Prognosis of subtypes of the mucinous breast carcinoma in Chinese women: a population-based study of 32-year experience (1983-2014)

Purpose The heterogeneous nature of the mucinous breast cancer (MBC), with its pure (PMBC) and mixed subtypes (MMBC), calls for precise prognosis assessment. Methods We analyzed 197 consecutive MBC patients, including 117 PMBC and 80 MMBC, who were treated from 1983 to 2014. The clinicopathological features, treatment choice, disease-free survival (DFS) and overall survival (OS) were compared among PMBC, MMBC and MMBC subgroups. Prognostic factors of PMBC and MMBC were identified. Results Compared to PMBC, MMBC had more lymph node metastasis (p = 0.043), Her2 positivity (p = 0.036), high Ki-67 index (defined as>20%, p = 0.026) and anti-Her2 targeted therapy (p = 0.016). The 5-year DFS of PMBC and MMBC were 90.4% and 86.2%, whereas the 5-year OS were 99.0% and 98.7%. No significant difference was found in DFS or OS among all MBC subtypes. High Ki-67 (p = 0.020) appeared as DFS factor in PMBC, while anti-Her2 targeted therapy (p = 0.047) as the DFS predictors in MMBC. Conclusion MMBC manifested similar 5-year survival to PMBC in Chinese woman, suggesting that intra-tumoral heterogeneity might not interfere with MBC short-term prognosis.

targeted therapy were largely unavailable [3,4,[8][9][10]. Few studies had investigated the tumor biology, treatment choice and survival outcomes of MMBC in Chinese population, especially with respect to the intra-and intertumoral histological heterogeneity represented by the different co-existing cancer components. The prognostic predictors for PMBC and MMBC also remained unclear. A recent study showed that both the mucinous and the coexisting components in MMBC were remarkably similar at the molecular level to PMBC, suggesting that MMBC be best classified as variants of mucinous cancers rather than of IDC [11]. Conversely PMBC appeared to possess phenotypic plasticity that could be converted by estrogen into MMBC with invasive lobular carcinoma (ILC) component [12]. Thus, we plan to compare the prognosis of PMBC versus MMBC in Chinese population when all measures of the modern comprehensive therapy were available.

Descriptive information of the study cohort
A total of 244 patients were identified as described in METHOD. After excluding 28 patients with < 50% focal mucinous lesion and 19 patients lost to follow-up, 197 patients were included in the analysis, comprising 1.9% of contemporary 10,192 breast cancer treated in PUMC Hospital. 171 patients (86.8%) were treated during the recent ten years (2005-2014) while 130 patients (66.0%) were treated during the recent five years (2010)(2011)(2012)(2013)(2014). 112 patients (56.9%) were pre-menopausal and 85 (43.1%) post-menopausal. There were 117 PMBC and 80 MMBC patients, the latter including 24 patients with ductal carcinoma in situ (DCIS) and IDC (with or without other types of carcinoma), 45 with only IDC, 9 with invasive micro-papillary carcinoma (IMPC) and 2 with ILC. With a median follow-up time of 41 months (3-385 months), 11 PMBC and 7 MMBC patients developed recurrence or metastasis, and 1 PMBC and 1 MMBC passed away (Figure 1).

survival outcomes and prognostic factors of Mbc subtypes
The 5-year DFS of PMBC and MMBC were 90.4% and 86.2%, whereas the 5-year OS were 99.0% and 98.7% respectively. The 5-year DFS and OS for MMBC subgroups were: 85.7% and 100.0% for MBC+DCIS+IDC, 83.5% and 97.6% for MBC+IDC, and 100.0% and 100.0% for MBC+IMPC. No significant difference was found in DFS or OS either between PMBC vs MMBC or among the above mentioned MMBC subgroups ( Figure 2, Table  3, 4). High Ki-67 index (defined as > 20%, p = 0.020) was identified as the significant DFS prognostic factor for PMBC, whereas anti-Her2 targeted therapy (p = 0.047) appeared to be the DFS predictor for MMBC (Table 5, 6). DFS stratified by Ki-67 in PMBC and by anti-Her2 targeted therapy in MMBC both showed significant differences ( Figure 2). ER, PR, hormone receptor status, immunophenotype and endocrine therapy might be potential DFS predictors according to univariate analysis. However, these factors were not significant in the multivariate analysis. None of the clinicopathological and treatment factors listed above could serve as OS predictors for MBC subtypes due to the limited OS events.

DIscUssION
MBC is one of the most commonly seen special types of breast cancer [1,2,4,8]. Experience in diagnosis and treatment of MBC was usually acquired from retrospective studies instead of prospective randomized trials. It was widely believed that MMBC had a poorer prognosis than PMBC [3,4,[8][9][10]. However, these retrospective studies were mainly based on data from Caucasian, and mostly performed during the 1960s to 1980s, when anti-Her2 targeted therapy, most of the endocrine therapy, chemotherapy and radiation therapy were unavailable. Thus the poorer outcome of MMBC might be due to insufficient treatment. Additionally, MMBC is not a single disease. Whether MMBC subgroups have different survival outcomes remains unclear. A recent study reported differences in breast cancer epidemiology, clinical characteristics and prognosis between Chinese and Caucasian women [13,14]. However, few studies have evaluated the survival outcome among MBC subtypes in Chinese women, who tend to develop breast cancer and MBC at a much younger age than Caucasian counterparts [1,2,5,15,16].
Although PMBC usually had normal diploid DNA         [5]. However, most of the MMBC patients included in that study were diagnosed at a much later stage than PMBC, while 70.5% of PMBC patients received chemotherapy. There was no data concerning the Her2 status so that the similar percentage of anti-Her2 targeted therapy between MBC vs non-MBC or between PMBC vs MMBC would be difficult to interpret. Compared with IDC, IMPC usually has larger size, more metastatic lymph nodes, increased lymphovascular invasion (LVI) and more aggressive behavior [22]. Poorer survival was also observed for breast carcinoma containing IMPC component [22]. Notably, a special subtype of PMBC with micropapillary epithelial growth pattern was identified as invasive micropapillary mucinous carcinoma (IMPMC) [23], or mucinous carcinomas with a micropapillary pattern (MUMPC) [24]. This heterogeneous PMBC had more LN metastasis, higher Her2 expression, LVI, and a poorer prognosis than pure PMBC [23,24]. Meanwhile, it showed decreased LN metastasis, lower nuclear grade, higher expression of ER and PR, less expression of Her2, and better prognosis compared to IMPC. Though controversial, it was proposed that PMBC, MUMPC/IMPMC and IMPC might represent clinical entities within a spectrum of heterogeneous diseases, with different percentage of mucin secretion and micropapillary components [23][24][25]. The MBC+IMPC in our study was different from MUMPC/IMPMC and did not exhibit higher LN metastasis, higher Her2 or Ki-67 expression, or poorer survival outcome.
Our study had several limitations. Firstly, it was a single-center study with limited case number, and two patients with MBC+ILC had to be excluded from the comparison. Secondly, although this retrospective study reviewed MBC patients distributed during 32 years' time span, majority (86.8%) of patients was treated in the recent decade (2005-2014), so it would make more sense to analyze the 5-year short-term survival. There might still be significant difference in long-term 10-year prognosis between PMBC and MMBC, because MBC is basically luminal subtype and have shown late recurrences after 10 years [24,26]. Thirdly, LN metastasis was not identified as the DFS predictor in our study, although it was identified in other studies to be candidate prognostic factor for PMBC [1,2,8,21,26,27]. Fourthly, Ki-67 expression was only documented in 79.5% of the PMBC and 87.5% of the MMBC, while p53 status in more than half of the cases was unknown.
In conclusion, our study revealed that MMBC had similar short-term survival as PMBC in Chinese patients, suggesting that intra-tumoral heterogeneity might not interfere with MBC prognosis in Chinese woman. Ki-67 proliferation index was identified as a DFS prognostic factor for PMBC, whereas anti-Her2 targeted therapy as the potential DFS predictor for MMBC. Further studies with increased cases number, prolonged follow-up and improved bio-markers need to be performed to gain a deeper understanding of MBC biology and prognosis with respect to intra-and inter-tumoral heterogeneity.

Ethics statement
This study was approved by the Ethics Committee of the Peking Union Medical College Hospital, Chinese Academy of Medical Sciences.

Patient selection, pathology review and follow-up
From January 1983 to December 2014, 244 consecutive MBC patients were treated primarily with breast cancer surgeries in PUMC Hospital according to the medical records searching. All patients' formalin-fixed paraffin-embedded (FFPE) pathological sections were reviewed and 28 patients with focal mucinous components < 50% of the total cancerous lesions were excluded from the study. All patients were followed by telephone call, by out-patient clinics records of follow-up examinations or by both measures. Another 19 patients who were lost to follow-up were also excluded.
There were 197 MBC patients, including 117 PMBC and 80 MMBC, in the study cohort. The clinicopathological characteristics, treatment choice, DFS and OS were compared both between 117 PMBC vs 80 MMBC, and among all MBC subgroups, including 24 MMBC with DCIS and IDC (with or without other types of carcinoma), 45 with only IDC and 9 with IMPC. Two patients with MMBC and ILC were excluded from the comparison due to the small case number. DFS factors of PMBC and MMBC were identified respectively. Identification of prognostic factors for MMBC subgroups were not performed also due to the limited case numbers (Figure 1).

statistical analysis
The quantitative variables were compared with t-test and the categorical variables were compared with chi-square tests. Survival outcomes including 5-year predicted DFS and OS were analyzed and compared by the Kaplan-Meier curve method. Kaplan-Meier univariate analyses and Cox multivariate analyses were performed to identify the prognostic factors for PMBC and MMBC respectively. The significance threshold was set at p < 0.05. SPSS software, version 18.0 (SPSS, Inc. Chicago, IL, US) was used for all of the statistical analyses.