The invasive lobular carcinoma as a prototype luminal A breast cancer: A retrospective cohort study

Background Although the invasive lobular carcinoma (ILC) is the second most frequent histologic subtype in Western countries, its incidence is much lower in Asia, and its characteristics are less well known. Methods We assessed the clinical characteristics and outcomes of 83 Korean patients (2.8%) with ILC for comparison with 2,833 (97.2%) with the invasive ductal carcinoma (IDC), including 1,088 (37.3%) with the luminal A subtype (LA-IDC). Results The mean age of all patients was 48.2 years, with no significant differences among the groups. Compared to IDC, ILC showed a larger tumor size (≥T2, 59.8% vs. 38.8%, P = 0.001), a lower histologic grade (HG 1/2, 90.4% vs. 64.4%, P < 0.001), more frequent estrogen receptor positive (90.4% vs. 64.4%, P < 0.001), progesterone receptor positive (71.1% vs. 50.1%, P < 0.001) and HER2 negative (97.5% vs. 74.6%, P < 0.001) status, and lower Ki-67 expression (10.3% ± 10.6% vs. 20.6% ± 19.8%, P < 0.001), as well as being more likely to be of the luminal A subtype (91.4% vs. 51.2%, P < 0.001). Six (7.2%) ILC and 359 (12.7%) IDC patients developed disease recurrence, with a median follow-up of 56.4 (range 4.9-136.6) months. The outcome of ILC was close to LA-IDC (HR 0.77 for recurrence, 95% CI 0.31-1.90, P = 0.57; HR 0.75 for death, 95% CI 0.18-3.09, P = 0.70) and significantly better than for the non-LA-IDC (HR 1.69 for recurrence, 95% CI 1.23-2.33, P = 0.001; HR 1.50 for death, 95% CI 0.97-2.33, P = 0.07). Conclusions ILC, a rare histologic type of breast cancer in Korea, has distinctive clinicopathological characteristics similar to those of LA-IDC.

Previous studies have demonstrated distinctive clinical and biologic characteristics for ILC as compared with IDC. For example, it is more likely to occur in older patients, be larger in size, be estrogen receptor (ER) and progesterone receptor (PgR) positive and have low to absent human epidermal growth factor receptor-2 (HER2) expression [9,10]. Traditionally, both ILC and IDC subtypes have received the same treatment, depending on their clinicopathological characteristics, and the prognosis is reported to be similar [10,11].
Recently, classification of breast cancers by gene expression profiling into particular subtypes has become established [12]. However, in clinical practice, the combination of expression of hormone receptors and HER2 by immunohistochemistry (IHC) is more commonly used to define breast cancers into the luminal A (ER + or PgR + , HER2 -), luminal B (ER + or PgR + , HER2 + ), HER2-overexpressing (ERand PgR -, HER2 + ), and triple-negative (TNBC: ER -, PgR -, HER2 -) subtypes, which demonstrate major differences in clinical outcomes, with the luminal A subtype showing the best prognosis [13,14]. The relative distributions of these four immunohistochemically defined subtypes in the lobular lesions have yet to be established in detail.
The purpose of the current study was to analyze the characteristics of an ILC series and compare the clinical and prognostic parameters with those of general IDC and of the luminal A subtype of IDC (LA-IDC).

Patients
All patients were treated at the National Cancer Center, Korea during the years from 2001 to 2008. A total of 83 consecutive cases diagnosed with pure ILC, including two cases with synchronous bilateral ILC, were enrolled in the study All ILC cases were classic subtype except for one case which was pleomorphic type. This particular case was triple negative by IHC. To compare clinicopathological characteristics and prognoses, 2,833 consecutive patients diagnosed with IDC during the same period were also selected.

Clinicopathological evaluation
We retrospectively evaluated conventional clinicopathological factors, including treatment modalities (type of operation, use of chemotherapy, hormone therapy, anti-HER2 therapy and radiotherapy) and the IHC results for five biological factors ( [15]. The pathological tumor stage was assessed according to the criteria described in the 6th edition of the American Joint Committee on Cancer (AJCC) staging manual [16]. The tumor grade was determined according to the Scarff-Bloom-Richardson classification modified by Elston and Ellis [17].
A cut-off value of 10% of positively stained nuclei was used to define ER and PgR positivity; HER2 was scored as 0-3+ by a pathologist (Y. Kwon) according to the method recommended for the Dako Hercep Test. Cases with IHC scores of 3+ or 2+ with gene amplification by fluorescence in situ hybridization (FISH) were considered positive for HER2. Cells with positive staining for Ki-67 and p53 were counted and expressed as a percentage. For p53, we scored the lesions as 0-3+ (0, negative; 1+, ≤25%; 2+, 25-50%; 3+, >50%). For the prognosis comparison, low expression was defined as Ki-67 < 20% and p53 ≤ 25% (median values for all evaluated tumors).
For the subgroup analysis, the definition of Luminal A was as follows: positive ER or PgR by IHC, negative HER2 represented by an IHC score of 0 or 1+, or 2+ if not amplified by FISH. The HER2 cases of an IHC score of 2+ but no FISH results were counted as unknowns. The definitions of the other subtypes were as follows: Luminal B, ER or PgR positive and HER2 positive; HER2 overexpressing, low ER and PgR scores but HER2 positive; TNBC, low ER and PgR scores and HER2-negative. Treatment, including surgery, adjuvant chemo, endocrine or anti-HER2 therapy, and radiotherapy, was applied equally to patients with ILC and IDC, dependent on the clinicopathological characteristics.

Statistical analysis
The primary endpoints of this study were disease-free survival (DFS) and overall survival (OS). The DFS period was defined as the interval from the date of diagnosis to the date of the first observation of disease recurrence, either loco-regional recurrence or distant metastasis, or the last follow-up date without any evidence of recurrence. Overall survival was calculated from the date of primary breast cancer diagnosis to the date of death or last follow-up.
To compare the clinicopathological characteristics between pairs of groups, we used the Student's t-test and the chi-square test. The DFS and OS rates were calculated using the Kaplan-Meier method, and the groups were compared using the log-rank test. For the multivariate analysis, Cox regression analysis was applied. Statistical analyses were performed using Stata 10.0 for Windows (Stata Corporation Station, TX, USA).
This study protocol was reviewed and approved by the Institutional Review Board of the National Cancer Center (NCCNCS-10-371), Korea, and it complied with the recommendations of the Declaration of Helsinki for biomedical research involving human subjects. The ethical review board supported that informed consent was not required for this study.

Patient characteristics
The clinicopathological characteristics of the 83 ILC patients and 2,833 IDC patients are summarized in Table 1. The mean ages were 48.3 years and 48.2 years, respectively, with no difference in the distributions of age at diagnosis between ILC and IDC ( Figure 1). Two patients with mixed lobular and ductal cancers were excluded from the analysis.
Significantly more tumors were positive for hormone receptors and had a negative HER2 status in the ILC group as compared to the IDC group (ER + , 90.4% vs. 64.4%, P < 0.001; PgR + , 71.1% vs. 50.1%, P < 0.001; HER2 -, 97.5% vs. 74.6%, P < 0.001), with a greater proportion of the luminal A subtype in the ILC group (91.4% vs. 51.2%, P < 0.001). In addition, the mean Ki-67 value was lower in Because 91.4% of ILC were of the luminal A subtype, we further compared ILC to the 1,088 LA-IDC. Whereas significant differences between ILC and LA-IDC were found for size, HG and Ki-67, the rates of nodal involvement and the expression of p53 were similar ( Table 1).
The treatment modalities in the ILC group were also comparable to those used in the LA-IDC group. One of two patients with a HER2 positive tumor among the ILC group who developed disease recurrence received anti-HER2 treatment upon recurrence.

Univariate analysis of DFS and OS of ILC compared to IDC patients
During the median follow-up of 56.4 (range 4.9-136.6) months, 365 patients experienced disease recurrence (6/83 ILC vs. 359/2833 IDC, P = 0.18) and 213 patients died (3/83 ILC vs. 210/2833 IDC, P = 0.28). One ILC patient experienced local recurrence, one contralateral breast cancer, and four distant metastasis. In the univariate analysis of OS, age, tumor size, nodal status, distant metastasis at diagnosis, individual ER, PgR, and HER2 statuses, p53, Ki-67, the type of operation, adjuvant hormone therapy and intrinsic subtype were prognostic factors. The 5-year OS rate was 93.6% for the ILC group and 92.5% for the IDC group (P = 0.38).
In this study, we classified the total 2,916 patients into ILC and four subtypes of IDC and compared the clinical outcomes. Figure 2 (a, b) presents the DFS and OS curves. The prognosis of ILC was similar to that of LA-IDC and was more favorable than with other subtypes of IDC; the 5-year DFS rates being 91.7% vs. 89.1% for LA-IDC, 80.7% for the luminal B subtype of IDC (LB-IDC), 78.9% for the triple-negative subtype of IDC (TN-IDC), and 75.9% for the HER2 overexpressing subtype (P < 0.001). The 5-year OS rates were 93.6%, vs. 93.4%, 92.8%, 85.8%, and 83.9%, respectively (P < 0.001).

Discussion
ILC constitutes 2-4% of all breast cancer in Korea, as presented in the current study, which is much lower  than the rate observed in most Western reports [1][2][3].
Although ILC occurs more often in older women in Western countries [9,10], our series demonstrated that age distributions were the same as those for overall IDC at diagnosis. Notably, the peak age of breast cancer patients in Korea is the late 40 s, which is 10 to 20 years younger than that in Western countries [4].
In the present study, the tumor size of ILC was larger than that of IDC, as observed in other studies [10,18,19]. Detection may be delayed because ILC is often clinically  impalpable or mammographically invisible due to a lack of desmoplasia in the stroma [20]. Despite the larger tumor size, the rate of lymph node involvement in ILC did not differ from that in general IDC, which may reflect the slow growth rate of ILC, and this finding is consistent with other reports [10,18]. Due to the difficulty associated with early detection, the larger tumor size in ILC adversely affected the outcomes of patients with poor DFS in the present study. We reported that ILC had lower histologic grade than IDC. Previously, Li et al. analyzed Surveillance, Epidemiology, and End Results Program data and demonstrated that ILC showed lower tumor grade than IDC specifically in 30-49 years old patient group [21]. Although Rakha et al. reported that histologic grade of ILC provided a strong predictor of outcome in breast cancer patients and should be provided routinely in pathology reports, we did not find such correlation [22]. One of the objectives of the present study was to characterize more comprehensively the biological phenotype of ILC. As previously reported [6,10], the majority of ILC showed ER or PgR positivity and HER2 negativity, which we defined as consistent with the LA-IDC subtype. Weigelt et al. also showed that most ILC fall into luminal A molecular subtype, although some ILC had cluster with either HER2 subtype or apocrine subtype [23]. Furthermore, ILC demonstrated lower p53 and Ki-67 expression compared to IDC and LA-IDC. All of these characteristics suggest that ILC likely originates from more differentiated luminal cells [24].
Previous studies have reported that the prognosis of ILC patients is similar to that of IDC patients [6,10,18], as confirmed in the present study. Arpino et al. analyzed 4,140 ILC patients and 45,169 not otherwise specified IDC patients and reported that the histologic type did not affect the prognosis despite the favorable biological phenotype of ILC [10]. However, the outcome of ILC in the present study was comparable to that of LA-IDC and significantly better than the outcomes of other, non-LA-IDC subtypes, when we further analyzed the prognosis by breast cancer subtype.
This study demonstrated a new aspect of ILC after construing the data including biologic markers other than general tumor characteristics in the consecutive breast cancer patients who received consistent therapeutic approaches at a single center. Similar clinicopathological characteristics and clinical outcomes between ILC and LA-IDC were discovered after we further compared ILC with the four subtypes. To our knowledge, this is the first report to show such similarities between ILC and LA-IDC.

Conclusions
ILC has distinct clinicopathological characteristics with a larger tumor at presentation, a lower HG, ER/PgR positive and HER2 negative status, and low Ki-67 expression, as compared to overall IDC. This study shows that most ILC are luminal A breast cancer, the prognosis of ILC is similar to that of LA-IDC, and both are better than the other subtypes.