This was a retrospective, single-center study including patients whose primary breast cancer was confirmed to be malignant by core needle biopsies. 18F-FDG LymphPET was performed before ALN assessment (ALNA). The LymphPET results were compared with the classical “gold standard” pathological assessment of ALN: SLNB alone or in combination with ALND for resectable tumor, and fine-needle aspiration (FNA) and neoadjuvant treatment for unresectable tumor.
Patients aged between 18 and 70 years with clinical T1-2N0-1M0 (cT1-2N0-1M0) breast cancer were enrolled from September to December. Tumor staging was based on the eighth edition of the American Joint Committee on Cancer staging manual. Compared with pathological N category (pN), which based on microscopic assessment of at least one node required, include imaging and diagnostic biopsy, the clinical N category (cN) based on physical exam, imaging, FNA or core needle biopsy, sentinel node biopsy. The cN0 means no regional lymph metastases (by imaging or clinical examination). The cN1 means metastases to movable ipsilateral level II axillary lymph node(s). In our study, both the primary and ALN status were evaluated by ultrasound (US). For cN0 patients, ALN was pathologically assessed using SLNB alone or SLNB and ALND. For cN1, ALN was pathologically confirmed using FNA.
The exclusion criteria including: uncontrolled diabetes, pregnant or breastfeeding, had already undergone neoadjuvant treatment, SLNB, or ALND, or had any clinical evidence of distant metastases at the time of enrolment. The study was approved by the Institutional Ethics Committee and signed informed consent was obtained from every patient prior to participation in this study.
LymphPET System and Examination
The LymphPET (PEMTECHTM, Shanghai, China) device contains movable double-planar confronted detectors with an axilla capability view and an adjustable distance between the two detector plates from 10 to 30 cm. Size of the sensitive detection area is 208×208 mm with a useful field of vision (FOV) of 204×204 mm, consisting of 16 modules in each plate with LYSO crystals of 1.2×1.2×12 mm, and coupled to a new Double Readout Module (DRM) system with J6 series SiPM arrays. The results show a transaxial resolution of 1.4 mm, uniformity of 10%, timing resolution of 500 ps, true coincidence rate peak of 390 kcps @20 MBq, and NEC rate peak of 319 kcps @12.6 MBq.
Patients were fasted at least 6 h prior to receiving a standardized injection of 4mCi 18F-FDG. Blood glucose levels were required to be lower than 10 mmol/L. After a resting period of 60 min to allow tracer distribution, LymphPET was performed with the patient standing up and abduction arm (Fig. 1).
LymphPET Image Analysis
In order to postulate a precise method of quantification adapted to this geometry, the concept of LUV, as a surrogate for SUV, or LymphPET Uptake Value, has been used for this special biplanar symmetry. The LUV, like SUV characterizes the activity intensity in the image, but unlike the SUV, the quantification of LUV does not suffer the same data corrections such as attenuation or scattered photons . Although validity of the LUV as a quantification metric has not yet been demonstrated due to its novelty, a similar concept is generally accepted [10-12] PUV (PEM Uptake Value) being demonstrated in a large single study published in the AJR11 a statistically significant correlation between PUV and SUV in the breast.
Image evaluation and quantification of maximum single-voxel LymphPET Uptake Value (maxLUV) was performed using commercial Medical Image Merge (version 6.5.4; MIM Software Inc, OH, USA), which is the professional image processing software certified by the United States Food and Drug Administration. Two separated experienced nuclear medicine physicians with 10 years of experience in PET / CT, who were blinded to study-related information apart from the laterality of breast cancer, analyzed the images and marked the regions of interest (ROIs) manually.
ALNs were considered positive under the following three conditions: (1) the positive focus was located in the axillary region but not in skin, muscle, or bone; (2) 18F-FDG uptake was higher than the reference background (fat tissue); and (3) the physiological lymphatic uptake was excluded. For quantitative analysis, the size of the lymph node (LN) was measured and an elliptic-shaped region of interest (ROI) was drawn manually. 18F-FDG uptake into this ROI was calculated as ALN maxLUV (maxLUV). The highest maxLUV was selected as study value in case multiple LNs were detected. Additionally, three separate ROIs measuring 1 cm in diameter (fat background) were located at the axillary adipose tissue, and the mean value of those areas was defined as maxLUVfat. Moreover, three 1-cm diameter ROIs were located at the biceps brachii and ectopectoralis muscles (muscle background) and the mean value was noted as maxLUVmuscle. Tumor-to-background ratio (TBR) was calculated as the coefficient between the positive value of maxLUV and the corresponding background value: TBR1=maxLUV/maxLUVfat; TBR2=maxLUV/maxLUVmuscle. Thus, three parameters were calculated for each patient: maxLUV, TBR1, and TBR2.
Surgery and Pathological Evaluation
For cN0 patients, ALN was pathologically assessed using SLNB alone or SLNB and ALND. For cN1, ALN was pathologically confirmed using FNA. If FNA was negative, SLNB was performed. If FNA was positive, neoadjuvant treatment was performed. For those patients with no more than two SLN positives, the decision whether to perform further axillary dissection depended on the operative type (breast-conserving therapy or mastectomy) and individual pathological characteristics. All LymphPET scans were performed before FNA.
In SLNB, nodes were identified by injection of blue dye (methylthioninium chloride, Jiangsu Jumpcan Pharmaceutical Co., Ltd.) and/or by intraoperative guidance of the gamma detector probe (neo2000, Neoprobe Corporation) using a radiocolloid (99mtechnetium sulfur colloid, CIS US Inc. Bedford, MA, USA). In our study, SLNs were defined as any blue-stained node, any node with a blue-stained lymphatic channel directly leading to it, any node with radioactive counts 10% or more of the most radioactive node, or any pathologically palpable nodes. Internal mammary SLNB would not be performed even if the nodes were detected using lymphoscintigraphy.
Touch imprint cytology was routinely performed on every SLN that was harvested. SLNs were cut along the long axis at a 2.0- to 3.0-mm interval intraoperatively, and each cut surface was touched, at least three times, onto a clean glass slide and stained with hematoxylin and eosin. Staining of serially-sectioned slices was performed in 100-μm intervals on every node. Slides were sent for cytopathological examination immediately after preparation. Moreover, additional slices were formalin-fixed and paraffin-embedded for further evaluation. According to pathological results, ALNs were classified as macro-metastasis (>2.0 mm), micro-metastasis (0.2-2.0 mm) and isolated tumor cells (ITC, <0.2 mm) according to the tumor-node-metastasis staging system.
It should be noted that both ITC and micro-metastases (<2 mm) were considered negative in the final statistical analysis.
Data were presented as range with means and standard deviation. Receiver operating characteristic (ROC) curves and area under the ROC curves (AUCs) were employed to evaluate the predictive value of 18F-FDG LymphPET, by determining the optimum cutoff for the three recorded parameters. Logistic regression analysis was used to determine independent factors predictive of ALN metastases. Multiple linear regression analysis was used to confirm whether maxLUV correlated with different clinical factors. For categorical variables, differences were calculated using the Chi-squared test or Fisher’s exact test where applicable. Statistical analysis was performed using the SPSS 20.0 software (IBM Inc., Chicago, IL, USA), and two-sided p values were reported with an alpha of 0.05 (p<0.05).