The Effect of HER2 Status on Metaplastic Breast Cancer A Propensity Score-Matched Analysis

Background The role of human epidermal growth factor receptor 2 (HER2) in metaplastic breast cancer (MBC) patients remains unclear. The present study aimed to evaluate the effect of HER2 status on MBC patients by propensity-score matching (PSM). Methods The SEER data from 2010 to 2016 were extracted. The breast cancer-specific survival (BCSS) of MBC patients, diagnosed from 2001 to 2016, was compared using Kaplan–Meier analysis. The multivariate Cox proportional model between groups was performed. PSM was used to make 1:1 case-control matching. Results We included 1887 patients with a median follow-up time of 28 months (range 1-83 months). 1749 (92.7%) and 138 (7.3%) patients presented in the HER2-negative group and HER2-positive group. 833 (44.1%) patients received post-mastectomy radiotherapy (PMRT). The HER2-positive group had younger patients, lower tumor grades, and more advanced tumor stages. The prognoses were related to age of diagnosis, race/ethnicity, TNM stage, and PMRT in multivariate Cox analysis. ER status and HER2 status had no impact on BCSS. In the Kaplan-Meier analysis, PMRT was associated with a better prognosis. Importantly, patients with HER2-negative status can benefit from PMRT, but not those with HER2-positive status. After PSM, on multivariate Cox analysis, the prognosis was related to HER2 status and PMRT. In the Kaplan-Meier analysis, PMRT was related to a better prognosis for HER2-negative patients. Conclusions Our findings supported that PMRT and HER2-positive status were associated with a better prognosis after PSM. However, HER2-negative, but not HER2-positive patients could benefit from PMRT.


BACKGROUND
Metaplastic breast cancer (MBC) was rare and the World Health Organization identified it as a unique pathological type in 2000 (1). MBC is a rare histologic subtype, accounting for about 2-5% of breast cancer (2). It was classified into 5 subtypes: squamous cell carcinoma, spindle cell carcinoma, matrix-producing carcinoma, carcinosarcoma, and metaplastic carcinoma with osteoclastic giant cells (3)(4)(5)(6)(7). With the improvement of pathologists' awareness of MBC, the incidence also increases (8). However, due to its rarity, the role of human epidermal growth factor receptor 2 (HER2) status in the treatments and prognoses of MBC is unclear.
Of note, although treatments of MBC are parallel to that of infiltrating ductal carcinoma (IDC) (9), the prognosis of MBC patients was worse than that of IDC even after receiving comprehensive treatment (10,11). However, there is no consensus on post-mastectomy radiotherapy (PMRT) in the management of MBC. On the one hand, some researchers reported that PMRT of patients showed a better prognosis than that non-PMRT (9,(12)(13)(14)(15)(16)(17). On the other hand, others debated that no connection was presented between PMTR and outcomes (18)(19)(20)(21). The management strategy and sample sizes of the study populations may result in this conflict.
HER2-positive (HER2 +) status in traditional breast cancers is an aggressive disease related to drugs resistance, regional recurrence, metastases, and outcomes (22). It had been proved that HER2+ patients that underwent radiotherapy and anti-HER2 therapy had better survival outcomes (23). However, our published report showed that HER2 + patients diagnosed with MBC receiving RT had not a superior breast cancer-specific survival (BCSS) than that not RT (24). This discrepancy may be due to several reasons. Firstly, the characteristics of MBC are different from traditional breast cancer. Secondly, there is no study to explore the role of HER2 status in MBC patients underwent PMRT. Therefore, to improve the comprehensive treatment of MBC, it is urgent to explore PMRT. Based on the above factors, the information of MBC patients was extracted from the Surveillance, Epidemiology, and End Results (SEER) registry to explore the effect of PMRT on MBC patients under different HER2 statuses.

Patients
Data from 2010 to 2016 were obtained from the SEER database. The demographic and clinicopathological information was obtained from the database. The international classification of diseases for oncology Version 3 (ICD-O-3) codes identified the  metaplastic histology, including 8560, 8562, 8570-8572, 8575,  and 8980-8982 (24-26). Finally, 1887 patients were included. Figure 1 showed the inclusion criteria.
The BCSS, defined as the date of diagnosis to the date of death from MBC, was considered the primary clinical outcome in our study.

Detection the Status of ER and HER2
In the SEER database, 1) If ER was reported on multiple tumor specimens, the highest value was recorded; 2) In case sample had any positive, that record is positive; 3) If ER status of all tested invasive specimens was negative, the status of ER was negative whatever ER status was in situ specimen; 4) The criterion of ERpositive status was that ≥ 1% cells stained positive; 5) HER2 negative status was defined as staining with a score of 0/1+ by IHC; 6) HER2 positive status was defined as staining with a score of 3+ by IHC; 7) The score of 2+ was interpreted as equivocal. The test of fluorescence in situ hybridization or silver in situ hybridization order to be performed. Only when the ratio of HER2 to CEP17 was >2.2, the results of HER2 amplification was interpreted as positivity.

Ethics Statement
Since the patient information in the SEER database has been deidentified, the study was exempted from the approval process of the institutional review committee. In addition, consent papers are not applicable.

Statistical Analysis
The differences between groups were analyzed by the c2 test. The univariate Cox proportional hazards model was implemented to evaluate the risk factors of BCSS, and then the variables with P-value < 0.1 and with clinically valuable were included in the multivariate Cox proportional hazards model. The Kaplan-Meier method plotted Survival curves, and the difference between the two group was tested by log-rank. Hazard ratios were showed with 95% confidence intervals (CIs). All statistical analyses were conducted using SPSS (version 24.0; IBM, Armonk, New York, USA). P < 0.05 was statistically significant.
Because of the retrospective design, there was a selection bias when patients were divided into HER2-negative and HER2-Abbreviations: MBC, Metaplastic breast cancer; HER2, Human epidermal growth factor receptor 2; IDC, Invasive ductal carcinoma; PMRT, Post-mastectomy radiotherapy; SEER, Surveillance, Epidemiology, and End Results; ICD-0-3, International Classification of Diseases for Oncology Version 3; PMCT, Postmastectomy chemotherapy; BCSS, Breast cancer-specific survival; ER, Estrogen receptor; HRs, Hazard ratios; CI, Confidence interval. positive groups. We compared the clinical and pathologic parameters between the groups and found that those parameters were different, including age of diagnosis, tumor grade, TNM, and PMCT. To reduce the confounding factors and treatment selection bias, propensity score matching (PSM) was conducted (27).
The characteristics of clinical and pathological between the two subgroups were showed in Table 1. Compared with HER2negative tumors, HER2-positive tumors were not different concerning race/ethnicity, tumor histology, tumor size, regional node involvement, and PMRT, but HER2-positive patients received more PMCT (P < 0.001). HER2-positive tumors had younger patients (HER2-negative 42.4% vs. HER2positive 53.6%, P = 0.007) and had higher tumor grade (P < 0.001) than HER2-negative tumors. After PSM, no difference existed between the two groups ( Table 1).

Prognostic Factors Associated with BCSS
Univariate analysis showed that those parameters were associated with BCSS, including the age of diagnosis, race/ ethnicity, insurance, tumor histology, tumor size, and regional node involvement. HER2 status was not related to better BCSS. Interestingly, patients could benefit from PMRT but not PMCT ( Table 2). After PSM, PMRT also benefits for MBC patients. Tumor size, regional node involvement were associated with a better BCSS.
the multivariate Cox proportional hazards model was conducted to explore the independent prognostic factors related to BCSS. the results showed that HER2 status was not associated with better BCSS (hazard ratio [

Kaplan-Meier Analysis for Patients Undergoing PMRT
The median follow-up time in the HER2 negative group was 28 months (range 1-82 months), and the median follow-up time in the HER2 positive group was 29 months (range 1-78 months). 5year survival rate in patients receiving PMRT was 78.0% and 74.2% in patients not receiving PMRT (P = 0.001, Figure 2A). After PSM, the median follow-up time in the HER2-negative group was 28 months (range, 1-82 months) and the median follow-up time in the HER2-positive group was 29 months (range, 1-78 months). 5-year survival rate was 83.4% in patients receiving PMRT and 64.1% in patients not receiving PMRT (P < 0.001, Figure 2B)

Subgroup Analysis for the Role of HER2 Status in PMRT
To explore the effect of PMRT on MBC patients under different HER2 statuses, this study conducted a subgroup analysis. Patients receiving PMRT had a higher survival rate when HER2 was negative than patients not receiving PMRT in the Kaplan-Meier analysis (P = 0.017, Figure 2C) even though after PSM (P = 0.006, Figure 2D). When HER2 status was positive, patients receiving PMRT had no better survival than those without PMRT (P = 0.298, Figure 2E) After PSM, HER2-negative patients could benefit from PMRT. However, HER2-positive patients undergoing PMRT were not associated with better prognoses. (P = 0.084, Figure 2F).

DISCUSSION
Our study explored the role of PMRT in the prognosis of MBC patients and verified the effectivity of HER2 status in prognosis.
After propensity score matching, our results showed that PMRT and HER2-positive status were associated with a better prognosis. However, only HER2-negative patients could benefit from PMRT. The effectiveness of radiotherapy (RT) on MBC is still controversial. Jung et al. (28) reported that RT was not associated with a better prognosis. Those patients' information was extracted from the Center for Breast Cancer Database and they diagnosed from 2001 to 2008. However, only 35 patients were diagnosed with MBC in those studies. Cecilia et al. (15) included stage I-III MBC patients diagnosed from 2010 to 2014. They illustrated that RT was associated with improved survival. The reasons for this effect could be the fact that, firstly, the sample size of the study varies greatly. Secondly, different eras might exist different results. As pathologists' understanding and surgeons' recognition of MBC has been improved, the prognosis might have also been improved. As we all know, to minimize local recurrence after patients undergoing lumpectomy, post-surgery radiotherapy is considered as a standard component of lumpectomy to treat patients with IDC. Dave et al. (29) and Yu et al. (10) found that patients receiving lumpectomy but not total mastectomy can benefit from radiotherapy. The National Comprehensive Cancer Network breast cancer guidelines recommended that the T1-2N1 stage patients should receive PMRT, while those with stage N2 might undergo PMRT (30). In addition, 5-year survival rates of the MBC patients ranged from 49 to 83%, which suggested that the effect of PMRT in those tumors is not clear. In the present study, PMRT was associated with a better prognosis for MBC patients.
According to previously published studies, the rate of MBC underwent CT ranged from 33 to 86% (31)(32)(33). The reasons might be that, firstly, the widely gapped rates might suggested that the effectivity of patients underwent CT remained unclear, but some studies with small sample size showed that patients receiving CT had a superior prognosis (34)(35)(36). Secondly, the high rate may be that the triple-negative phenotype was the common molecular subtype of MBC, which is characterized by more aggressive cancer (37). The next but not the last reason is that in the NCCN guideline, its treatment was paralleled to that of IDC (38). Nevertheless, CT can not affect the prognosis of MBC patients, which is supported by most researchers (28,(39)(40)(41). 64.2% of patients received CT but they had no better outcomes than that not receiving CT, in the present study, which was consistent with the previous study (15,42). The presence of more than one metaplastic component may be one of reasons for chemotherapy-resistant.
Although the triple-negative phenotype was the common molecular subtype in MBC, HR-positive and HER2 overexpression tumors do exist (43).
A published study reported that HER2 status was associated with a better prognosis for MBC patients (37). This is in contrast to invasive ductal and lobular carcinoma of the breast (22). Interestingly, some small sample reports suggest that antiestrogen therapy does not improve the disease-free and overall survival of HR-positive MBC (8,32,44). In our study, HER2positive status was associated with better outcomes, this conclusion is consistent with a recent study by Schroeder et al. that was published. Additionally, little is known about the presentations and prognoses of HER2 positive MBC, due to lack of reports of tumor HER2 receptor status. There was a particularly significant gap when consider the availability and use of HER2-directed therapy. In addition, by investigating the response of MBC to HER2 targeted therapy, we can understand the vulnerability to antibodies (37). Owing to the rarity of HER2 over-expression tumors, clinicopathologic features need to be fully determined. The incidence of MBC is unknown, so the association of these therapeutic factors with MBC is unknown. Her2-positive breast cancer is an invasive disease, and until recently the overall survival rate for this subtype of breast cancer had been the worst (45,46). Overall survival in this subtype had been greatly improved due to the use of HER2-targeted therapies by antibody-based approaches (e.g., trastuzumab, pertuzumab) and small-molecule inhibitors (lapatinib, neratinib) (47,48). However, the involvement of HER2 over-expression in MBC prognoses is unknown. Previous studies have found the rate of HER2 over-expression ranging from 0% to 25% (49,50). In our study, 7.3% of MBC patients had HER2 over-expression, which is consistent with previous studies. According to the current consensus guidelines, the degree of HER2 overexpression or amplification was thought to be intermediate between typical breast cancer and MBC, as reported in previous studies (51).
Our study has several key strengths. The role of HER2 status and PMRT in the prognosis of MBC is unclear. From our results, the prognosis was improved in MBC patients receiving PMRT. In addition, HER2 status can redefine the role of PMRT in the prognosis of MBC.
Our study has several limitations. First, due to its retrospective study, it is characterized by the nature of observation and the possibility of selection bias. Second, the SEER database lacks information on hormone therapy, anti-Her-2 therapy, and baseline characteristics including working status, comorbidity, and socio-economic environmental parameters. Third, the SEER database can not provide detailed chemotherapy and radiotherapy information, so it is impossible to conduct further case-control studies. However, our results will help researchers understand the role of HER2 in the prognosis of MBC.

CONCLUSIONS
Our findings supported that, after PSM, PMRT and HER2positive status were associated with a better prognosis. However, only HER2-negative patients could benefit from PMRT.

DATA AVAILABILITY STATEMENT
The datasets presented in this study can be found online at https://doi.org/10.6084/m9.figshare.19800526.

AUTHOR CONTRIBUTIONS
We sincerely appreciate our department members for providing great support. Thanks are due to TH, LL, and HC for their conception and design of this study and to YZ for her help with the methodology. During the period of writing and revising our manuscript, FD and JS had given us many good suggestions, thanks sincerely. All authors contributed to the article and approved the submitted version.

FUNDING
This study was supported from the National Science Foundation Committee (NSFC) of China (Grant number: No. 81702397 to LL). This project is supported by Hainan Province Clinical Medical Center.