Adenosquamous Carcinoma of the Breast: A Population-Based Study Using the SEER Database

Background: The present study is aimed at summarizing the clinicopathological characteristics, prognosis, and management of breast adenosquamous carcinoma (ASC). Methods: A population-based study was performed using retrospectively extracted data from the Surveillance, Epidemiology and End Results database for breast cancer patients with histological diagnoses of ASC, inltrating duct carcinoma (IDC) and squamous cell carcinoma (SCC) from 2004 to 2016. End-points were overall survival (OS) and breast cancer-specic mortality (BCSM). Propensity Score Matching (PSM) was employed to minimize selection bias of baseline characteristics. Univariable and multivariable analyses were used for identifying valuable prognostic factors. Results: ASC presented similar tumor size but low histological grade and less lymph node metastasis compared to IDC. ASC expressed less positive rate of hormone receptors and barely HER2, which was similar with SCC (estrogen receptor (ER): ASC 27.74% and SCC 21.53%, progesterone receptor (PR): ASC 18.06% and SCC 12.85%, HER2: ASC 4.44% and SCC 7.53%). ASC patients underwent the same treatment as IDC (chemotherapy 36.99% vs. 41.86%, BCS 50.58% vs 52.83%, P >0.05), only with less radiotherapy (39.88% vs. 48.34%, P<0.05). Median follow-up data of 78 months showed that the prognosis of IDC patients was better than that of ASC patients (all P <0.05 for BCSM and OS). After adjustment for clinicopathological and therapeutic factors in Cox proportional hazards models, ASC was no longer an independent poor prognosis factor. In matched groups, no signicant difference in BCSM nor OS was observed between ASC and IDC groups. In HR-negative patients, the prognosis of ASC was similar with that of IDC, and both were superior to SCC. In HR-positive patients, the ve-year survival rate of ASC was only about 60%,


Background
Adenosquamous carcinoma (ASC) is a rare and unique form of invasive mammary carcinoma that occurs less than 0.2% of all cases of breast cancer [1]. The earliest example of a breast tumour with adenosquamous features was reported in 1912 by Konjentzny. It was only in 1987, when Rosen and Ernsberger consolidated and described 11 such cases, that the term 'ASC' was established and entrenched [2]. Until now, in the World Health Organization (WHO) breast cancer classi cation (2019), ASC was characterized as a variant of metaplastic mammary carcinoma, and other subtypes of metaplastic breast cancer were bromatosis-like metaplastic carcinoma, squamous cell carcinoma, spindle cell carcinoma, metaplastic breast cancer with mesenchymal differentiation [3].
Though several case reports and case series were published over the years [4,5], ASC remains poorly de ned by immunohistochemistry and its genetic pro le is still unclear [6]. From these limited studies, it appears that ASC differs from its counterparts in this heterogeneous category by its relative clinical indolence, also re ects histologically in its cytomorphology [7]. Under normal conditions, squamous carcinoma (SCC) of the breast must be considered in the differential diagnosis as many reports have included tumors with varying proportions of squamous carcinoma and adenocarcinoma, very few are completely pure without other kind of glandular cell differentiation [8].
Currently, ASC has no consistent therapeutic strategy, the principles of treatment for ASC are either similar with those for SCC or for in ltrating duct carcinoma (IDC) according to the experience of doctors.
However, accurate information concerning the comparison of breast ASC, breast SCC and breast IDC has been unavailable.
In this way, the aim of our study is to perform a comparison of the prognosis among breast ASC, breast SCC and breast IDC, and to further identify the underlying prognostic clinicopathological factors.

Method Data source and patient selection
The SEER program is a national database and a primary source of cancer statistics maintained by The

Statistical analysis
The differences of demographic and clinicopathological features among IDC, ASC and SCC groups were analyzed by one-way ANOVA and Chi-square test. Propensity Score Matching (PSM) method (STATA PSMATCH2 [9]) was used to eliminate demographic and clinic-pathological mixed bias in IDC and ASC groups. Overall survival (OS) was de ned as the time from admission to the date of death from any cause. Breast cancer-speci c mortality (BCSM) was de ned as the period from the operative date to death due to breast cancer. The OS curves and BCSM curves of each group were estimated by Kaplan-Meier survival analyses, and the curves were analyzed by the log-rank test. In the multivariate analysis, a COX's Proportional Hazard Model was employed to estimate whether a factor was a signi cant independent prognostic factor of survival. All statistical tests were two-sided; P values less than 0.05 were considered as statistically signi cant. The statistical analyses were performed using STATA version 15.1 for Windows (StataCorp LLC).

Result
Differences of demographic and clinicopathological features among IDC, ASC and SCC After omitting censored data, an original of 557,203 female breast cancer patients were enrolled in our study. In total patients, 173 patients (3.10‱) were diagnosed as adenosquamous carcinoma of breast (ASC group) and 372 patients (6.68‱) were identi ed as squamous cell carcinoma (SCC group). Mean age was signi cantly different among the three groups, with a greater age among SCC participants; mean age were not signi cantly different between ASC and IDC groups. (F= 32.03, P=0.001, ANOVA, IDC vs ASC P =0.06; ASC vs SCC P =0.05; Bonferroni). More patients of other race and higher proportion of married in IDC group did not constitute a meaningful result. Compared with IDC, ASC had similar tumor size but low histological grade and less lymph node metastasis, while SCC was just the opposite. More distant metastasis of SCC leads its advanced AJCC stage at the time of diagnosis. The molecular markers of ASC were close to SCC, such as less positive rate of hormone receptors (estrogen receptor (ER): ASC 27.74% and SCC 21.53%, progesterone receptor (PR): ASC 18.06% and SCC 12.85%), barely expression of HER2 (ASC 4.44% and SCC 7.53%), which were totally different to IDC (all P < 0.05). In terms of molecular subtype of breast cancer, triple-negative and Luminal A were more common in ASC due to the absence of HER2. ASC patients underwent the same treatment as IDC (chemotherapy 36.99% vs. 41.86%, BCS 50.58% vs 52.83%, P >0.05), only with less radiotherapy (39.88% vs. 48.34%, P<0.05). The comparisons of features among the three groups were shown in Table 1.

Survival analysis among IDC, ASC and SCC patients
All breast cancer patients enrolled in our study were followed-up for a median of 78 months (range of 1 to 155 months). By the end of the follow-up period, 108397 IDC patients had died, 54543 patients died of breast cancer, with the corresponding, 48 and 170 patients in ASC and SCC group had died, of which 26 and 103 patients due to recurrence and metastasis of breast cancer. In the total sample, the OS and BCSM of three histological group had noticeable difference from those of each other (OS: IDC vs ASC P=0.001; ASC vs SCC P=0.001, BCSM: IDC vs ASC P=0.008; ASC vs SCC P=0.001, log-rank test) (Fig.1A1B).

Survival analysis between IDC and ASC patients in PS matched groups
The propensity score matching method (Match Ratio 1:1; Logit model; the nearest neighbor matching approach) was employed to eliminate the bias of demographic and clinicopathological features between ASC and IDC groups. Because almost no expression of HER2 in ASC, we assumed the missing HER2 in ASC records before 2010 as negative to retain as many matched cases as possible. After matching, the hypothesis test showed that except for race, there was no statistical difference in the mean standard deviation of each variable between the two groups ( Table 2). The kernel density functions showed that the general features between ASC group and IDC group (143 patients from the original ASC and IDC group respectively) were similar (Fig.1C1D). After PSM, 22 of 143 patients in IDC group had died, 13 of whom owing to breast cancer. Accordingly, 19 patients died from breast cancer in 34 death cases of ASC.
The OS and BCSM curve of ASC and IDC groups interwove with each other (P=0.645 for OS and P=0.596 for BCSM, log-rank test) (Fig.1E1F). The prognosis of ASC seemed not inferior to that of IDC.
Clinical outcomes of IDC, ASC and SCC in different breast cancer subtype groups Molecular subtypes of breast cancer play an essential role in guiding clinical treatment and predicting prognosis. In ASC group, the absence of HER2 expression led us to divide ASC into triple negative and luminal A only through hormone receptor expression. In HR-negative subgroup, we found that the OS and BCSM of ASC patients were close to that of IDC (P=0.736 for OS and P=0.226 for BCSM, log-rank test) (Fig.2A2C). The prognosis of IDC and ASC with negative HR receptor was better than that of SCC with the same immunophenotype (all P < 0.05 for OS and BCSM between groups, log-rank test). On the contrary, in HR-positive subgroup, the prognosis of ASC was poor, which was similar to that of SCC (OS: IDC vs ASC P=0.001; ASC vs SCC P=0.193, BCSM: IDC vs ASC P=0.001; ASC vs SCC P=0.470, log-rank test) (Fig.2B2D). The ve-year survival rate of ASC with HR-positive was only about 60%, which was far less than that in the HR-negative subgroup (Fig.2C2D).

Cox proportional hazards models for OS and BCSM
To further investigate the effect of baseline characteristics on prognosis of breast cancer, the multivariate Cox proportional hazards model was utilized to tted for OS and BCSM. As shown in Table 3, as the consensus that had been achieved, demographic factors such as older age, black race and unmarried were the poor prognostic factors for breast cancer, clinicopathological features such as higher histological grade, larger tumor size, more lymph node metastasis and negative expression of HR/HER2 related to poor prognosis of breast cancer. Standard mastectomy/breast conserving surgery and adjuvant radiotherapy/chemotherapy brought survival bene ts to the patients (all P<0.05 for HR). However, after adjusting other prognostic factors, histology type of ASC was no longer an independent prognostic factor in multivariate analysis (HR=0.93 for BCSM, 95% CI 0.35-2.47, P=0.880; HR=1.06 for OS, 95% CI 0.53-2.12, P=0.866) ( Table 3). We also analyzed the variables potentially in uencing OS and BCSM of ASC by Cox proportional hazards model and Table 4 showed that older age (age≥60) and advanced AJCC stage (III and IV) were independent factors of poor prognosis in ASC (all P<0.05 for HR). BCS had the same therapeutic effect as mastectomy for OBC patients (HR =1.45 for BCSM P=0.530, HR =0.93 for OS P=0.857). Chemotherapy and radiotherapy also failed to bring signi cant survival bene ts to ASC patients (all P>0.05 for HR).

Discussion
Most studies of ASC of the breast had been small series or single case reports because of its rarity [10]. Therefore, clinicopathological features and outcomes of this entity remained unclear. In the present study, we described clinical characteristics of patients with ASC of the breast and identi ed variables affecting BCSM and OS using data from SEER. Only 173 patients recorded in SEER diagnosed as ASC between 2004 and 2016 were extracted from the database. Compared with 556,658 cases with IDC of the breast contemporaneously, the prevalence of ASC of the breast was very low.
According to our results, age at diagnosis of patients ranged from 47 to 79 years, and the mean age of all patients was 63 years, which meant ASC was more commonly found in middle-aged and older female. In this cohort, white patients accounted for the largest proportion (~ 78.49%), which was consistent with the distribution of races in the Western population.
In our study, ASC patients had lower histological grade and less lymph node metastasis than IDC patients, however, after matching, these characteristics didn't give them better survival outcomes than IDC patients. On the contrast, though SCC patients had similar tumor size, histological grade and lymph node metastasis to IDC patients, they came up with the worst survival outcomes among these three histological types of breast cancer. Compared with ASC in other site, breast ASC patients predicted favorable prognosis. For instance, the prognosis of gastric ASC was worse than that of gastric AC [11].
Besides, lung ASC had higher grade malignancy, stronger lymph nodal invasiveness, more frequent brain metastases and poorer prognosis than lung adenocarcinoma (ADC) and SCC [12]. However, in other studies, there were also ASC in some site behaved similar with common type. As an example, patients with gallbladder ASC were similar to those with AC of the gallbladder in clinical characteristics and features, although the ASC patients were more prone to in ltration of multiple adjacent organs and lymphatic metastasis [13]. In addition, esophageal ASC behaves more like AC in response to chemoradiation and survival based on treatment modality [14]. Under these circumstances, we recommended clinical doctors not evaluating prognosis of ASC patients only by tumor size, histological grade or lymph node metastasis.
Then we focused our attention on molecular markers, nearly all ASC patients were HER2 negative, which gave an explanation to why Luminal A and TNBC accounted for larger proportion of all molecular classi cations. To our surprise, ER/PR expression in ASC patients seemed more like that in SSC patients rather than that in IDC patients. Since ASC and SCC were partly similar in pathology, they were both positive in cytokeratin 5/6 (CK5/6) [8], cytokeratin 10/13(CK10/13) or p63 [5,8], however, there was much distance between survival outcomes of ASC patients and SCC patients. In that case, we recommended clinical doctors noticing pathological differentiate diagnosis.
Besides, we found that different molecular classi cations could exert a profound in uence on survival prognosis of ASC patients. Five-year survival rate of ASC subgroup with hormone receptor positive was far less than that of the HR-negative subgroup, this result was contrary to IDC patients. There was a case report suggesting that when the expression of hormone receptors was positive in ASC, CD44v could play an important role in the transition of LGASC precursor lesions into malignant processes [7]. CD44v, a widely accepted cancer stem cell (CSC) marker in breast cancer, was considered to promote the tumor progression in various cancers [15]. However, this could have a better explanation, which required a deeper study.
We found that most ASC patients received surgery (~ 94.18%), with a BCS to mastectomy ratio of 1.14:1. Besides, they also received radiotherapy (~ 42.31%) and chemotherapy (~ 34.62%). Probably due to lack of understanding of ASC, half doctors still chose mastectomy rather than BCS, combined with the exclusion of multiple factors offset cox, mastectomy didn't lead to a better survival prognosis than BCS, so BCS was still the appropriate choice for ASC under the reasonable indications.

Conclusions
The present study has shown patients with ASC of the breast to be not exactly the same as those with IDC of the breast in clinical characteristics and features. Although the ASC patients were less prone to lymphatic metastasis, the prognosis of ASC was similar to that of IDC. Molecular markers may play an important role in dividing ASC patients into better or worse prognosis groups. Both BCS and mastectomy can effectively improve the prognosis of these patients. Further studies with larger sample sizes from multiple institutions are needed to con rm clinicopathological features and survival rates of ASC.

Competing interests
The authors declare that they have no competing interests.

Funding
This work was supported by the Fund: Shanghai Yangpu district science projects (grants number YP18M03) for data collection and analysis.

Author contributions
Literature search ZYG, JL, XYL, CW and JJL. Study design CX and ZGZ. Methodology ZYG, YF and CX. Writing ZYG, CX and ZGZ. Review and editing XLC, CX and ZGZ. All authors have critically reviewed the nal version of the manuscript and approved its content. The corresponding author had nal responsibility for the decision to submit for publication.