Introduction

Lung cancer is the most frequent malignancy throughout the world since 1985 (1) and the leading cause of cancer-related death in both men and women (2). Curative treatment can be offered to lung cancer patients with early diagnosis, in whom computed tomography (CT) is the first diagnostic tool (3). One of the most important prognostic factors in lung cancer is tumor stage. Hence, proper staging is very important when determining prognosis and choosing appropriate treatment. T-staging is based on CT findings, although this modality may be unable to distinguish a tumor from atelectasis or show disease extent. Fluorine 18-fluorodeoxyglucose positron emission tomography/CT (¹⁸F-FDG PET/CT) is an effective method for the diagnosis, staging, evaluation of treatment response, follow-up for recurrence and re-staging of lung cancer. In our retrospective study, we investigated the contribution of ¹⁸F-FDG PET/CT to lung cancer diagnosis, staging and management, as well as the prognostic and survival effects of the maximum standardized uptake value (SUV_max) of the primary lesion in an ¹⁸F-FDG PET/CT.

Materials and Methods

The present study included 50 patients who have been referred to Ege University Faculty of Medicine, Department of Nuclear Medicine between February 2012 and February 2014, and who have undergone ¹⁸F-FDG PET/CT scanning for the diagnosing and staging of lung cancer. Patients were injected intravenously with 250-400 MBq of ¹⁸F-FDG at least four hours after fasting for ¹⁸F-FDG PET/CT imaging. Approximately one hour after the tracer injection, PET and CT scans were obtained using a PET-CT scanner (biograph high-definition 16-slice CT, Siemens Healthcare, Erlangen, Germany), and the PET and CT images were loaded onto three-dimensional workstations. Visual and semi-quantitative evaluation of the lesions observed on ¹⁸F-FDG PET/CT were carried out. In case of non-physiologically enhanced areas of activity associated with soft/bone tissue on CT, the finding was identified as “PET lesion”, and these PET lesions were interpreted as malignant if either the SUV_max value >2.5 or when the FDG uptake was significantly higher than the background tissue. Adrenal gland lesions with a higher FDG uptake than the liver were considered as malignant. A total of 30 patients or their close relatives were accessed to assess survival. One of those 30 patients had synchronous double primary lung cancers. The correlation between the SUV_max value of the primary lesion and patient survival was evaluated. The findings of ¹⁸F-FDG PET/CT were compared with radiologic and histopathologic results as well as the final clinical decisions and the determined stages.

Statistical Analysis

The IBM SPSS V 21.0 software was used for statistical analysis. Kolmogorov-Smirnov and Shapiro-Wilk (S-W) tests were utilized for normality analysis of data distribution, and Mann-Whitney U test was applied to determine the differences between the two groups in case of non-normal distribution. A Spearman’s rho correlation test was utilized to investigate the correlation between the parameters, and the Kaplan Meier method was used for survival analysis. The differences in survival curves taking different variables into account were tested by using the log-rank test.

Results

This study included 50 patients (46 males, four female, mean age 63.0±8.6) who underwent ¹⁸F-FDG PET/CT. A histopathologic diagnosis of lung cancer was obtained in all patients. There were 51 tumors in 50 patients since one patient had synchronous double primary lung cancers that have been diagnosed as adenocarcinoma. The histopathologic results of the lung lesions revealed 23 adenocarcinomas, 15 squamous cell carcinomas (SCC), four small-cell lung cancers (SCLC), two large-cell neuroendocrine carcinomas, one adenoid cystic carcinoma, one carcinoid tumor and five non-SCLC ​[(NSCLC)​, with no specific classifications]. Patient and tumor characteristics are summarized in Table 1. In terms of detecting malignancy, the ¹⁸F-FDG PET/CT yielded true positive results in 48 and false negative findings in three cases out of the 51 tumors. The tumor size and histopathologic features of these three false negative lesions were 0.6 cm adenocarcinoma (SUV_max: 1.4), 1.3 cm adenocarcinoma (SUV_max: 1.6) and 1 cm carcinoid tumor (SUV_max: 0.8). In the patient who had synchronous double primary lung cancers, the adenocarcinoma of 0.6 cm (SUV_max: 1.4) had a false negative finding whereas the 2.7 cm-sized (SUV_max: 18.6) adenocarcinoma was detected. The sensitivity, accuracy and positive predictive value of ¹⁸F-FDG PET/CT in detecting primary malignancy were calculated as 94%, 94% and 100%, respectively. The mean SUV_max values of adenocarcinoma (n=23) and SCC (n=15) lesions was 16.8±13.5 and 17.9±5.6, respectively. These two types of lung cancer were similar in terms of SUV_max values (p=0.2).

The correlation between tumor diameter and SUV_max was evaluated for all tumors. The mean tumor diameter of the 51 tumors was 5.0±2.9 cm. Tumor sizes ranged from 0.6 cm to 12.3 cm. A statistically significant positive correlation (r=0.4) was identified between tumor size and SUV_max (p=0.002). Two patients had post-obstructive atelectasis due to the tumor, and the ¹⁸F-FDG PET/CT was able to distinguish between the tumor and the atelectasis owing to an intense FDG uptake in the tumor area. Thus, the tumor size was correctly calculated (Figure 1). The histopathologic results of the lymph nodes were compared with nodal staging according to ¹⁸F-FDG PET/CT. Histopathologic evaluation of the lymph nodes was performed on 21 patients. The ¹⁸F-FDG PET/CT findings were interpreted as true negative in nine, false positive in six, true positive in two and false negative in four patients. In this regard, 11/21 (52%) of the patients were correctly staged by ¹⁸F-FDG PET/CT, while 4/21 (19%) patients were incorrectly down-staged and 6/21 (28%) were incorrectly up-staged by PET-CT. Within the group of 15 histopathologically N0 patients, the ¹⁸F-FDG PET/CT was N0 in nine (true negative) while it was false positive in the remaining six patients. ¹⁸F-FDG PET/CT identified subcarinal, right hilar and left interlobar hypermetabolic lymph nodes in one of the six patients with a false positive result (Figure 2). Thus, the patient was staged as N3 on ¹⁸F-FDG PET/CT. Histopathologically, these lymph nodes were assessed as anthracosis and the patient was down-staged. When assessed on a patient-by-patient basis, hilar and interlobar lymph nodes were most commonly recorded as false-positives on ¹⁸F-FDG PET/CT (in 66% of patients with false-positive lymph nodes). A total of 20 distant metastatic sites (eight bone, five liver, three surrenal gland, two brain, one spinal, one pons metastases) were detected in 14 patients (28% of all patients) by ¹⁸F-FDG PET/CT . Of these metastases, 12/20 (60%) could only be detected by ¹⁸F-FDG PET/CT. When evaluated on a patient-by-patient basis, in 14% (7/50) of all patients, distant metastases were detected only by ¹⁸F-FDG PET/CT.

¹⁸F-FDG PET/CT led to a change in treatment plan based on conventional methods in 17/50 (34%) patients. Through conventional staging, 11 of these 17 patients were classified as “at a potentially operable” stage (stage 1-3A), five were classified as “at a potentially inoperable” stage (stage 3B-4), and one patient had a lesion that was assessed to be benign by conventional methods. Within the group of 11 patients classified as “at a potentially operable” stage, four had N3 lymph node metastases and five had distant metastases as identified by ¹⁸F-FDG PET/CT. As a result, these patients underwent chemotherapy thus avoiding unnecessary surgery in 9/50 (18%) patients. In the other two patients, N1 lymph node metastasis was detected by ¹⁸F-FDG PET/CT, and neoadjuvant chemotherapy was started prior to the operation. Within the group of five patients who were classified as “at a potentially inoperable” stage, three were identified with M1 disease by conventional staging. The areas considered as distant metastases in these three patients were interpreted as benign by ¹⁸F-FDG PET/CT and thus were accepted as candidates for curative treatment. The other two patients were categorized as stage 3B by conventional methods and were thus planned for chemotherapy and localized radiotherapy. However, distant metastases were detected by ¹⁸F-FDG PET/CT in these two patients who underwent chemotherapy and palliative radiotherapy. Another patient had a lesion that was interpreted as benign by conventional methods while the ¹⁸F-FDG PET/CT evaluated this mass as malignant, and the patient was started on chemotherapy. The impact of ¹⁸F-FDG PET/CT on patient management is listed in Table 1. Finally, the correlation between SUV_max value of the primary lesion and patient survival was evaluated in 30 patients, one of whom was the patient who had synchronous double primary lung cancers. Primarily, the median SUV_max (median SUV_max= 15.6) was calculated and accepted as the “cut-off” value. The mean survival time was 10.3±2.2 months in patients with SUV_max <15.6, and 15.9±1.6 months in those with SUV_max ≥15.6. There was no significant difference between the two groups in terms of survival (p=0.118) (Figure 3).

Discussion

Lung cancer is the most common cause of cancer-related death worldwide, and early diagnosis is important for the application of curative treatments. ¹⁸F-FDG PET/CT is used commonly to differentiate benign and malignant lung lesions. Gupta et al. (4) reported the sensitivity and specificity of ¹⁸F-FDG PET/CT in benign-malignant distinction of indeterminate solitary pulmonary nodules as 93% and 88%, respectively. ¹⁸F-FDG PET/CT might yield a false positive result in inflammatory conditions, tuberculosis or granulomatous lesions. A false negative ¹⁸F-FDG PET/CT result is generally associated with nodules measuring less than 1 cm, carcinoid tumors or bronchoalveolar carcinomas (BAC) (5,6,7,8,9). In our study, ¹⁸F-FDG PET/CT provided a true positive result in 94% and a false negative result in 6% of 51 lesions. The tumors with a false negative ¹⁸F-FDG PET/CT included two adenocarcinomas (0.6 cm and 1.3 cm) and one carcinoid tumor (1 cm). The small size and presence of a carcinoid tumor may have contributed to the false negative results. In the present study, the sensitivity, accuracy and positive predictive value of ¹⁸F-FDG PET/CT in the detection of primary malignancy were calculated as 94%, 94% and 100%, respectively. These results are consistent with that reported in the literature. Several studies indicated that the mean SUV and SUV_max differ according to histologic subtypes of lung cancer. In particular, it has been reported that the SUV_max of carcinoid tumors and BAC are low (10,11). Vesselle et al. (12) found that the SUV_max of BAC was lower than all other subtypes, and that non-BAC adenocarcinomas had lower SUV_max values than SCC. The majority of studies in the literature report that the SUV_max of adenocarcinomas is lower than that of SCCs (13,14). However, in our study, no statistically significant difference was identified between adenocarcinoma (16.8±13.5) and SCC (17.9±5.6) in terms of mean SUV_max (p=0.2). Several studies showed that tumor size is a prognostic factor for survival in NSCLC (15). The correlation between tumor size and SUV_max has been previously assessed (16,17). Zhu et al. (18) reported a moderate positive correlation between SUV_max value and tumor size (r=0.642, p<0.001). In our study, a positive correlation (p=0.002) was identified between tumor size and SUV_max, in line with findings of the earlier studies.

The detection of both lymph node and distant metastases is essential for proper staging in lung cancer. The sensitivity of ¹⁸F-FDG PET/CT in lymph nodes greater than 1 cm is high, although the accuracy and specificity rates are low (19). In a study by Detterbeck et al. (20), the false positive rate of PET in mediastinal lymph nodes was reported to be 13-22%, and the false negative rate as 5-8%. In our study, histopathologic lymph node evaluation was carried out in 21 of the 50 patients, and 52% of the 21 patients were correctly staged by ¹⁸F-FDG PET/CT whereas the rate of patients incorrectly down-staged or up-staged by ¹⁸F-FDG PET/CT were detected as 19% and 28%, respectively. In our study, the false positive and false negative lymph node rates in ¹⁸F-FDG PET/CT were found to be higher than that reported in the literature. This finding may be attributed to the fact that histopathologic evaluation was not performed in all lymph nodes evaluated as either positive or negative by ¹⁸F-FDG PET/CT. Furthermore; sarcoidosis, amyloidosis, anthracosis, tuberculosis and organized pneumonia may cause false positive results in lymph nodes. In one patient with a false positive lymph node on ¹⁸F-FDG PET/CT, histopathologic sampling revealed anthracosis. In addition, pulmonary infections or granulomatous diseases such as tuberculosis or sarcoidosis that are frequent in our country might have contributed to these outcomes. Further investigation of lymph node and distant organ metastases, particularly in patients being considered for surgical therapy, could prevent unnecessary surgery. Van Tinteren et al. (21) reported that performing an ¹⁸F-FDG PET/CT in addition to conventional methods as part of preoperative staging of NSCLC might avoid unwarranted surgery in one-fifth of patients. In a study on patients with NSCLC, an ¹⁸F-FDG PET/CT led to a change in disease stage as determined by conventional methods in 50.6% (41.1% up-staged, 9.5% down-staged) of patients, and alteration in treatment planning in 42.3% (22). In our study, the ¹⁸F-FDG PET/CT resulted in a change in the treatment plan as decided by conventional methods in 34% of all patients, and unnecessary surgery was prevented in 18% of the patients. In 6% of the patients, the ¹⁸F-FDG PET/CT was interpreted as negative for areas identified as metastasis through conventional methods, leading patients to possible curative therapy. In various studies, SUV_max of the primary tumor detected by ¹⁸F-FDG PET/CT was used to predict lung cancer prognosis and assess patient survival (23,24). Ahuja et al. (25) found that patients with primary tumors showing a high FDG uptake had lower survival rates than those with a low uptake, while Hoang et al. (26) found no significant association between FDG uptake of the primary tumor and survival in patients with advanced NSCLC. In a meta-analysis published by IASLC in 2008, the SUV_max of the primary tumor was a strong determinant of prognosis in NSCLC. However, the need to support this finding with multivariate analysis was emphasized (27). The present study evaluated the relationship between SUV_max of the primary tumor and patient survival and no significant difference was identified between those with SUV_max below 15.6 (10.3±2.2 months) and above 15.6 (15.9±1.6 months) in terms of mean survival time (p=0.118). Several previous studies support our findings, although a larger number of authors argue that SUV_max could be used to predict prognosis and assess survival in case of lung cancer. Our results might be related to the limited number of patients in the study as well as the differences between the two groups (e.g. age, genetic factors, stage, tumor pathology).

Conclusion

Despite the limitations of our study, such as limited number of patients, most of the results are consistent with previous studies. In our study, it is concluded that ¹⁸F-FDG PET/CT contributes to both the diagnosis and management of lung cancer by providing valuable information in addition to conventional methods.

Ethics

Ethics Committee Approval: The study protocol was approved by the Ethics Committee of Clinical and Laboratory Researches of Ege University Faculty of Medicine with decision number: 14-4.2/16 dated: 12.06.2014.

Informed Consent: Informed consent was waived due to the retrospective design of the study.

Peer-review: Externally peer-reviewed.

Authorship Contributions

Concept: E.B., Design: E.B., A.A., Data Collection or Processing: E.B., G.Ç., A.A., Analysis or Interpretation: E.B., G.Ç., A.A., Literature Search: E.B., A.A., Writing: E.B., A.A.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.