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Introduction

Positron emission tomography (PET) is widely used for cancer detection, staging and monitoring the response to therapy. [18F] fluorodeoxyglucose (18F-FDG) and 3′-deoxy-3′-[18F] fluorothymidine (18F-FLT) are commonly used PET tracers for imaging glucose metabolism and cell proliferation, respectively (1–12). In a mouse model of human lung cancer, it has been previously demonstrated that intratumoral distribution between 18F-FLT and 18F-FDG was mutually exclusive. 18F-FLT accumulated primarily in proliferating cancer cells, whereas 18F-FDG accumulated in hypoxic cancer cells that are less proliferative (13–16). To the best of our knowledge, intratumoral distribution of 18F-FLT and 18F-FDG in patients with lung cancer has not been previously reported.

Differential diagnosis of malignant pulmonary lesions may be challenging. Computed tomography (CT) is the method of choice for the diagnosis of pulmonary lesions. PET/CT imaging reflects the biological and metabolic aspects of pulmonary lesions (17). 18F-FDG PET/CT has been widely used for the diagnosis of pulmonary lesions; however, false-negative as well as false-positive results are frequently observed (17,18). 18F-FLT is a positron radioactive tracer that reflects cancer cell proliferation. Therefore, 18F-FLT may be a useful tool for the diagnosis of pulmonary lesions (19).

In the present study, it was hypothesized that the mutually exclusive distribution pattern between 18F-FLT and 18F-FDG described in animal tumor models may apply to patients with lung malignancies as well. To examine this hypothesis, patients with pulmonary lesions that initially underwent a 18F-FDG PET/CT scan and subsequently a 18F-FLT PET/CT scan were studied.

Materials and methods

Patients

The present study was approved by the Institutional Review Boards of the Inner Mongolia Medical University (Hohhot, China) and the Soochow Medical University (Jiangsu, China). Written informed consent was obtained from all patients prior to participation. The Institutional Review Board of the University of Louisville (Louisville, KY, USA) approved data transfer and use. From June 2013 to August 2015, a total of 55 patients (Table I) with pretreated lung lesions were recruited to the present study (31 males and 24 females; age range, 17–68 years). Histological examination of the lesions was performed in every patient. The diameter of the lesions ranged between 8 and 50 mm.

Table I. Patients' clinical data and PET/CT results.

Table I.

Patients' clinical data and PET/CT results.

Patient no.	Age/sex	SUVmax FDG/FLT	Pathological diagnosis	FDG/FLT PET/CT SUVmax
1	55/F	5.3/2.7	Adenocarcinoma	Mismatch
2	48/F	3.5/1.4	Tuberculoma	Match
3	61/F	4.2/2.1	Squamous carcinoma	Match
4	65/F	2.8/1.1	Tuberculoma	Mismatch
5	59/F	2.3/1.0	Organizing pneumonia	Match
6	63/F	5.8/2.1	Adenocarcinoma	Mismatch
7	60/F	2.3/1.2	Tuberculoma	Mismatch
8	62/F	4.8/2.2	Adenocarcinoma	Mismatch
9	64/F	3.2/2.0	Adenocarcinoma	Mismatch
10	64/F	1.6/0.9	Tuberculoma	Mismatch
11	67/F	1.9/0.9	Hamartoma	Match
12	49/F	2.1/1.5	Tuberculoma	Match
13	57/F	3.8/2.8	Adenocarcinoma	Mismatch
14	59/F	4.1/2.6	Adenocarcinoma	Mismatch
15	47/F	2.0/1.4	Tuberculoma	Match
16	49/M	2.6/1.6	Tuberculoma	Mismatch
17	53/M	3.7/2.4	Adenocarcinoma	Mismatch
18	60/M	7.9/2.8	Squamous carcinoma	Match
19	65/M	1.5/0.9	Organizing pneumonia	Match
20	67/M	3.5/2.0	Inflammatory pseudotumor	Mismatch
21	57/M	6.8/2.4	Squamous carcinoma	Match
22	60/M	3.7/2.6	Squamous carcinoma	Match
23	58/M	4.2/2.0	Squamous carcinoma	Mismatch
24	62/M	3.6/2.5	Adenocarcinoma	Mismatch
25	63/M	3.4/2.6	Adenocarcinoma	Mismatch
26	66/M	1.8/1.0	Inflammatory pseudotumor	Mismatch
27	45/M	1.6/1.3	Tuberculoma	Mismatch
28	59/M	2.6/1.2	Tuberculoma	Match
29	17/M	2.9/1.5	Tuberculoma	Match
30	48/M	3.0/1.8	Squamous carcinoma	Mismatch
31	62/M	1.1/1.0	Tuberculoma	Match
32	62/M	2.4/1.6	Tuberculoma	Match
33	62/M	2.1/0.8	Organizing pneumonia	Mismatch
34	50/M	3.1/0.9	Tuberculoma	Match
35	52/M	1.5/1.0	Hamartoma	Match
36	57/M	3.6/2.2	Adenocarcinoma	Mismatch
37	54/M	3.2/1.9	Adenocarcinoma	Mismatch
38	52/M	1.6/0.7	Tuberculoma	Match
39	44/M	1.0/0.7	Hamartoma	Mismatch
40	49/M	5.4/1.8	Squamous carcinoma	Mismatch
41	62/M	1.5/0.7	Tuberculoma	Match
42	68/M	3.5/2.0	Adenocarcinoma	Mismatch
43	47/F	4.1/1.1	Tuberculoma	Match
44	49/M	3.1/1.4	Tuberculoma	Match
45	58/M	6.8/2.4	Squamous carcinoma	Mismatch
46	60/M	2.6/1.1	Tuberculoma	Mismatch
47	67/M	8.2/3.5	Adenocarcinoma	Mismatch
48	55/F	3.2/1.8	Adenocarcinoma	Match
49	48/F	1.5/1.0	Organizing pneumonia	Mismatch
50	61/F	5.8/2.5	Adenocarcinoma	Mismatch
51	65/F	2.7/2.1	Adenocarcinoma	Mismatch
52	59/F	1.9/0.8	Inflammatory pseudotumor	Match
53	63/F	1.6/0.8	Hamartoma	Mismatch
54	60/F	1.8/1.1	Inflammatory pseudotumor	Match
55	62/F	1.5/0.9	Hamartoma	Match

[i] PET/CT, positron emission tomography-computed tomography; FLT, 3′-deoxy-3′-[¹⁸F]fluorothymidine; FDG, [¹⁸F]fluorodeoxyglucose; SUV_max, maximal standardized update value; F, female; M, male.

Radiopharmaceuticals

[18F] fluoride was generated in-house using a cyclotron. 18F-FDG and 18F-FLT were synthesized automatically using FX-FN conventional modules at the PET/CT facility of the Inner Mongolia Medical University (Hohhot, China). 18F-FDG and 18F-FLT were pyrogen-free and qualified for clinical use, with radiochemical purity >98%.

PET/CT imaging protocol

PET/CT images were obtained using a GE Discovery ST PET/CT scanner. Prior to 18F-FDG PET scanning, patients were instructed to fast for >6 h and their blood glucose levels were determined to be <6 mmol/l. Whole body 18F-FDG PET/CT scans were performed 1 h after intravenous administration of 3.7 MBq/kg 18F-FDG. Subsequently, 3 days after 18F-FDG imaging, local thoracic 18F-FLT PET/CT scans were performed, 1 h after the injection of 18F-FLT (3.7 MBq/kg). Spiral CT scans (voltage, 120 kV; current, 160–220 mA) were conducted for attenuation correction and anatomy referral.

A board of three certified physicians in nuclear medicine assessed the PET/CT images. Visual analysis to score lesion radioactivity uptake of each tracer was performed (20). The maximal standardized uptake value (SUVmax) was used to spatially compare the intralesional distribution of 18F-FDG and 18F-FLT.

Histological examination of the lesions was performed for all patients by board-certified pathologists at the Department of Pathology (Affiliated Hospital of Inner Mongolian Medical University). Routine hematoxylin and eosin (H&E) staining was performed. Briefly, slides containing 5 µm paraffin sections were placed on a slide holder, deparaffinized and rehydrated. Sections were treated with hematoxylin solution, dipped 8–12 times in acid ethanol to destain, and stained for 30 sec with eosin. H&E stain imaging was developed with a light microscope at ×100 magnification.

Statistical analysis

SPSS software (version 17.0; SPSS, Inc., Chicago, IL, USA) was used to analyze the data using a χ2 test. P<0.05 was considered to indicate a statistically significant difference.

Results

The clinical information and PET/CT results of the patient cohort are summarized in Table I. Among the 55 cases, 24 lesions were confirmed as primary lung malignancies (16 cases with adenocarcinoma, 8 cases with squamous cell carcinoma) and 31 lesions were benign (18 cases with tuberculosis, 5 with hamartoma, 4 with inflammatory pseudo-tumor and 4 with organizing pneumonia).

Spatial intratumoral distribution of 18F-FLT and 18F-FDG mismatched in 19/24 malignant lesions (79%) and matched in 5 (21%). Fig. 1 presents an apparent mismatch in intratumoral distribution of 18F-FLT and 18F-FDG in a 67-year-old male patient with pretreated lung adenocarcinoma. Increased 18F-FDG uptake combined with decreased 18F-FLT accumulation in a patient with squamous carcinoma is presented in Fig. 2. Intratumoral distribution of 18F-FLT and 18F-FDG in lung malignancies was identified to be mainly heterogeneous and mutually excluded.

Figure 1. Scan images of a 67-year-old male patient with pretreated adenocarcinoma (39×23 mm) in the right middle lobe of the lung. An 18F-FLT PET/CT scan was performed 3 days after an 18F-FDG scan. (A) 18F-FDG PET/CT scan image. (B) 18F-FLT PET/CT scan image. (C) CT scan providing additional anatomical information. (D) Enlarged 18F-FDG scan image of the area indicated by the arrow. (E) Enlarged 18F-FLT scan image of the area indicated by the arrow. (F) Hematoxylin and eosin staining indicated lung adenocarcinoma. PET/CT, positron emission tomography-computed tomography; 18F-FLT, 3′-deoxy-3′-[18F] fluorothymidine; 18F-FDG, [18F] fluorodeoxyglucose; R, right.

Figure 2. Scan images of a 49-year-old male patient with pretreated squamous carcinoma (30×17 mm) in the left upper lobe in the lung. An 18F-FLT PET/CT scan was performed 3 days after an 18F-FDG scan. (A) 18F-FDG PET/CT scan image. (B) 18F-FLT PET/CT scan image. (C) CT scan providing additional anatomical information. (D) Enlarged 18F-FDG scan image of the area indicated by the arrow. (E) Enlarged 18F-FLT scan image of the area indicated by the arrow. (F) Hematoxylin and eosin staining indicated squamous carcinoma. PET/CT, positron emission tomography-computed tomography; 18F-FLT, 3′-deoxy-3′-[18F] fluorothymidine; 18F-FDG, [18F] fluorodeoxyglucose; R, right.

Regarding the 31 benign lesions, intralesional mismatched distribution of 18F-FLT and 18F-FDG was observed in 12 cases (39%). Fig. 3 presents scan images of mismatched 18F-FLT and 18F-FDG intralesional distribution in a 49-year-old male patient with lung tuberculoma. Matched intralesional distribution of 18F-FLT and 18F-FDG was observed in 19/31 benign lesions (61%). An indicative example of a patient with an inflammatory pseudotumor demonstrating negative 18F-FLT and positive 18F-FDG PET scans is presented in Fig. 4.

Figure 3. Scan images of a 49-year-old male patient with untreated tuberculoma (13×18 mm) in the right upper lobe in the lung. An 18F-FLT PET/CT scan was performed 3 days after an 18F-FDG scan. (A) 18F-FDG PET/CT scan image. (B) 18F-FLT PET/CT scan image. (C) CT scan providing additional anatomical information. (D) Enlarged 18F-FDG scan image of the area indicated by the arrow. (E) Enlarged 18F-FLT scan image of the area indicated by the arrow. (F) Hematoxylin and eosin staining indicated tuberculoma. PET/CT, positron emission tomography-computed tomography; 18F-FLT, 3′-deoxy-3′-[18F] fluorothymidine; 18F-FDG, [18F] fluorodeoxyglucose; R, right.

Figure 4. Scan images of a 59-year-old female patient with inflammatory pseudotumor (50×40 mm) in the right upper lobe in the lung. An 18F-FLT PET/CT scan was performed 2 days after an 18F-FDG scan. (A) 18F-FDG PET/CT scan image. (B) 18F-FLT PET/CT scan image. (C) CT scan providing additional anatomical information. (D) Enlarged 18F-FDG scan image of the area indicated by the arrow. (E) Enlarged 18F-FLT scan image of the area indicated by the arrow. (F) Hematoxylin and eosin staining indicated inflammatory pseudotumor. PET/CT, positron emission tomography-computed tomography; 18F-FLT, 3′-deoxy-3′-[18F] fluorothymidine; 18F-FDG, [18F] fluorodeoxyglucose; R, right.

These results indicate that mismatched intralesional accumulation of 18F-FLT and 18F-FDG was more frequently observed in malignant compared with benign lung lesions. The difference in intralesional distribution of 18F-FLT and 18F-FDG between malignant and benign lesions was statistically significant (P<0.05).

Discussion

It has previously been reported based on studies using mouse non-small cell lung cancer models that 18F-FDG accumulates in hypoxic regions, whereas 18F-FLT accumulates in well-oxygenated proliferating cells. Additionally, it has been demonstrated that the intratumoral distribution of 18F-FDG and 18F-FLT is mutually exclusive (13,14). In the present study, the association between 18F-FDG and 18F-FLT uptake was further elucidated in patients with lung cancer.

In the present study, it was demonstrated that intratumoral 18F-FDG and 18F-FLT accumulation is mutually exclusive. It was observed that regions with increased 18F-FDG accumulation were mainly associated with decreased 18F-FLT uptake. This is consistent with previous preclinical results in mouse lung cancer models (13–16). Intratumoral heterogeneity of 18F-FDG and 18F-FLT accumulation reflected the heterogeneous distribution of hypoxic (increased 18F-FDG uptake) and highly proliferative (increased 18F-FLT uptake) cancer cells; in agreement with previously reported preclinical results (13–16).

18F-FDG PET/CT is widely used in clinical practice for the detection of malignancies. However, it is not a cancer-specific tracer as it accumulates in hypoxic tissues regardless of malignant phenotype (10,13,15). Even though benign lesions present mainly low 18F-FDG uptake, in certain cases increased 18F-FDG accumulation is observed in inflammatory diseases including tuberculosis. Activated macrophages and other inflammatory cells may result in enhanced 18F-FDG accumulation in benign conditions including pneumonia, bronchiectasis, pulmonary tuberculosis, fungal infections, sarcoidosis, histoplasmosis and granuloma (21,22). Macrophages and other inflammatory cells, frequently observed in necrotic regions of inflammatory lesions, accumulate increased levels of 18F-FDG possibly due to the hypoxic microenvironment (23).

In the present study, 18F-FDG and 18F-FLT PET/CT scan uptake, performed with a 3-day interval, were compared in patients with lung lesions. A mismatch in the intralesional 18F-FLT and 18F-FDG accumulation was observed particularly in lung malignancies compared with benign lesions (Table I). Therefore, on the basis of the results of the present study, it is suggested that this mismatch may serve as an indicator of lung malignancy.

In well-differentiated slow-growing tumors, including bronchiole alveolar carcinomas, false-negative 18F-FDG PET results have been reported (20,24). This may be attributed to to the absence of hypoxic microenvironment of slow-growing malignancies“18F-FDG is mainly considered as a hypoxia-specific rather than a tumor avid tracer (13,15,16). This explains why 18F-FDG exhibited relatively low specificity in distinguishing malignant from benign lesions.

It has been demonstrated that the combination of 18F-FLT and 18F-FDG, either as separate PET scans performed on subsequent days or as one scan using a 18F-FLT and 18F-FDG cocktail, may be superior to an 18F-FDG scan for accurate disease detection (14). 18F-FDG mainly accumulates in hypoxic regions, whereas 18F-FLT accumulates in highly proliferating cells (6,7,13,14). The use of 18F-FLT and 18F-FDG cocktail PET may have an advantage compared with individual tracer PET. A clinical trial for 18F-FLT and 18F-FDG cocktail PET scanning for cancer detection and management is currently underway (25).

The results of the present study demonstrate that mismatched intratumoral distribution of 18F-FLT and 18F-FDG is a common feature of patients with lung cancer and may serve as an indicator of lung malignancy.

Acknowledgements

The authors would like to thank Dr Cheng Wang, Mr. Baoliang Bao and Dr Chunmei Wang (Department of Nuclear Medicine, Affiliated Hospital of Inner Mongolian Medical University, Hohhot, China) for their technical assistance. The present study was supported by the Natural Science Foundation of Inner Mongolia (grant no. 2013MS1188), the Scientific Research Project of the Affiliated Hospital of Inner Mongolian Medical University (grant no. 2014NYFYYB008), the Inner Mongolian Major Basic Science Research Program (grant no. 201503001) and the Inner Mongolian Science and Technology Innovation Project (grant no. 2015cztcxyd03). Part of the present study was presented at The Society of Nuclear Medicine and Molecular Imaging 2015 Annual Meeting (Baltimore, MD, USA; 4–10 June 2015) (26).

References