis of metastatic breast cancer (MBC) remain poorly identied in China. Methods: We attained clinical data of 3161 MBC patients initially diagnosed between December 1991 and September 2019 from the China National Cancer Center and nally included 2263 MBC patients in this study, among which 809 patients presented with lung metastases at rst MBC diagnosis. The risk factors for BCLM were determined using multivariate logistic regression analysis and the prognostic factors of BCLM patients were assessed by univariate and multivariate Cox regression analyses. Results: Patients with triple-negative subtype (42.3%) harbored the highest incidence proportions of lung metastases. Age ≥ 50 years, ECOG 2 and triple-negative subtype were remarkably associated with higher incidence of lung metastases, while N3, liver and bone metastases were signicantly correlated with lower odds of lung metastases at diagnosis. The median survival of BCLM patients was 41.7 months, with triple-negative subtype experiencing the worst prognosis of 26.8 months. ECOG 2, N3, HR-/HER2+ subtype, triple-negative subtype, liver metastases and bone metastases were signicantly correlated with poor survival of BCLM patients. Conclusions: Our study provides essential information on clinicopathological features, incidence and survival outcomes of BCLM patients at initial diagnosis of MBC in China.


Background
Lung metastasis is the second most frequent distant metastases of breast cancer 1, 2 , clinically presenting in 15-25% of metastatic breast cancer (MBC) patients 3,4 . Autopsy data of 197 women dying with MBC over a period of 50 years revealed that 80.7% of patients had lung or pleura metastases 5 . A population-based study indicated that the median survival of 3372 patients with lung metastases at primary breast cancer diagnosis was 21 months 6 . Although the prognosis of MBC patients with metastases con ned to lungs is not so poor as brains or livers 7 , most patients are considered incurable and the treatment is still intractable. With an occult onset, lung metastases from breast cancer usually present asymptomatically and progress aggressively without appropriate care 8 . Systemic treatments including chemotherapy, targeted therapy and hormone therapy are recommended for patients with breast cancer lung metastases (BCLM) 9 and pulmonary metastasectomy is considerable for properly selected cases 10 . The early detection of lung metastasis and the precise estimation of outcome may bene t breast cancer patients in clinical practice, thus achieving long-term survival. However, the risk factors that affect the incidence and prognosis of BCLM remain unclear.
In this article, we explored the risk factors associated with the morbidity and mortality of BCLM in newly diagnosed MBC patients in China, which may help identify cases with higher odds of lung metastases and worse survival. Early intervention and multidisciplinary treatment for BCLM patients are of utmost importance.

Study population
We attained clinical data of 3161 MBC patients initially diagnosed between December 1991 and September 2019 from the China National Cancer Center. We removed patients with unknown tumor receptor status (n = 579), unknown distant metastases (n = 65) and follow-up no more than 1 month since the initial diagnosis of MBC (n = 254) from this cohort, nally leaving 2263 patients for incidence analysis. Among these, 809 cases presented with lung metastases upon initial MBC diagnosis. Telephone calls or clinical visits were used to follow up patients further to June 30, 2019 or date of their deaths.

Study Variables
Study variables, including age at initial MBC diagnosis, Eastern Cooperative Oncology Group (ECOG) grade, TNM stage of primary breast cancer, tumor receptor status, metastatic sites and overall survival (OS) from the onset of metastasis to death were retrospectively collected from medical records. Cancers with 1%-100% estrogen receptor or progesterone receptor routine immunohistochemistry (IHC) staining were considered hormone receptor-positive (HR+). Human epidermal growth factor receptor 2 (HER2) overexpression was de ned as IHC3 + or in the case of IHC2+, uorescent in-situ hybridization (FISH) positive. Breast cancer subtypes were divided as HR+/HER2-, HR-/HER2+, HR+/HER2 + and triple-negative (HR-/HER2-). Tumor staging was based on the 8th American Joint Committee on Cancer (AJCC) TNM staging system.

Statistical analysis
Chi-square or Fisher's exact test were used for category variables to compare the clinicopathological features among different subtypes in patients with lung metastases. Incidence of lung metastases was de ned as the number of BCLM patients divided by the total number of MBC patients. We performed multivariate logistic regression to explore factors associated with the presence of lung metastases upon initial diagnosis of MBC. We calculated odds ratios (ORs) and 95% con dence intervals (CIs) in the model. Kaplan-Meier method was utilized to estimate the survival within subsets and log-rank test was used to analyze the differences. We conducted univariate and multivariate Cox regression analyses to investigate the independent predictive factors signi cantly associated with the prognosis of BCLM patients. All the statistics were analyzed using SPSS statistical software version 23.0 package. A twosided P value of 0.05 or less was signi cantly different.

Survival
The median survival among the whole MBC cohort was 45.4 months, with a median follow-up of 61.6 months. Figure 1 showed that the prognosis of patients with lung metastases upon MBC diagnosis (median OS, 41.7 months) was signi cantly worse than those without lung metastases (median OS, 47.9 months, P = 0.001). Figure

Discussion
In this retrospective study, we analyzed the incidence and prognosis of patients with synchronous lung metastases at initial MBC diagnosis in China. We identi ed 809 patients with BCLM upon newly diagnosis of MBC, accounting for 35.7% of all MBC patients. Compared with other groups, patients with triple-negative subtype had the highest percentage of lung metastases, consistent with previous ndings [11][12][13][14] . The incidence of lung metastasis in triple-negative breast cancer (TNBC) could reach up to 40% 15 , similar with 42.3% in our data. Additionally, the prognosis of BCLM patients differed remarkably in tumor subtypes, varying between 26.8 months of triple-negative subtype and 49.0 months of HR+/HER2subtype.
Our study con rmed the results that TNBC was more aggressive and preferred to develop lung metastases. The molecular mechanisms underlying TNBC metastasis to lung might offer therapeutic targets for clinical prevention and management. Minn et al. 16 identi ed fascin as a mediator promoting basal-like breast cancer metastasis to lung, due to its close association with cell motility. Iriondo et al. 17 observed that inhibition of transforming growth factor-β1-activated kinase-1 (TAK1) could suppress lung metastasis in TNBC, which might provide a novel target for impairing TNBC lung metastasis. A single mutation on microrchidia family CW-type zinc nger 2 (MORC2) promoted TNBC lung metastasis by regulating heterogeneous nuclear ribonucleoprotein M (hnRNPM)-mediated CD44 splicing, which indicated that the knockdown of hnRNPM might reduce lung metastatic potential of TNBC cells with mutant MORC2 18 . Another research revealed that the overexpression of transcription and export complex 2 subunit (ENY2) could promote TNBC progression and lung metastasis both in vitro and in vivo 19 .
Further mechanisms clarifying TNBC lung metastasis are certainly worth exploring, which may provide potential targets for new drugs.
Our data also indicated that patients with older age and worse performance status were more likely to present with lung metastases at initial MBC diagnosis. The increasing risk of lung metastases associated with aging was consistently found in population-based studies 6,20 . On the contrary, previous studies observed that younger patients had a higher risk of liver metastases 5,21 . Interestingly, our study also revealed that patients with liver or bone metastases had lower odds of lung metastases at diagnosis. The predictive features associated with different metastatic sites may help clinicians distinguish patients with distinct organ-speci c metastases during the clinical practice. We also identi ed prognostic factors for survival of BCLM patients and found that worse performance status, later N-stage, HR-/HER2 + subtype, triple-negative subtype and the simultaneous presence of liver or bone metastases were signi cantly correlated with poor outcome. Multiple sites of rst metastases had signi cantly unfavorable prognosis than single site rst metastases [27][28][29] . In our data, the extrapulmonary metastases of bone and liver had 1.33 and 2.57 times of mortality risk than lung-only metastases at MBC diagnosis, respectively. Brain metastases also worsen the outcome of BCLM patients but the difference did not reach signi cance, probably due to the late onset of brain metastases during the clinical course, with an incidence of only 6.90-7.56% in newly MBC diagnosis patients [29][30][31] .
There were some limitations in our study. Firstly, discordance in tumor phenotype has been reported in multiple studies 32 , but we did not have enough information on the receptor status of metastatic tumors, which might cause some bias in the analysis of incidence and survival outcomes when strati ed by breast cancer subtype. Additionally, the number of lung lesions was an important risk factor for BCLM patients 33 , but it was not documented in detail in our database. Finally, the retrospective nature of this research and relatively small population require future studies to con rm the results.

Conclusions
Our study provides essential information on clinicopathological features, incidence and survival outcomes of BCLM patients at initial diagnosis of MBC in China. The risk factors identi ed here help to screen breast cancer patients with high odds of lung metastases and BCLM patients with high risk of mortality. The early detection of metastases and proper evaluation of prognosis in clinical practice are bene cial to optimize the disease outcomes.

Availability of data and materials
The data used in our study are accessible from the corresponding author on request.

Competing interests
The authors have no con ict of interest.

Funding
This study was supported by grants from National Key R&D Program of China (2018YFC1312101) and Chinese Academic of Medical Sciences Initiative for Innovative Medicine (CAMS-12M-1-010).