Benefit of adjuvant chemotherapy in node-negative T1a versus T1b and T1c triple-negative breast cancer

National comprehensive cancer network guidelines recommend delivery of adjuvant chemotherapy in node-negative triple-negative breast cancer (TNBC) if the tumor is > 1 cm and consideration of adjuvant chemotherapy for T1b but not T1a disease. These recommendations are based upon sparse data on the role of adjuvant chemotherapy in T1a and T1b node-negative TNBC. Our objective was to clarify the benefits of chemotherapy for patients with T1N0 TNBC, stratified by tumor size. We performed a retrospective analysis of survival outcomes of TNBC patients at two academic institutions in the United States from 1999 to 2018. Primary tumor size, histology, and nodal status were based upon surgical pathology. The Kaplan–Meier plot and 5-year unadjusted survival probability were evaluated. Among 282 T1N0 TNBC cases, the status of adjuvant chemotherapy was known for 258. Mean follow-up was 5.3 years. Adjuvant chemotherapy was delivered to 30.5% of T1a, 64.7% T1b, and 83.9% T1c (p < 0.0001). On multivariable analysis, factors associated with delivery of adjuvant chemotherapy were tumor size and grade 3 disease. Improved overall survival was associated with use of chemotherapy in patients with T1c disease (93.2% vs. 75.2% p = 0.008) but not T1a (100% vs. 100% p = 0.3778) or T1b (100% vs. 95.8% p = 0.2362) disease. Our data support current guidelines indicating benefit from adjuvant chemotherapy in node-negative TNBC associated with T1c tumors but excellent outcomes were observed in the cases of T1a and T1b disease, regardless of whether adjuvant chemotherapy was delivered.


Introduction
Most early-stage, node-negative breast cancer patients face an excellent outcome with appropriately-selected locoregional and systemic therapy [1]. Triple-negative breast cancer (TNBC) represents a high-risk phenotype associated with a more advanced stage distribution and higher mortality rate compared to non-TNBC, even when detected early [2][3][4]. Chemotherapy is the standard adjuvant systemic treatment offered for TNBC and because these tumors tend to be biologically more aggressive, the threshold for offering adjuvant chemotherapy to node-negative patients is lower for TNBC compared to non-TNBC patients. However, the minimum tumor size for which a node-negative TNBC patient should be routinely offered adjuvant chemotherapy has not yet been definitively established.
Selective retrospective analyses suggest that TNBC patients with node-negative disease and primary tumors no larger than one centimeter achieve excellent 5-year locoregional and distant control, regardless of whether they receive adjuvant chemotherapy [1,5,6]. In contrast, others have shown that adjuvant chemotherapy is associated with improved outcomes even among cases of sub-centimeter disease [7]. Robust data regarding outcomes for T1a/ T1bN0 TNBC are sparse, because of challenges regarding early detection of TNBC as TNBC is more difficult to detect mammographically compared to non-TNBC [8][9][10].
Adjuvant chemotherapy is included as standard treatment in 2021 National Comprehensive Cancer Network (NCCN) management algorithms for all node-positive TNBC and for node-negative TNBC when the primary tumor is larger than one centimeter. NCCN guidelines are ambiguous for cases of node-negative T1b TNBC, with a recommendation that adjuvant chemotherapy be "considered"; adjuvant chemotherapy is not usually recommended for T1aN0 disease [11]. In view of chemotherapy toxicity, cost, and risk of overtreatment, we sought to review our experience by investigating the survival benefits associated with adjuvant chemotherapy among women diagnosed with node-negative T1 TNBC stratified by tumor size.

Patient population
The study design and data collection methods were approved by the Weill Cornell Medicine (WCM) and Henry Ford Health System (HFHS) Institutional Review Boards. HFHS includes patients treated at two sites in metropolitan Detroit, Michigan and WCM includes patients treated at two sites in Manhattan, New York. We reviewed the electronic medical records of TNBC patients ages 18 and older seen at WCM and HFHS from December 1999 to June 2018. Patients meeting inclusion criteria for this study were those with pathologically confirmed TNBC defined as immunohistochemistry revealing estrogen receptor < 1%, progesterone receptor < 1%, HER2/neu immunohistochemistry (IHC) 1 + or 0; cases of HER2/neu 2 + were included if they were negative for amplification by fluorescence in situ hybridization (FISH) according to the guidelines of the American Society of Clinical Oncology [12].
Patients with tumors that were pathologic stage T1N0 (T1a: > 1 mm but ≤ 5 mm; T1b: > 5 mm but ≤ 10 mm; T1c: > 10 mm but ≤ 20 mm), undergoing primary surgical therapy without the receipt of any neoadjuvant treatment were reviewed. Patients with unknown or unverified hormone receptor and/or HER2 status, an incomplete clinical record or those in whom delivery of adjuvant chemotherapy could not be confirmed were excluded. Patient, disease, and treatment characteristics were retrospectively reviewed and entered into a RedCap database. Primary tumors and lymph nodes were staged based on pathology reports according to the pathological anatomic stage of the eighth edition of the American Joint Committee on Cancer's AJCC Cancer Staging Manual [13].

Statistical analysis
The statistical programming language R version 3.6.1 (R Foundation for Statistical Computing) was used. Chi-squared tests assessed association between categorical variables; student's t tests were used to compare difference of continuous variables within groups. Multivariable logistic regression was performed to evaluate demographic and clinical variables associated with receipt of adjuvant chemotherapy including age at diagnosis, tumor size, presence of grade 3 disease, lymphovascular invasion, receipt of adjuvant radiation therapy, and type of breast surgery. The primary endpoints were overall survival, local recurrence-free survival, distant recurrence-free survival, and overall recurrence-free survival. The Kaplan-Meier plot and the unadjusted 5-year survival probability were evaluated. Log-rank test and Cox proportional-hazard (CPH) modeling wre used to assess the survival differences between patients who did and did not receive postoperative chemotherapy. Time 0 was defined as the date of diagnosis, defined as date of biopsy-proven malignancy. Additionally, after exclusion of patients with unknown adjuvant chemotherapy and radiation status, joint CPH modeling was performed to analyze the impact of adjuvant chemotherapy and adjuvant radiation therapy on overall survival. Survival data were censored at 15 years. Additional survival analysis was performed on a subset of patients ages 18-80 at diagnosis with at least one year of follow-up time.

Results
We identified 756 TNBC cases at WCM and HFHS. Clinicopathologic characteristics of the 282 patients with T1N0 disease at each site are shown in Table 1. Regarding the two study sites, the population at HFHS was composed of more Black American patients compared to WCM (57.1% vs. 11.1%; p < 0.0001), reflecting differences in the population demographics of Detroit compared to Manhattan. There were also differences between the two sites regarding histology; however, at both sites the majority of patients had invasive ductal carcinoma (84.52% vs. 93.43%; p < 0.0001). A higher proportion of grade 3 disease was seen at WCM than at HFHS (81.31% vs. 71.43%; p = 0.048). Additionally, patients at WCM were more likely to undergo contralateral prophylactic mastectomy than at HFHS (19.19% vs. 4
For the multivariable analysis, a strong correlation was demonstrated between type of breast surgery and receipt of adjuvant radiation where 76.4% of patients having mastectomy did not have adjuvant radiation and 82.4% of patients who did not have mastectomy had radiation (p < 0.0001). Therefore, we built two separate models, one utilizing adjuvant radiation as a covariate and another with type of breast surgery as a covariate. On multivariable analysis, tumor size (OR 5.66, CI 2.787-12.194; p < 0.0001), grade 3 disease (OR 2.75, CI 1.244-6.141; p = 0.0126), and postoperative radiation therapy (RT) (OR 2.66, CI 1.329-5.392; p = 0.0059) were associated with receipt of adjuvant chemotherapy. With inclusion of mastectomy as a covariate, only tumor size (OR 5.94, CI 3.006-12.457; p < 0.0001) and grade 3 disease (OR 2.59, CI 1.218-5.589; p = 0.0014) were associated with receipt of adjuvant chemotherapy (Table 3).

Adjusted multivariate outcomes
Joint modeling was performed on patients in whom adjuvant chemotherapy and RT status was known to account for the effect of both adjuvant chemotherapy and adjuvant RT on overall survival, given the substantial difference in receipt of adjuvant RT among patients who received adjuvant chemotherapy. With joint modeling of both adjuvant chemotherapy and RT, the delivery of RT did not change our results; overall survival was improved only in patients with T1c disease with receipt of adjuvant chemotherapy.

Discussion
In this multi-institutional study, we sought to determine the benefit of adjuvant chemotherapy in early-stage, nodenegative TNBC. With the limitation of retrospective data, the results generated from patients treated over the last two decades at two academic medical centers demonstrated that adjuvant chemotherapy was associated with improved 5-year overall survival in patients with stage T1c node-negative TNBC but not among those with smaller tumors. The majority of screen-detected breast cancers are hormone receptor positive, resulting in a paucity of data detailing survival outcomes for cases of small, node-negative TNBC. Nonetheless, the favorable prognosis of patients with early-stage TNBC has been demonstrated by others (Table 6) [1,5,6,14]. In 2012, Memorial Sloan Kettering reported a series of 194 T1a/b N0 TNBC from 1999 to 2006 and demonstrated excellent 5-year locoregional and distant control among those that received and those that did not receive adjuvant chemotherapy [6]. Similarly, a 2014 prospective multi-institutional cohort study from the National Comprehensive Cancer Network database involving 363 T1a/b N0 TNBC patients treated 2000-2009 reported excellent prognosis for T1aN0 and T1bN0 patients regardless of whether adjuvant chemotherapy was delivered [1].
Most studies looking at outcomes for cases of T1N0 TNBC are hampered by relatively small sample sizes of patients with T1a and T1b tumors. For example, a 2019 series of 45 TNBC and 71 hormone receptor-negative/ HER2 + patients with early-stage, node-negative disease (T1mi/a/bN0M0) reported no difference in survival for those receiving chemotherapy compared to those not receiving adjuvant chemotherapy [5]. In July 2020, An and colleagues published a single-center study of 351 TNBC patients with T1N0 disease, 88% of whom received adjuvant chemotherapy. Adjuvant chemotherapy improved recurrence-free survival only in T1c disease, not in T1b and T1a. No difference in recurrence-free survival was noted for patients with T1c disease receiving different chemotherapy regimens. However, it should be noted that this study included only 19 T1a and 67 T1b TNBC patients [7]. Ren and colleagues reported a 2019 single-institutional study of 354 T1N0 TNBC patients and found that adjuvant chemotherapy improved recurrence-free survival for T1c but not T1a or T1b patients. Of note, however, only seven T1a and 44 T1b patients were included in this study [15]. More recently in 2020, Zhai and colleagues reported on 7739 cases of T1N0 TNBC and also found that adjuvant chemotherapy was associated with improved overall survival only in T1c patients [16].
In an effort to address the fact that most individual studies are underpowered to detect possible benefit from adjuvant Table 5 5-year unadjusted  overall survival probability of  T1, T1a, T1b, and T1c nodenegative triple-negative breast  cancer patients treated with and  without adjuvant chemotherapy The 5-year survival probability was estimated from the Kaplan-Meier curve and the hazard ratio is derived from univariate Cox proportional hazards modeling; the p-value is associated with the hazard ratios through the Cox modeling chemotherapy in patients with T1a/bN0 TNBC, a nine-study meta-analysis was recently published, demonstrating that adjuvant chemotherapy was beneficial for the pooled cohort of over 750 T1bN0 patients [17]. Other national registry data from the United States and the Netherlands also indicated that adjuvant chemotherapy may improve outcomes for cases of node-negative TNBC associated with T1b tumors [16,18]. A limitation of the large-scale registries, however, is the lack of standardized treatment approaches across the multiple institutions contributing data.
In this study, we noted that a significantly larger proportion of patients who received adjuvant chemotherapy were also recipients of postoperative adjuvant RT (70.16% vs. 50.65%; p = 0.007). A recent cohort study by de Boniface et al. comprised nearly 50,000 women examined survival after breast conservation versus mastectomy. At a median follow-up of 6.28 years, they found that breast conservation therapy with RT led to improved survival compared to mastectomy [19]. These data suggest that RT might confer a survival advantage, perhaps to due abscopal tumor effects [20]. In our study, to address this difference regarding receipt of RT, joint modeling was performed to account for the possible impact on survival. Our analysis did not change our results that adjuvant chemotherapy improved overall survival in patients with T1c disease but did not significantly improve outcomes in patients with T1a and T1b disease. Out study strengthens the existing literature regarding the role of adjuvant chemotherapy in early-stage TNBC because we evaluated the management of patients seen in two large tertiary referral cancer programs, both of which are certified by the National Accreditation Program for Breast Centers. We found that node-negative TNBC patients with tumors no larger than one centimeter have excellent survival rates and may be spared the toxicity of systemic therapy. While Oncotype and Mammaprint are available to predict chemotherapy response and likelihood of metastasis for hormone receptor-positive and HER2-negative breast cancers, Mammaprint does not risk stratify TNBC patients as precisely [21,22]. TNBC remains a heterogeneous group of tumors that can be further categorized into subtypes based on gene expression analysis [23]. These subtypes have varying expression of other receptors and immune cells, conferring varying responses to chemotherapy. Specific to neoadjuvant chemotherapy, basal-like tumors exhibit the highest rates of pathologic complete response to carboplatin regimens, while luminal-androgen receptor lesions have the lowest pathologic complete response to all regimens [24]. These findings suggest that there may be utility in gene expression analysis for identifying patients who would benefit most from adjuvant chemotherapy and determining optimum regimens. In addition, ongoing research to develop new systemic therapies for TNBC, including targeted therapy for tumors with high expression of epidermal growth factor receptor, androgen receptor, and PDL-1 and immunotherapy with PARP inhibitors have the potential to impact adjuvant treatment decisions for TNBC and improve patient outcomes [25][26][27]. Ongoing work to examine the prognostic value of tumor infiltration leukocytes (TILs) both for survival outcomes as well as chemotherapeutic effects may also help to better identify early-stage cancers that may benefit most from adjuvant chemotherapy [28,29]. Clinical judgment regarding cases associated with higher-risk features (e.g., young age at diagnosis, histologic features consistent with more aggressive disease such as metaplasia) remain important in individualizing treatment plans.

Limitations
There are limitations inherent to our study, given the retrospective nature and the prolonged time during which data were collected as adjuvant chemotherapy regimens have evolved. We also acknowledge the small sample sizes of subsets within the T1N0 category, albeit larger than reported in several other studies. Regarding the possible effect of adjuvant radiation therapy, we recognize that our sample size precluded exploration of the possibility that adjuvant RT may confer a survival advantage. Our analysis was also limited by the inability to provide details regarding chemotherapy schedules and content. Lastly, we recognize that given the retrospective nature of our study, selection bias may exist regarding which patients were offered adjuvant chemotherapy. There was also a small but statistically significant difference in the amount of follow-up time, in which patients receiving adjuvant chemotherapy had greater median follow-up than patients not receiving adjuvant chemotherapy. We did not collect data regarding performance status and comorbidities, and therefore cannot ascertain whether this may account for the overall benefit seen among patients with larger tumors.

Conclusion
Our findings support current guidelines indicating overall survival benefit from adjuvant chemotherapy in node-negative TNBC associated with T1c tumors. We found excellent survival outcomes in T1a/b node-negative patients regardless of whether adjuvant chemotherapy was delivered. Additional research is necessary regarding more precise methods to risk stratify patients with node-negative TNBC and tumors no larger than one centimeter in size.