Utility of 18 F-FDG PET/CT for Predicting Pathologic Complete Response in Luminal HER2-Negative Breast Cancer Patients Receiving Neoadjuvant Chemotherapy CURRENT

Purpose: Pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) is a predictor of improved outcomes in breast cancer. In patients with luminal-type breast cancer, the response to NAC is variable and mostly limited. This study was an investigation of the predictive relevance of parameters of 18 F-FDG PET/CT for the pCR to NAC in patients with luminal HER2–negative breast cancer. Methods: A total of 109 consecutive hormone receptor-positive and HER2-negative breast cancer patients who were treated with NAC were enrolled in this prospective cohort study. The relationships between pretreatment 18 F-FDG PET/CT and clinical outcomes including pathologic response to NAC were evaluated. Results: All patients finished their planned NAC cycles and eight patients (7.3%) achieved pCR. In the receiver operating characteristic (ROC) curve analysis, pSUVmax exhibited high sensitivity and specificity for predicting pCR. Furthermore, multivariate logistic regression analysis revealed pSUVmax as a predictive factor for pCR (hazard ratio = 17.452; 95% CI = 1.847 – 164.892; p = 0.013). Conclusion: The results of this study suggest that 18 F-FDG PET/CT pSUVmax is a predictive factor for pCR of luminal HER2-negative breast cancer to NAC. AC:anthracycline + cyclophosphamide; CI:confidence interval; ER:estrogen receptor; 18 F-FDG PET/CT:Fluorine-18 fluorodeoxyglucose positron emission tomography; HER2:human epidermal growth factor 2; HR:hormone receptor; IDFS:invasive disease-free survival; IHC:Immunohistochemistry; ISH:in situ hybridization; NAC:neoadjuvant chemotherapy; pCR:Pathologic complete response; PR:progesterone receptor; pSUVmax:SUVmax of the primary breast tumor; ROC:receiver operating characteristic; SUV:standardized uptake value; T:docetaxel; TC:docetaxel + cyclophosphamide

core needle biopsy in this setting. Meanwhile, several large studies and a meta-analysis revealed that pathologic complete response (pCR) itself predicts survival of patients with aggressive breast cancers, including HER2-positive and triple negative subtypes [[7, 8]]. Therefore, the achievement of pCR after NAC has been accepted as a predictive marker of long-term oncologic outcomes and became a surrogate endpoint for prognosis in this setting [[9, 10]].
Meanwhile, for patients with hormone receptor (HR)-expressing breast cancer, the most common subtype, achieving pCR is infrequent and did not statistically correlate with survival in a meta-analysis [[11]]. However, patients with high grade/HER2-negative or luminal B tumors more frequently achieve pCR, which correlates with better survival and suggests the clinical value of differentiating luminal B from luminal A tumors before NAC [[11, 12]]. Because subtype determination by molecular assay is expensive [ [13]] and assays of specimens acquired by core needle biopsy do not always correlate with those of whole tumor specimens, new parameters are needed to refine NAC strategies or to estimate survival outcome before NAC.
Fluorine-18 fluorodeoxyglucose positron emission tomography ( 18 F-FDG PET/CT) provides quantitative data on the level of metabolic activity by calculating the degree of 18 F-FDG uptake, represented by the standardized uptake value (SUV), and has shown efficacy in diagnosing, staging, and monitoring various cancers. In breast cancer patients, its efficacy in evaluating chemotherapeutic effects has been reported: a correlation was observed between the intensity of FDG uptake and tumor characteristics such as tumor grade, HR status, and HER-2 status [ [14][15][16]] and the early metabolic response after one or two courses of NAC was shown to predict pCR, particularly for aggressive subtypes [ [17,18]]. However, few studies have evaluated the clinical implications of 18 F-FDG PET/CT in HR-positive-and HER2-negative breast cancer patients. Accordingly, the present study evaluated the utility of SUVmax on PET/CT to predict pCR in breast cancer patients treated with NAC followed by surgery, especially those with the HR-positive and HER2-negative subtype.

Study Design
To identify new predictive or prognostic markers for breast cancer from tumor or plasma specimens and functional images such as FDG-PET, we designed a prospective cohort study of breast cancer patients who underwent preoperative chemotherapy at Kyungpook National University Hospital (KNUH), South Korea. The criteria for NAC and study inclusion were tumor size > 2 cm or node positive (stage IIA-IIIC) resectable breast cancer with adequate organ function; patients with cT0 or multiple tumors were excluded from the current study. Patients underwent pretreatment and/or posttreatment 18 F-FDG PET/CT combined with conventional radiologic images. NAC regimens were selected based on the presence of lymph node involvement as follows: four cycles of anthracycline + cyclophosphamide (AC4) followed by 4 cycles of docetaxel (T4) for node-positive and 4 cycles of docetaxel + cyclophosphamide (TC4) or AC4 for node-negative tumors. Curative-intent surgery was scheduled to be performed within 6 weeks after the last cycle of NAC and postoperative treatment was adequately done based on the domestic and/or international guidelines. Follow-up imaging was performed semi-yearly for the initial 3 years, and then yearly or at the time of events. This study was approved by the Institutional Review Board of KNUH (KNUH_07-0033) .

Subjects
Among the 775 breast cancer patients who underwent NAC between January 2009 and December 2015, 109 female patients (median age 47 years; range 29-68 years) with an immunohistochemicallydefined luminal HER2-negative tumor (HR-positive and HER2-negative) were selected. The primary tumor features including clinical stage with tumor size and lymph node involvement, estrogen receptor (ER) status, progesterone receptor (PR) status, HER2 status, and Ki67 expression index are presented in Table 1. All patients (excluding two with cT4 who underwent additional T4 after AC4) received the planned regimens followed by curative surgical resection; one patient received fewer than the planned courses (6 out of 8 cycles) because of intolerance.

Pathologic Assessment
The tumor histology and biologic parameters were evaluated on both core-needle biopsy at initial diagnosis and the surgical specimen. Immunohistochemistry (IHC) was performed on formalin-fixed, paraffin-embedded tissue and ER and PR expression were scored according to the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines and graded by the Allred system [ [19]]. HER-2 positivity was defined as 3 + by IHC and/or by gene amplification using in situ hybridization (ISH). The Ki67 expression index was categorized with a cutoff of 14%. Molecular subtype was estimated based on these four IHC results and/or HER2 ISH, as defined in a prior report [[20]]. Histologic grade was determined using the modified Scarff-Bloom-Richardson grading system but was not calculated at baseline because the small-sized specimens acquired by core biopsy were insufficient to interpret mitosis count [ [21]]. After surgery, pCR was defined as the absence of invasive cancer cells in both breast and axillary nodes.

PET Imaging
18 F-FDG PET/CT was performed at baseline before NAC and/or before surgery and all patients fasted for at least 6 hours before 18 F FDG administration, which was confirmed by serum glucose concentration (less than 150 mg/dl). All imaging studies were obtained with a hybrid PET/CT scanner and PET data were reconstructed iteratively according to the standard procedure described previously. [22] The SUV was defined as: The SUVmax on 18 F-FDG-PET imaging was measured for both breast and axilla by two experienced nuclear medicine physicians, but only SUVmax of the primary breast tumor (pSUVmax) was used in analyzing the relationship with pCR and other clinical outcomes, as SUVmax of the axilla correlated with clinical N stage. We defined axillary lymph nodes as not detected (ND) in cases with clinically negative nodes or discordance with other images or pathological findings.

Statistical Analysis
Data are presented as numbers (%) or mean ± standard deviation unless otherwise stated. To evaluate the association of pSUVmax with variable parameters, subjects were divided into two groups based on the mean value of pSUVmax. Frequencies were compared using the chi-square test for categorical variables, and logistic regression models were used for identifying predictive factors for pCR among expected clinical and pathological variables including pSUVmax. Relapses were categorized as local, regional, and systemic recurrence, and invasive disease-free survival (IDFS) was calculated as the time between the date of diagnosis to the date of systemic recurrence and analyzed by the Kaplan-Meier method; the differences were assessed using the log-rank test, and each hazard ratio (HR) and 95% confidence interval (CI) was calculated using a cox-regression analysis. To explore the optimal cutoff value of pSUVmax to predict pCR, we performed receiver-operating characteristic (ROC) analysis. A P value of less than 0.05 was considered to be statistically significant. Analyses were conducted with IBM SPSS 22.0 (SPSS Inc., Chicago, IL, USA) and GraphPad Prism 7 (GraphPad Software, Inc., La Jolla, CA, USA).

Survival Analysis
During the follow-up period (median, 34.6 months; range, 0.5-85.3 months), eighteen patients (16.5%) experienced relapse (4 locoregional and 16 distant). Also, among 12 observed deaths, 11 were breast cancer-related (Table 2). Kaplan-Meier survival analysis demonstrated that advanced TNM stage, low ER expression, and high Ki67 were significantly associated with a worse IDFS (p = 0.001, 0.005, and 0.028, respectively) (Fig. 3). Multivariate survival analysis revealed that only clinical TNM stage was a prognostic factor for IDFS (HR and 95% CI, not calculated; p = 0.010; Table 4) However, pSUVmax and achievement of pCR were not associated with survival among the patients with HER2-negative luminal breast cancer in the current study.  Meanwhile, achieving pCR is associated with better prognosis in patients with aggressive tumor subtypes and thus pCR has been accepted as a surrogate marker for long-term survival. However, this prognostic value was not found in a study involving luminal subtype tumors [11]. Similarly, in the current study, a high pCR rate in the group with high pSUVmax did not connote better survival.
Instead, the pathologic stage of the residual tumors was significantly associated with survival when the patients achieving pCR were excluded (data not shown). These findings may indicate that luminal breast cancers are heterogeneous, having different levels of glucose metabolism, and the tumors with high pSUVmax may be more responsive but have a different clinical course compared to the others. The small sample size and relatively lower incidence of pCR compared to that of other NAC studies limit definite conclusions. The lower incidence of pCR can be explained by the higher proportion of luminal A subtype in the study population. Nevertheless, despite the unproven role of PET scanning and its decreasing use in our region, this study may stimulate new insights into PET scanning.
Moreover, the number of enrolled patients with HER2-negative luminal early breast cancer is high compared to that of other studies of the role of PET in the neoadjuvant setting, and, to our knowledge, this study is the first to establish the role of initial pSUVmax as a noninvasive predictive marker of pCR to NAC.

Conclusions
In this study, patients with luminal breast cancer generally have a low incidence of pCR to NAC and therefore are infrequent candidates for NAC. However, the results of the current study suggest that PET imaging may be a good modality for selecting the initial therapeutic plan and possibly optimizing the chance of breast preservation in patients with luminal type breast cancer.