Outcomes of her 2-positive early-stage breast cancer in the trastuzumab era : a population-based study of Canadian patients

Breast cancer is a heterogeneous disease. Gene expression studies have identified five distinct molecular subtypes of breast cancer: luminal A, luminal B, human epidermal growth factor receptor 2 (her2)–enriched, basal-like, and normal breast-like1. Other subtypes (such as claudin-low) continue to be defined2. The use of gene expression profiling is currently limited in many clinical settings, and so histopathologic markers such as estrogen receptor (er), progesterone receptor (pr), and her2 are used as surrogates for the molecular subtypes, which divide breast tumours into distinct phenotypes with distinct outcomes. Studies comparing outcomes between the different subtypes have shown that her2 overexpression, which is found in 15%–20% of human breast cancers3, is associated with increased risk of locoregional recurrence4–9 and increased breast cancer mortality9–10. Overexpression of her2 is thus generally considered to be a negative prognostic feature with an increased adjusted risk of breast cancer mortality that is approximately doubled11. However, the development of agents specifically targeting her2 has transformed the management of patients with these ABSTRACT Breast cancer is heterogenous, with variable expression of the estrogen receptor (er), progesterone receptor (pr), and human epidermal growth factor receptor 2 (her2). Overexpression of her2 is generally considered a negative prognostic feature, but whether outcomes for her2-positive early breast cancer remain different from those for other subtypes in the era of trastuzumab-based adjuvant therapy is unknown.


INTRODUCTION
Breast cancer is a heterogeneous disease.Gene expression studies have identified five distinct molecular subtypes of breast cancer: luminal A, luminal B, human epidermal growth factor receptor 2 (her2)-enriched, basal-like, and normal breast-like 1 .Other subtypes (such as claudin-low) continue to be defined 2 .The use of gene expression profiling is currently limited in many clinical settings, and so histopathologic markers such as estrogen receptor (er), progesterone receptor (pr), and her2 are used as surrogates for the molecular subtypes, which divide breast tumours into distinct phenotypes with distinct outcomes.
Studies comparing outcomes between the different subtypes have shown that her2 overexpression, which is found in 15%-20% of human breast cancers 3 , is associated with increased risk of locoregional recurrence [4][5][6][7][8][9] and increased breast cancer mortality [9][10] .Overexpression of her2 is thus generally considered to be a negative prognostic feature with an increased adjusted risk of breast cancer mortality that is approximately doubled 11 .However, the development of agents specifically targeting her2 has transformed the management of patients with these ABSTRACT Breast cancer is heterogenous, with variable expression of the estrogen receptor (er), progesterone receptor (pr), and human epidermal growth factor receptor 2 (her2).Overexpression of her2 is generally considered a negative prognostic feature, but whether outcomes for her2-positive early breast cancer remain different from those for other subtypes in the era of trastuzumab-based adjuvant therapy is unknown.
tumours.One of the first of these targeted agents was trastuzumab (Herceptin: Genentech, San Francisco, CA, U.S.A.), a monoclonal humanized antibody that binds the extracellular domain of her2.That binding interferes with the signal transduction cascade initiated by her2 overexpression and possibly stimulates an immune response to tumour cells overexpressing the receptor 12 .
Since the advent of adjuvant trastuzumab as the standard of care for her2+ tumours, few studies have compared outcomes in early breast cancer by molecular subtype (or histopathologic features).In a study of stage ii and iii breast cancer treated with modern-era systemic therapy, including trastuzumab for most patients with her2+ disease (86%), her2 overexpression was associated with a risk of locoregional recurrence that was significantly lower than that observed for her2-negative (her2-) tumours 4 .Thus, it is hypothesized that the use of adjuvant trastuzumab has improved outcomes in her2+ disease to reach similarity with outcomes in her2-disease.

Study Design
We conducted a retrospective chart review for all patients with a diagnosis of stage

Data Collection
We collected data on patient demographics, tumour characteristics, cancer stage at diagnosis, specifics of treatment (dates of surgery, hormonal therapy, radiation, and chemotherapy), recurrence, date and location of recurrence, and date of death.For locally advanced breast cancer, the tumour size was determined from magnetic resonance imaging, clinical measurement, mammographic measurement, or surgical pathology (in order of descending priority); nodal status was denoted as "Nx."

Classification of Groups
Patients were categorized into 3 groups: hr+, her2+, and tn.Tumour er and pr status was determined on the basis of immunohistochemistry, with staining of 1% or more of tumour cell nuclei being considered positive 21 .Immunohistochemistry was also used to determine her2 status, with a polyclonal antibody (TAB250 and CB11) being raised against the her2/ neu oncoprotein.Test results of 0 to 1+ were considered negative, and 3+ was considered positive.Fluorescence in situ hybridization was performed for all equivocal (2+) immunohistochemistry results; a ratio of 2.0 or higher for her2 gene signal to chromosome 17 signal was considered positive, in accordance with prospective randomized clinical trials of adjuvant trastuzumab [17][18][19] and with eligibility criteria for funding for adjuvant trastuzumab by Cancer Care Ontario 22 .

Endpoints
The primary and secondary endpoints of this study were 5-year rfs and os respectively.The rfs was calculated from date of diagnosis to date of first relapse or death from any cause."Date of diagnosis" was defined as the date of first definitive treatment, including surgical or medical therapy.Survival times were censored to the date of last contact for subjects who were lost to follow-up.The os was calculated as duration from cancer diagnosis to date of death from any cause.

Statistical Analysis
For the three subtypes, baseline characteristics (such as age) were compared using an analysis of variance test for continuous variables; a Fisher exact test was used for categorical variables (such as sex and tumour stage).The Kaplan-Meier product limit method was used to estimate os and rfs.The logit transformation was used to estimate 95% confidence intervals (cis) for the percentage of patients surviving at a particular time.Age-adjusted logistic regressions were used to derive odds ratios and 95% cis.Cox proportional hazards models were used to derive adjusted (for age, stage, histologic grade, chemotherapy treatment, and lymph node status) hazard ratios (hrs) and 95% cis for os and rfs by disease subtype.

Association Between Subtype and Other Prognostic Indicators
We observed a significant difference between the three breast cancer subtypes in the overall distribution of patient age (p < 0.001), tumour stage (p < 0.001), histologic grade (p < 0.001), and tumour size (p = 0.001, Table i).The tn patients were younger (p < 0.001) and had larger (p = 0.001), higher-stage (p = 0.001), and higher-histologic grade tumours at diagnosis (p < 0.001).They more frequently received adjuvant radiation therapy (p = 0.021).
For her2+ patients who received trastuzumab, the 5-year rfs and os were 75% and 89% respectively; for those who did not, the survival percentages were 76% and 88% respectively.
On multivariable analysis adjusted for age, stage, histologic grade, chemotherapy treatment, and lymph node status, the tn tumour subtype was associated with a risk of mortality that was significantly increased compared with that for the hr+ subtype, which served as the reference group (hr: 4.37; 95% ci: 1.56 to 12.24); patients with the her2+ subtype had an outcome similar to that in patients with the hr+ subtype (hr: 1.07; 95% ci: 0.31 to 3.67; Table iii).

DISCUSSION
The treatment of breast cancer has progressed significantly since the early 1990s.One significant advance was the advent of adjuvant trastuzumab for patients with early-stage breast cancer.Our study is one of the first to examine the relative outcomes for the subtypes of breast cancer since adjuvant trastuzumab became the standard of care for Canadian patients with her2+ early-stage breast cancer.Our results indicate that the 5-year os and rfs for the entire cohort were excellent, but variable by receptor status (a surrogate for molecular subtype).
At baseline, we found a distribution of breast cancer subtypes similar to that reported by other immunohistochemical studies [23][24][25][26] .We also observed that both her2+ and tn tumours were more prevalent among younger women and more frequently exhibited pathologic characteristics associated with more aggressive tumour behaviour.As such, we found that outcomes in both of the latter subtype groups were worse than those in the hr+ (luminal) subtype group.
In our study, 18.7% of breast cancers overexpressed her2, which is similar to the incidence of 18%-20% reported in the literature 3  of 1.07.Previous studies in which her2+ patients did not receive trastuzumab showed worse outcomes for those patients relative to hr+ patients.For example, Dawood et al. 25 showed that the 5-year os was 94% for luminal A (defined as er+ or pr+, her2-, grade 1-2), 85% for luminal B (defined as er+ or pr+, her2+; or er+ or pr+, her2-and high grade), and 80% for her2-type (defined as er-, pr-, and her2+).They also demonstrated that the 5-year rfs 93% A, 82% for luminal B, and 78% for her2type.In a multivariate model with luminal A tumours as reference, the hr for breast cancer death was 1.90 for luminal B and 1.36 for her2-type 25 .Thus, our suggests although her2-overexpressing breast cancer is associated with poor prognosis relative to hr+ breast cancer, integration of appropriate systemic chemotherapy together with trastuzumab may mitigate the risk and improve outcomes in patients with this subtype of breast cancer.
Trastuzumab was underutilized for the her2+ patients in our study, being used in only 76 of the 94 in this group (80.9%).Our results are similar to those observed by Noonan et al. 27 , who, in a study of early-stage her2+ breast cancer in Newfoundland and Labrador, found that only 76% of patients with her2+ breast cancer received adjuvant trastuzumab.Of the 18 her2+ patients in our study who did not receive adjuvant trastuzumab, 10 (55.5%) had subcentimetre tumours (presumably the reason that adjuvant trastuzumab was omitted), and 3 (16.7%)declined systemic treatment.No reason was provided for the omission of trastuzumab in the 5 remaining patients (27.8%).
In our study, 15 her+ patients had sub-centimetre tumours.Of those patients, only 5 (33.3%) received adjuvant trastuzumab.Of the 15, 13 had node-negative disease.Only 3 of the her2+ patients with sub-centimetre, node-negative disease (23.1%) received adjuvant trastuzumab.Both of the patients with sub-centimetre, node-positive her2+ tumours received adjuvant trastuzumab (100%).Patients with sub-centimetre, node-negative her2+ tumours who received trastuzumab had rates of 5-year rfs and os that were similar to rates in patients who did not receive trastuzumab (88.9% vs. 100% and 100% vs. 100%).However, that finding is limited by the small number of patients included in this analysis (n = 13, with 10 patients receiving trastuzumab, and 3 not receiving it).Current OnCOlOgy-VOlume 20, number 6, DeCember 2013 Copyright © 2013 Multimed Inc.Following publication in Current Oncology, the full text of each article is available immediately and archived in PubMed Central (PMC).
Our study also highlights that her2+ disease in itself represents a heterogeneous group.Analysis of the her2+ group revealed that outcomes differed by er and pr status.The 5-year os and rfs rates trended better in patients with er+ or pr+ disease than in patients with er-and pr-disease [5-year os: 92.1% (95% ci: 70.4% to 98.1%) vs. 86.9%(95% ci: 73.0% to 93.9%); 5-year rfs: 79.8% (95% ci: 62.0% to 89.8) vs. 71.4% (95% ci: 56.4% to 82.1%)].That finding is consistent with other studies that have shown a heterogenous biology for her2+ tumours.Carey et al. 26 previously showed that patients with her2+, er− tumours were particularly prone to early and frequent relapse and experienced particularly poor survival.That knowledge may become useful for the selection of patients who need more aggressive treatment.
Our study has several potential limitations, including its retrospective design and the relatively small number of patients with certain tumour subtypes, which limited power for statistical comparisons between receptor subgroups.Accordingly, our comparison of survival outcomes may not have reached statistical significance.Because of the small number of her2+ patients, our analysis combined patients who were her2+ and hr+ (that is, luminal B) with patients who were her2+ and er-and pr-into a single subgroup; however, as shown both in our study and in others 26 , those two groups are likely have different outcomes.Finally, classification based on er, pr, and her2 status is only an approximation for the underlying molecular breast cancer subtypes; however, because cost and technical issues have typically rendered gene expression profiling impractical as a routine diagnostic tool, the use of standard histopathologic surrogates-as in our study-is likely more relevant for practicing clinicians at this time.
Our study is one of the first to examine outcomes by breast cancer subtype since the introduction of adjuvant trastuzumab for her2+ tumours.Strengths of the study include its collection of information about adjuvant treatment, the inclusion of patients with her2+ sub-centimetre tumours, and its perspective on use of trastuzumab in a population-based setting in Canada.The study provides insights into the patient population selected for treatment, ability to deliver therapy, and breast cancer outcomes in a non-trial setting.

CONCLUSIONS
In the era of her2-targeted therapy, patients with her2+ tumours experience outcomes similar to those for her2-, hr+ breast cancer.By contrast, her2+, hr-breast cancer may represent a subtype with a particularly high risk of recurrence and mortality.Future studies need to focus on the development of improved adjuvant therapies for the her2+, hr-and the tn breast cancer subtypes, which are associated with the worst prognosis.

CONFLICT OF INTEREST DISCLOSURES
The present study was an unsponsored and unfunded research project.

Current OnCOlOgy-VOlume 20, number 6, DeCember 2013
. On multivariate analysis, patients with her2+ tumours had rates of os and rfs similar to those in hr+ patients, with a hr Copyright © 2013 Multimed Inc.Following publication in Current Oncology, the full text of each article is available immediately and archived in PubMed Central (PMC).aOf94her2+ patients, 39 (41.5%) had er+ or pr+ tumours, and 55 (58.5%) had er-, pr-tumours.bBecause of rounding, percentages may not add to exactly 100%.cByFisherexact test because of small sample sizes.er=estrogenreceptor, positive (+) or negative (-); pr = progesterone receptor, positive (+) or negative (-); her2 = human epidermal growth factor receptor 2, positive (+) or negative (-); ai = aromatase inhibitor.e543CurrentOnCOlOgy-VOlume20, number 6, DeCember 2013Copyright © 2013 Multimed Inc.Following publication in Current Oncology, the full text of each article is available immediately and archived in PubMed Central (PMC).

table ii
Five-year overall survival and relapse-free survival by tumour subtype and receptor status

table iii
Multivariate analysis a of the association between tumour subtype and overall survival or relapse-free survival Controlled for age, tumour stage, tumour grade, adjuvant chemotherapy, and positive lymph node status.er= estrogen receptor, positive (+) or negative (-); pr = progesterone receptor, positive (+) or negative (-); her2 = human epidermal growth factor receptor 2, positive (+) or negative(-) a