18F- FDG PET/CT-derived parameters predict clinical stage and prognosis of esophageal cancer

Background Although 18F- FDG PET/CT is validated in baseline workup of esophageal cancer to detect distant metastases, it remains underused in assessing local staging and biology of the primary tumor. This study aimed to evaluate the association between 18F- FDG PET/CT-derived parameters of esophageal cancer, and its clinico-pathological features and prognosis. Methods All patients (n = 86) with esophageal adenocarcinoma or squamous cell cancer operated between 2005 and 2014 were analyzed. Linear regression was used to identify clinico-pathologic features of esophageal cancer associated with the tumor’s maximal Standardized Uptake Value (SUVmax), Total Lesion Glycolysis (TLG) and Metabolic Tumor Volume (MTV). ROC curve analysis was performed to precise the optimal cutoff of each variable associated with a locally advanced (cT3/4) status, long-term survival and recurrence. Kaplan Meier curves and Cox regression were used for survival analyses. Results High baseline SUVmax was associated with cT3/4 status and middle-third tumor location, TLG with a cT3/4 and cN+ status, whereas MTV only with active smoking. A cT3/4 status was significantly predicted by a SUVmax > 8.25 g/mL (p < 0.001), TLG > 41.7 (p < 0.001) and MTV > 10.70 cm3 (p < 0.01) whereas a SUVmax > 12.7 g/mL was associated with an early tumor recurrence and a poor disease-free survival (median 13 versus 56 months, p = 0.030), particularly in squamous cell cancer. Conclusions Baseline 18F- FDG PET/CT has a high predictive value of preoperative cT stage, as its parameters SUVmax, TLG and MTV can predict a locally advanced tumor with high accuracy. A SUVmax > 12.7 g/mL may herald early tumor recurrence and poor disease-free survival.


Background
Esophageal cancer is associated with aggressive lymphatic spread, resulting often in locally advanced or metastatic disease upon diagnosis [1]. Metabolic imaging with 18F-Fluorodeoxyglucose Positron Emission Tomography/ Computerized Tomography ( 18 F-FDG -PET/CT) has been integrated into the preoperative workup of esophageal cancer for the detection of distant suspicions lesions [1][2][3], interval metastases [4] or assessment of response to neoadjuvant treatment [5]. Esophageal cancer workup should use three-modality staging with Computerized Tomography (CT), endoscopic ultrasound (EUS) and 18 F-FDG PET/CT [2,3], as failure to identify locally advanced tumors (cT3/4 or N+) may lead to omission of neoadjuvant treatment before surgery, compromising patient survival [2,6]. To this day, 18 F-FDG PET/CT is not primarily used for local staging of the primary tumor due to its poor spatial resolution. However, Malik et al. recently demonstrated a significant predictive value of Metabolic Tumor Volume (MTV) in differentiating early-stage (cT1/2) from locally advanced (cT3/4) lesions [7]. Obtaining accurate cTN staging information through 18 F-FDG PET/CT may be of prime importance particularly in cases where EUS is unavailable or if the tumor is obstructive (up to 19% of patients) [7]. Furthermore, although the predictive value of 18 F-FDG PET/CT derived parameters maximal standardized Uptake Value (SUV max ), Total Lesion Glycolysis (TLG) and MTV on long-term survival of esophageal cancer has been extensively reported in the literature [8], the absence of universally accepted thresholds and the paucity of data for each histological type limit the predictive value for the individual patient.
The aim of our study was to assess the clinicopathological correlations and staging value of 18 F-FDG PET/CT derived parameters SUV max , TLG and MTV, as well as their predictive value in patient survival and tumor recurrence.

Methods
All patients operated for esophageal adenocarcinoma or squamous cell cancer, with curative intent, from 2005 to 2014 in our tertiary referral center and a baseline 18 F-FDG PET/CT in the preoperative workup were included in this study. Demographic, clinical and histological data were retrieved from our prospectively maintained database.
In all patients, routine preoperative staging was performed by esophagogastroduodenoscopy, EUS and thoraco-abdominal CT scan. Since 2005 18 F-FDG PET/ CT was integrated in the baseline preoperative workup, according to current recommendations [2]. TNM stage was defined according to the 7th TNM classification [9]. Neoadjuvant treatment was administered for locally advanced lesions (cT3/4 and/or N+), with 5FU-platin or carboplatin-paclitaxel based chemotherapy and external beam radiation of 41-54 Gy. R0 resection was defined as the presence of tumor within 1 mm of resection margins. Postoperative follow-up included a thoraco-abdominal CT scan every 4 months for the first two postoperative years and further workup in cases of suspected recurrence [10]. Early recurrence was defined as any documented recurrence in the first 12 postoperative months. Follow-up data were last updated in November 2018, to assure a minimum follow-up of 4 years for all patients.

Baseline 18 F-FDG PET/CT and derived parameters
Since the beginning of this study, we introduced our own PET/quality control program used in several national and international PET studies [11] until our center participated to the quality control program by the European Association of Nuclear Medicine (EANM)-EARL as PET/CT Center of Excellence in October 2011, for which we have been accredited each year so far. For 18 F-FDG PET/CT, patients fasted for at least 6 h before and blood glucose was measured before administration of the radiotracer and required to be < 8.5 mmol/L, otherwise the scan was rescheduled. Each patient received 5.5 MBq/kg until 08/2011 or 3.5 MBq/kg thereafter of 18 F-FDG intravenously and remained in a calm and warm area for 1 h post injection. Thereafter, the patient was asked to void and subsequently was placed in the scanner. Images were acquired on PET/CT scanner (Discovery LS until 08/2011 and thereafter Discovery D690 TOF; GE Healthcare, Waukesha, WI) with scatter and point-spread function recovery corrections. The CT scan (140 kV, 80 mA pith 1.5 until 08/2011 and thereafter 120 keV, 80-200 AutomA/SmartmA, pitch 1.375) was used for attenuation correction. The CT scan was followed by a PET over the same body region (3-5 min/ bed position acquired in 2-D mode until 08/2011 thereafter 1 min 30 s-2 min/bed position acquired in 3-D mode). Images were reconstructed using OSEM (2 iterations, 26 subsets) with a 5.4-mm FWHM post-filter and 3.91-mm FWHM loop filter until 08/2011 and thereafter using OSEM (3 iterations, 16 subsets) with 5-mm postfilter and PSF and TOF recovery corrections. Two nuclear medicine physicians (AP, JOP) closely reviewed the images using for analysis an Advantage Workstation (version 4.6, GE Healthcare, Waukesha, WI) using PET VCAR software to compute SUV mean , SUV max , TLG and MTV.
SUV max was defined as the point of maximal radiotracer uptake value within the delineated tumor volume (g/mL). MTV represents the metabolically active volume of the main tumor (cm 3 ), whereas TLG was computed as the product of MTV multiplied by the tumor's SUVmean . In order to define the contouring margin of primary tumor, a volume of interest around the tumor was drawn carefully to incorporate the target lesion in transaxial, sagittal and coronal planes. For tumor delineation we used a 42% threshold, as it is the most commonly used in the literature [7,12].

Statistical analysis
Linear regression was performed to assess correlation between several clinicopathological variables and baseline SUV max , TLG and MTV. For each PET/CT derived parameter, a Receiver Operating Characteristic (ROC) curve analysis was performed to assess whether an optimal cutoff could be associated with locally advanced lesions (cT3/4), overall survival and early tumor recurrence. Overall and disease-free survival were analyzed with the Kaplan-Meier method and log-rank test as well as and a Cox regression analysis. Based on previously published differences in FDG uptake between adenocarcinoma and squamous cell cancer [3,13], subgroup analyses were separately carried out for each histological type. Co-variates with a p-value< 0.2 on a univariate level were entered to a backward elimination process, allowing to build the final multivariate model with the lowest Akaike Information Criterion (AIC) value. Significance level was set at p < 0.05 and all tests were two-sided. Statistical analysis was performed with RStudio (version 3.2.3, RStudio Team 2015, Boston, USA) and SPSS (version 23.0, Chicago, USA).

Results
From the 141 patients operated in the study period, 89 had a baseline 18 F-FDG PET/CT in their workup (63%). Three of them were excluded from analysis because of histology other than adenocarcinoma or squamous cell; thus, the current series consists of 86 patients. Baseline patient characteristics are outlined in Table 1. Adenocarcinoma represented 53% and squamous cell histology 47% of all lesions; 94% of all tumors were FDG-avid in 18 F-FDG PET/CT.

Baseline 18 F-FDG PET/CT -derived parameters and initial tumor staging SUV max (maximal standardized uptake value)
Median baseline SUV max was 12.1 g/mL (range 2.8-48.0) for all tumors. Middle third tumor location, advanced cT and cN stage as well as squamous cell histology were associated with higher SUV max values on a univariate level, however only tumor location and cT stage remained significant on multiple regression ( Table 2). cT3/4 tumors had an expected SUV max 6.61 higher than a cT1-2 lesion (β coefficient 6.61, 95%CI 2.40, 10.81, p = 0.002), and middle third tumors an expected SUV max 7.01 higher than GEJ lesions (β coefficient 7.01, 95%CI 0.71-13.32, p = 0.029). The multivariable model presented a good fit to the data (R 2 = 0.2804, F-statistic 4.676 on 6 and 72 DF, p < 0.0001).
When the two histological subtypes were analyzed separately, there was no significant association of SUV max with DFS for adenocarcinoma. For squamous cell carcinoma, a baseline SUV max > 12.7 g/mL along with pT and pN stage independently predicted worse DFS (Table 4). The distinct metabolic profile of the two histological types is illustrated in Fig. 4. No association was found between baseline SUV max and overall survival (OS), neither on the Kaplan-Meier (Fig. 3)

Discussion
In this study, higher baseline SUV max of esophageal cancer was significantly related to a middle-third tumor location and a cT3/4 stage, whereas higher TLG was related to cT3/4 and cN+ stage. Baseline SUV max > 8.25 g/mL, TLG > 41.7 and MTV > 10.7 cm 3 were associated with cT/4 stage, whereas SUV max ≥ 12.7 g/mL predicted early recurrence and poor disease-free survival.
Currently, 18 F-FDG PET/CT is mostly used for the detection of distant metastases as it can identify suspicious lesions as small as 1 cm [2,14,15]. Walker et al. reported 18 F-FDG PET/CT -detected distant lesions precluding curative treatment in 21% of patients [15], even though the specificity of an FDG 'hot spot' remains low [16]. Until recently, limited spatial resolution of PET for esophageal wall layers and adjacent structures had restrained this modality as a detector of distant metastases [15]. Recent data, however, reinforce the role of 18 F-FDG PET/CT in better defining cTNM stage and the tumor's biology, the latter being FDGavid in 84-92% of cases especially if it infiltrates the submucosa [7,15,17].
It is generally admitted that all 18 F-FDG PET/CT-derived parameters are higher in advanced and aggressive tumors, however no universally accepted cutoffs exist to this day, limiting their cTNM staging value in the individual patient. Malik et al. suggested SUV max > 4.1 (sensitivity 85%, and specificity 48%) and MTV > 23.4 cm 3 (sensitivity 64%, specificity 67%) as optimal thresholds to distinguish cT1/2 from cT3/4 lesions [7]. In our study, SUV max and TLG presented a higher predictive value than MTV in preoperative staging; ROC curve analysis yielded as significant cutoff values to predict preoperative cT3/4 status with high accuracy a SUV max >  3 . This discrepancy may be explained by the predominance of adenocarcinoma in the Malik study (75% of patients, versus 53% in the present study); adenocarcinoma has being described as less FDG-avid with lower SUV max values compared to squamous cell cancer, probably in relation to increased expression of the HK-II biomarker [3,13]. However, as in our study tumor histology was not independently associated with SUV max (Table 2), no separate cutoffs of SUV max were justified for each type. One might argue that EUS is sufficient to identify locally advanced lesions (cT3/4 or N+) and thus to direct the patient to neoadjuvant treatment before surgery. However, previous data from our institution suggest a rather low rate of accurate usT (51%) and usN (72%) staging, with the highest rates of understaging among active smokers [18]. Indeed, a three-modality workup strategy ( 18 F-FDG PET/CT, CT and EUS) offers the highest probability (84%) to correctly select patients for surgery, a fortiori when 18 F-FDG PET/CT is the first exam performed [3]. Complementary use of these exams could improve staging accuracy, directing patients with locally advanced lesions to neoadjuvant treatment and avoiding its unnecessary toxicity for early-stage tumors.
Although several studies have reported poor long-term prognosis associated with high baseline SUV max its prognostic value for the individual patient remains limited, as great variability is seen in the suggested cutoffs (ranging from 3 to 9 g/mL) [16,[19][20][21][22]. This variability might be linked to patient-related factors (e.g. cardiac output, tumor histology) but also to the PET/CT configuration and interobserver variability. To overcome these limitations, tumor-to-blood Standard Uptake Ratio (SUR) has been recently published as a promising predictor of survival [23,24]. Hofheinz et al. [23] suggest the superiority of baseline SUR over SUV max to predict overall survival in squamous cell cancer patients treated with definitive chemoradiation, whereas Bütof et al. [24] propose the restaging SUR value along with baseline MTV as a reliable survival predictor. In a recent meta-analysis, Han et al. reinforce the prognostic value of MTV and TLG for overall survival, although no specific cutoff value is suggested and both histological types are jointly taken into account [8]. In the present study none of the 18 F-FDG PET/CT derived parameters demonstrated significant association with overall survival.
The added value of our study lies in the identification of SUV max > 12.7 g/mL as an independent predictor of early tumor recurrence, within the 1st postoperative year. This result is of major clinical importance, as it might help early identification of patients with resectable esophageal cancer, who may not benefit from surgical resection as their risk of short-term recurrence is significantly increased. Indeed, we observed a significantly shorter DFS for squamous cell tumors with a baseline SUV max > 12.7 g/mL; as definitive chemoradiation is a valid treatment option for this histological type, the above threshold may provide valuable prognostic information for the individual patient and guide accordingly therapeutic management. Schreurs et al. previously reported worse DFS for patients with baseline SUV max > 3.67 g/mL, although no correlation with overall survival was found in that study either [3]. Markers of aggressive biology (GLUT-1, p53, Ki-67, HK-II) were studied in relation to SUV max , although no clear immunohistochemical profile was found for high-FDG uptake tumors Fig. 3 Baseline SUV max as a predictor of disease-free and overall postoperative survival. a SUV max > 12.7 g/mL was a significant predictor of poor disease-free survival (median DFS 13 versus 56 months, p = 0.030), b but not of overall survival compared to the others [3]. Thus, even though it is generally admitted that high baseline SUV max may herald tumor aggressiveness and early recurrence, the underlying mechanism remains poorly understood. Our team previously reported active smoking as an independent predictor of early recurrence after esophagectomy [25], which is being confirmed in the present analysis along with a baseline SUV max > 12.7 g/mL, and may express pathologic DNA-methylation patterns and tumor proliferation genes [26]. This study has some limitations that need to be addressed. Retrospective analysis has an inherent drawback in data completeness, even though our institutional database is maintained prospectively, with a stringent followup of all patients. The use of two different PET/CT scanners over the years might have introduce some bias in SUV max measurements especially in the smallervolume lesions, because of a better resolution recovery in the newer PET/CT scanner (from September 2011 and thereafter). However, both scanners measured similar SUV for lesions above 1.2 cm diameter (about 4 cm 3 ). Since the mean MTV in our series was well above this threshold (48 cm 3 ± 83 cm 3 ), it can be estimated that this scanner change had little effect on the results. Moreover, we compared the SUV max across both scanners in lesions with a MTV ≥30 cm 3 (where SUV max would be similar with both scanners) vs. lesions < 30 cm 3 , where the new scanner could measure SUV max better. No differences were observed across scanners in the large lesions (MTV ≥ 30 cm 3 : SUV max 20.6 ± 7.8 g/mL for the new scanner vs. 17.3 ± 12.0 g/mL for the previous one, p = 0.36). This was also the case for the smaller lesions (MTV < 30 cm 3 : SUV max 9.9 ± 4.9 g/mL for the new scanner vs. 11.9 ± 4.4 g/mL for the previous one, p = 0.17). Results of this comparative analysis between the two CT scans are provided as supplementary material, and enhance our belief that the scanner change could not influence significantly our results.
Although there was practically no heterogeneity in 18 F-FDG PET/CT results over the years, inaccuracies in the preoperative workup may have occurred for both T and N staging especially with EUS, confounding associations with 18 F-FDG PET/CT parameters. As mentioned above, histological type and patient-related factors may influence 18 F-FDG PET/CT-derived parameters and limit the use of universally accepted cutoffs. In the present study, due to the small number of patients per histological type our subgroup analyses can be considered as exploratory, needing validation in larger cohorts. Prospective validation of the suggested cutoffs should take into account the specificities of histological types studied.

Conclusions
18 F-FDG PET/CT derived parameters SUV max > 8.25 g/ mL, TLG > 41.7 g and MTV > 10.70 cm 3 were significantly associated with locally advanced cT3/4 stage and a baseline SUV max > 12.7 g/mL with early tumor recurrence and poor disease-free survival, particularly for squamous cell cancer. These findings add to the existing knowledge of 18 F-FDG PET/CT's value in esophageal cancer management and reinforce its predictive value in terms of tumor recurrence and survival. A prospective study is currently running in our institution to correlate these values with high-resolution CT and MR imaging characteristics of the primary tumor.
Additional file 1. Comparison of the SUVmax values for large (MTV>=30cm 3 ) and small (MTV<30cm 3 ) tumors, for the two CT scanners used in the study.