Comparison of the diagnostic value of 3-deoxy-3-18F-fluorothymidine and 18F-fluorodeoxyglucose positron emission tomography/computed tomography in the assessment of regional lymph node in thoracic esophageal squamous cell carcinoma: a pilot study

Advances in the Imaging of Esophageal and Gastroesophageal Junction Malignancies

Accurate imaging is key for the diagnosis and treatment of esophageal and gastroesophageal junction cancers . Current imaging modalities, such as computed tomography (CT) and 18F-FDG (2-deoxy-2-[18F]fluoro-D-glucose) positron emission tomography (PET)/CT, have limitations in accurately staging these cancers. MRI shows promise for T staging and residual disease assessment. Novel PET tracers, like FAPI, FLT, and hypoxia markers, offer potential improvements in diagnostic accuracy. 18F-FDG PET/MRI combines metabolic and anatomic information, enhancing disease evaluation. Radiomics and artificial intelligence hold promise for early detection, treatment planning, and response assessment. Theranostic nanoparticles and personalized medicine approaches offer new avenues for cancer therapy.

Comparison of 18F-fluorothymidine Positron Emission Tomography/Computed Tomography and 18F-fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Patients with Breast Cancer

The uptake of ¹⁸F-fluorothymidine (¹⁸F-FLT) depends on cells' proliferative rates. We compared the characteristics of ¹⁸F-FLT positron emission tomography/computed tomography (PET/CT) with those of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET/CT for breast cancer. We prospectively diagnosed patients with breast cancer who underwent ¹⁸F-FLT PET/CT and ¹⁸F-FDG PET/CT. Subsequently, significant differences and correlation coefficients of the maximum standardized uptake value (SUVmax) in primary breast cancer and axillary lymph nodes were statistically evaluated. We enrolled eight patients with breast cancer. In six treatment-naive patients, the SUVmax for primary lesions showed a significant difference (mean, 2.1 vs. 4.1, p = 0.031) and a strong correlation (r = 0.969) between ¹⁸F-FLT and ¹⁸F-FDG. Further, although the SUVmax for the axillary lymph nodes did not show a significant difference between ¹⁸F-FLT and ¹⁸F-FDG (P = 0.246), there was a strong correlation between the two (r = 0.999). In a patient-by-patient study, there were cases in which only ¹⁸F-FDG uptake was observed in lymph nodes and normal breasts. Bone metastases demonstrated lower accumulation than bone marrow on the ¹⁸F-FLT PET/CT. In conclusion, a strong correlation was observed between the ¹⁸F-FLT PET/CT and ¹⁸F-FDG PET/CT uptake. Differences in the biochemical characteristics of ¹⁸F-FLT and ¹⁸F-FDG were reflected in the accumulation differences for breast cancer, metastatic lesions, and normal organs.

Impact of PET/CT on management of patients with esophageal cancer - results from a PET/CT registry study

PURPOSE

To investigate the impact of positron emission tomography/computed tomography (PET/CT) on clinical management in patients with esophageal cancer and its link to overall survival (OS) in a real-world setting.

METHODS

A patient cohort with advanced esophageal cancer undergoing PET/CT was prospectively enrolled in a registry study between 04/2013 and 06/2019. Intended patient management prior and after PET/CT was documented based on standardized questionnaire data. Management changes after PET/CT were recorded including major changes concerning the treatment goal (curative vs. palliative) and minor changes (therapy adjustments). OS was analyzed for subgroups with squamous cell carcinomas (SCC) or adenocarcinomas (AC) and stratified for extent of metastatic disease and treatment goals.

RESULTS

257 patients (53 female;65.5 ± 10.0 yr.) were included. After PET/CT, major changes of intended therapy were observed in 34/257 patients (13.2%), from curative to palliative (8.2%), palliative to curative (1.9%) and from "not finally determined" to a curative (1.9%) or palliative (1.2%) concept. Minor changes were found in 62/257 patients (24.1%). Invasive procedures and additional imaging were intended in 70/257 (27.2%) and 94/257 (36.6%) patients before PET/CT and 20/257 (7.8%) and 8/257 (3.1%) patients after PET/CT. Curative therapy concepts based on PET/CT were associated with a longer OS (3.5 yr.[95%CI 3.1-3.8 yr.]) as compared to palliative concepts (0.9 yr.[95%CI 0.6-1.2 yr.];p < 0.0001). Patients with SCC had a worse prognosis (2.4 yr.[95%CI 2.0-2.9 yr.]) as compared to patients with AC (3.2 yr.[95%CI 2.7-3.7 yr.];p = 0.01).

CONCLUSIONS

In patients with advanced esophageal cancer, PET/CT has a significant impact on clinical management by improving the selection of individualized treatment strategies and avoiding additional diagnostic procedures.

Comparison of different images in gross target volume delineating on VX2 nasopharyngeal transplantation tumor models

Background: To determine the optimum conditions for diagnosis of nasopharyngeal carcinoma, we established VX2 rabbit model to delineate gross target volume (GTV) in different imaging methods. Methods: The orthotopic nasopharyngeal carcinoma (NPC) was established in sixteen New Zealand rabbits. After 7-days inoculation, the rabbits were examined by CT scanning and then sacrificed for pathological examination. To achieve the best delineation, different GTVs of CT, MRI, ¹⁸F-FDG PET/CT, and ¹⁸F-FLT PET/CT images were correlated with pathological GTV (GTVp). Results: We found 45% and 60% of the maximum standardized uptake value (SUV_max) as the optimal SUV threshold for the target volume of NPC in ¹⁸F-FDG PET/CT and ¹⁸F-FLT PET/CT images, respectively (GTV_FDG45% and GTV_FLT60%). Moreover, the GTV_MRI and GTV_CT were significantly higher than the GTVp (P ≤ 0.05), while the GTV_FDG45% and especially GTV_FLT60% were similar to the GTVp (R = 0.892 and R = 0.902, respectively; P ≤ 0.001). Conclusions: Notably, the results suggested that ¹⁸F-FLT PET/CT could reflect the tumor boundaries more accurately than ¹⁸F-FDG PET/CT, MRI and CT, which makes ¹⁸F-FLT PET-CT more advantageous for the clinical delineation of the target volume in NPC.

Systematic review and meta-analysis of the accuracy of 18F-FDG PET/CT for detection of regional lymph node metastasis in esophageal squamous cell carcinoma

Background

We performed a systematic review and meta-analysis to assess the accuracy of 18F-fluorodeoxyglucose positron emission tomography with computer tomography (18F-FDG PET/CT) for detection of regional lymph node metastasis in esophageal squamous cell carcinoma in per-patient and per-nodal station basis.

Methods

Electronic databases were researched for studies assessing the sensitivity and specificity of PET/CT to detect the regional lymph node metastasis published between January 2006 and December 2017 on esophageal squamous cell carcinoma. STATA software was performed to assess the sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odd ratio (DOR) and summary receiver operating characteristic (SROC) curve. The Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) and Deeks' Funnel Plot Asymmetry Test were performed to evaluate the study quality and publication bias of included studies.

Results

Nineteen studies were eligible for meta-analysis, comprising 1,089 patients with esophageal cancer who underwent 18F-FDG PET/CT before surgery. According to the content of the article, we divided the selected studies into per-patient basis group and per-nodal basis group (one of the articles was involved in both groups). For the per-nodal station basis group (12 studies, 5,681 stations), the pooled sensitivity and specificity estimates of 18F-FDG PET/CT for detecting regional lymph node metastasis were 66% [95% confidence interval (CI): 51-78%] and 96% (95% CI: 92-98%), respectively. The corresponding values on a per-patient basis group (8 studies; 506 patients) were 65% (95% CI: 49-78%) and 81% (95% CI: 69-89%) in sensitivity and specificity, respectively.

Conclusions

Overall, 18F-FDG PET/CT have a moderate to low sensitivity and a high to moderate specificity for detection of regional nodal metastasis in esophageal cancer. Therefore, since the false rate is considerable, extending the extent of lymph node dissection or radiotherapy target volume is necessary after diagnosis of regional nodal metastasis by 18F-FDG PET/CT.

Diagnostic value of 18F-FDG-PET/CT in benign lung diseases

Introduction

There are many diseases which, despite not being malignant, show high metabolic activity and cause false-positive results.

Aim

To evaluate the results of positron emission tomography (PET) in patients who underwent resection after preliminary diagnosis of malignancy based on fluorodeoxyglucose (FDG) uptake value, in whom the lesions were later classified as pathologically benign.

Material and methods

The analysis included the records of 106 (12.3%) patients out of 862 patients who underwent surgery between January 2012 and December 2015 after being initially diagnosed with malignant lung lesions based on PETCT results, in whom the lesions were later classified as pathologically benign. Diagnoses, PET findings, types of surgery, and demographic data of the patients were recorded.

Results

The mean age of the patients was 55.5 (26-79) years. The mean diameter and SUVmax of the lesions were 2 ±2.14 (0.5-13) and 3.55 ±4.35 (0-22.2) cm, respectively. The pathology results were analyzed in five different groups. The SUVmax in the hamartoma group was significantly lower than in the other groups (p < 0.001), while the SUVmax in the granulomatous disease group was significantly higher than in the other groups (p < 0.001).

Conclusions

The possibility of false positive PET results must be kept in mind when diagnosing and treating lung cancer. In particular, in the case of suspected granulomatous disease, all available pre- and intraoperative diagnostic procedures must be used.

Mediastinal node staging by positron emission tomography-computed tomography and selective endoscopic ultrasound with fine needle aspiration for patients with upper gastrointestinal cancer: Results from a regional centre

AIM

To investigate the impact of endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) and positron emission tomography-computed tomography (PET-CT) in the nodal staging of upper gastrointestinal (GI) cancer in a tertiary referral centre.

METHODS

We performed a retrospective review of prospectively recorded data held on all patients with a diagnosis of upper GI cancer made between January 2009 and December 2015. Only those patients who had both a PET-CT and EUS with FNA sampling of a mediastinal node distant from the primary tumour were included. Using a positive EUS-FNA result as the gold standard for lymph node involvement, the sensitivity, specificity, positive and negative predictive values (PPV and NPV) and accuracy of PET-CT in the staging of mediastinal lymph nodes were calculated. The impact on therapeutic strategy of adding EUS-FNA to PET-CT was assessed.

RESULTS

One hundred and twenty one patients were included. Sixty nine patients had a diagnosis of oesophageal adenocarcinoma (Thirty one of whom were junctional), forty eight had oesophageal squamous cell carcinoma and four had gastric adenocarcinoma. The FNA results were inadequate in eleven cases and the PET-CT findings were indeterminate in two cases, therefore thirteen patients (10.7%) were excluded from further analysis. There was concordance between PET-CT and EUS-FNA findings in seventy one of the remaining one hundred and eight patients (65.7%). The sensitivity, specificity, PPV and NPV values of PET-CT were 92.5%, 50%, 52.1% and 91.9% respectively. There was discordance between PET-CT and EUS-FNA findings in thirty seven out of one hundred and eight patients (34.3%). MDT discussion led to a radical treatment pathway in twenty seven of these cases, after the final tumour stage was altered as a direct consequence of the EUS-FNA findings. Of these patients, fourteen (51.9%) experienced clinical remission of a median of nine months (range three to forty two months).

CONCLUSION

EUS-FNA leads to altered staging of upper GI cancer, resulting in more patients receiving radical treatment that would have been the case using PET-CT staging alone.

PET/Computed Tomography Scanning and Precision Medicine: Esophageal Cancer

Esophageal cancer commonly has a poor prognosis, which requires an accurate diagnosis and early treatment to improve outcome. Other modalities for staging, such as endoscopic ultrasound imaging and computed tomography (CT) scans, have a role in diagnosis and staging. However, PET with fluorine-18 fluoro-2-deoxy-d-glucose/CT (FDG PET/CT) scanning allows for improved detection of distant metastatic disease and can help to prevent unnecessary interventions that would increase morbidity. FDG PET/CT scanning is valuable in the neoadjuvant chemotherapy assessment and predicting survival outcomes subsequent to surgery. FDG PET/CT scanning detects recurrent disease and metastases in follow-up.

Current clinical status of 18F-FLT PET or PET/CT in digestive and abdominal organ oncology

Positron emission tomography (PET) or PET/computed tomography (CT) using ¹⁸F-3'-fluoro-3'-deoxythymidine (¹⁸F-FLT) offers noninvasive assessment of cell proliferation in human cancers in vivo. The present review discusses the current status on clinical applications of ¹⁸F-FLT-PET (or PET/CT) in digestive and abdominal oncology by comparing with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG)-PET (or PET/CT). The results of this review show that although ¹⁸F-FLT uptake is lower in most cases of digestive and abdominal malignancies compared with ¹⁸F-FDG uptake, ¹⁸F-FLT-PET can be used to detect primary tumors. ¹⁸F-FLT-PET has shown greater specificity for N staging than ¹⁸F-FDG-PET which can show false-positive uptake in areas of inflammation. However, because of the high background uptake in the liver and bone marrow, it has a limited role of assessing liver and bone metastases. Instead, ¹⁸F-FLT-PET will be a powerful tool for monitoring response to treatment and provide prognostic information in digestive and abdominal oncology.

ypN0: Does It Matter How You Get There? Nodal Downstaging in Esophageal Cancer

BACKGROUND

ypN0 following induction treatment for advanced esophageal cancer improves survival. Importance of how ypN0 is achieved is unknown. This study evaluates survival in "natural" N0 (cN0/ypN0) and "downstaged" N0 (cN+/ypN0) patients.

METHODS

Among patients treated with induction treatment and surgery, 83 CT scans were retrieved in digital format and re-evaluated by a radiologist, blinded to pathological nodal status: 28 natural N0, 37 downstaged N0, and 18 ypN+. Impact of N0 classification on survival and associations with survival were identified.

RESULTS

Survival varied with ypN: 3-year survival was 84 % for natural N0 patients, 59 % for downstaged N0, and 20 % for ypN+ (p < .001). Compared with natural N0 patients, risk of cancer mortality was 3.8 for downstaged N0 and 7.6 for ypN+ (p = .01). Survival was also stratified by ypT: compared with ypT0 natural N0, who had the best survival, intermediate survival was seen in ypT+ natural N0 [hazard ratio (HR), 1.3] and ypT0 downstaged N0 (HR, 1.8), and poor survival in ypT+ downstaged N0 (HR, 9.5) and ypN+ (HR, 12.0) (p = .026).

CONCLUSIONS

Natural N0 and downstaged N0 patients are different clinical entities: downstaging cN+ with induction treatment producing downstaged N0 improves survival only if there is concomitant primary cancer downstaging to ypT0. Intermediate survival is seen in downstaged N0 patients with complete tumor response. Natural N0 patients experience intermediate survival with incomplete response (ypT+). Complete response in natural N0 patients produces the best survival. Means of obtaining ypN0 status matters and requires a complete response for downstaged N0 patients to benefit from induction treatment.

18F-FLT PET imaging of cellular proliferation in pancreatic cancer

Pancreatic ductal adenocarcinoma is known for its poor prognosis. Since the development of computerized tomography, magnetic resonance and endoscopic ultrasound, novel imaging techniques have struggled to get established in the management of patients diagnosed with pancreatic adenocarcinoma for several reasons. Thus, imaging assessment of pancreatic cancer remains a field with scope for further improvement. In contrast to cross-sectional anatomical imaging methods, molecular imaging modalities such as positron emission tomography (PET) can provide information on tumour function. Particularly, tumour proliferation may be assessed by measurement of intracellular thymidine kinase 1 (TK1) activity level using thymidine analogues radiolabelled with a positron emitter for use with PET. This approach, has been widely explored with [(18)F]-fluoro-3'-deoxy-3'-L-fluorothymidine ((18)F-FLT) PET. This manuscript reviews the rationale and physiology behind (18)F-FLT PET imaging, with special focus on pancreatic cancer and other gastrointestinal malignancies. Potential benefit and challenges of this imaging technique for diagnosis, staging and assessment of treatment response in abdominal malignancies are discussed.

Positron emission tomography-computed tomography in oesophageal cancer staging: a tailored approach

BACKGROUND

Positron emission tomography-computed tomography (PET-CT) scanning is used routinely in the staging of oesophageal cancer to identify occult metastases not apparent on CT and changes the management in typically 3-18% patients. The authors aim to re-evaluate its role in the management of oesophageal cancer, investigating whether it is possible to identify a group of patients that will not benefit and can safely be spared from this investigation.

METHODS

Consecutive patients with oesophageal cancer undergoing PET-CT staging between 2010 and 2013 were identified from a specialist modern multidisciplinary team database. Without knowledge of the PET-CT result, patients were stratified into low-risk or high-risk groups according to the likelihood of identifying metastatic disease on PET-CT based on specified criteria routinely available from endoscopy and CT reports. Clinical outcomes in the two groups were investigated.

RESULTS

In 383 undergoing PET-CT, metastatic disease was identified in 52 (13.6%) patients. Eighty-three patients were stratified as low risk and 300 as high risk. None of the low-risk patients went on to have metastatic disease identified on PET-CT. Of the high-risk patients, 17% had metastatic disease identified on PET-CT.

CONCLUSIONS

In one of the largest studies to date investigating the influence of staging PET-CT on management of patients with oesophageal cancer, the authors report a classification based on endoscopy/CT criteria is able to accurately stratify patients according to the risk of having metastatic disease. This could be used to avoid unnecessary PET-CT 22% of patients, saving cost, inconvenience and reducing potential delay to definitive treatment in this group.

State-of-the-art molecular imaging in esophageal cancer management: implications for diagnosis, prognosis, and treatment

Molecular imaging techniques are increasingly being used in addition to standard imaging methods such as endoscopic ultrasound (EUS) and computed tomography (CT) for many cancers including those of the esophagus. In this review, we will discuss the utility of the most widely used molecular imaging technique, (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET). (18)F-FDG PET has a variety of potential applications ranging from improving staging accuracy at the time of initial diagnosis to assisting in radiation target volume delineation. Furthermore, (18)F-FDG PET can be used to evaluate treatment response after completion of neoadjuvant therapy or potentially during neoadjuvant therapy. Finally, we will also discuss other novel molecular imaging techniques that have potential to further improve cancer care.

Diagnostic value of ¹⁸F-FDG PET/CT for lymph node metastasis of esophageal squamous cell carcinoma

PURPOSE

This study investigated the diagnostic accuracy of hybrid positron-emission tomography/computed tomography (PET/CT) for lymph node (LN) metastasis of esophageal cancer. We also investigated the correlation between the size of metastatic nests and the detection by PET/CT.

METHODS

Two hundred and fifty-eight patients with esophageal squamous cell carcinoma who underwent esophagectomy with two- or three-field radical lymphadenectomy were analyzed retrospectively. We compared the diagnosis of preoperative PET/CT to the postoperative histopathological examination by each anatomical field (n = 1,231) in all 258 patients. The metastatic LNs resected from PET/CT positive fields were classified as belonging to the PET/CT-N-positive group (n = 229) and those from negative fields as belonging to the PET/CT-N-negative group (n = 352). The cross-sectional areas of metastatic nests were measured in each metastatic LN.

RESULTS

Of the 1,231 fields, 275 (22 %) were positive for metastasis, including 581 LNs from 408 regional LN stations. The sensitivity and specificity of PET/CT examined by each anatomical field were 25.8 and 97.8 %, respectively. The median area of metastatic nests was 17.7 mm2 in the PET/CT-N-positive group, and 7.7 mm2 in the PET/CT-N-negative group (p < 0.001).

CONCLUSIONS

A significant correlation was suggested between the nest size and detection by PET/CT. Because of its low sensitivity, PET/CT alone is insufficient to determine the surgical procedures, especially when considering reduction surgery.

Repopulation of tumor cells during fractionated radiotherapy and detection methods (Review)

Repopulation of tumor cells during radiotherapy is believed to be a significant cause for treatment failure. The phenomenon of tumor repopulation during fractionated radiotherapy was found from clinical observations that identified that the local control rate decreased with a prolonged treatment time. A series of animal experiments with varied overall treatment time and fractionated doses were performed to demonstrate tumor cell repopulation during radiotherapy in various mouse xenograft models. However, conventional detection methods are challenging, as it is difficult to separate viable cells from those destined for apoptosis during fractionated radiotherapy. In essence, the mechanism of tumor repopulation involves the continuing proliferation of clonogenic tumor cells. In vivo imaging, tracking and targeting of the repopulation of these cells has been of clinical interest so as to administer a higher dose to the tumor repopulation regions. Currently, functional imaging methods, including 3'-deoxy-3'-¹⁸F-fluorothymidine positron emission tomography (¹⁸F-FLT PET), are showing promise in assessing the proliferation activity of tumors in vivo. This review mainly focuses on the phenomenon of tumor repopulation during radiotherapy and its conventional and novel detection methods, particularly on the feasibility of ¹⁸F-FLT PET for the detection of tumor-cell repopulation.

Recent Trends in PET Image Interpretations Using Volumetric and Texture-based Quantification Methods in Nuclear Oncology

Image quantification studies in positron emission tomography/computed tomography (PET/CT) are of immense importance in the diagnosis and follow-up of variety of cancers. In this review we have described the current image quantification methodologies employed in (18)F-fluorodeoxyglucose ((18)F-FDG) PET in major oncological conditions with particular emphasis on tumor heterogeneity studies. We have described various quantitative parameters being used in PET image analysis. The main contemporary methodology is to measure tumor metabolic activity; however, analysis of other image-related parameters is also increasing. Primarily, we have identified the existing role of tumor heterogeneity studies in major cancers using (18)F-FDG PET. We have also described some newer radiopharmaceuticals other than (18)F-FDG being studied/used in the management of these cancers. Tumor heterogeneity studies are being performed in almost all major oncological conditions using (18)F-FDG PET. The role of these studies is very promising in the management of these conditions.

Evaluating the role of fluorodeoxyglucose positron emission tomography-computed tomography in multi-disciplinary team recommendations for oesophago-gastric cancer

BACKGROUND

National guidelines recommend that fluorodeoxyglucose positron emission tomography-computed tomography (PET-CT) is performed in all patients being considered for radical treatment of oesophageal or oesophago-gastric cancer without computerised tomography scan (CTS) evidence of metastasis. Guidance also mandates that all patients with cancer have treatment decisions made within the context of a multi-disciplinary team (MDT) meeting. Little is known, however, about the influence of PET-CT on decision making within MDTs. The aim of this study was to assess the role of PET-CT in oesophago-gastric cancer on MDT decision making.

METHODS

A retrospective analysis of a prospectively held database of all patients with biopsy-proven oesophageal or oesophago-gastric cancer discussed by a specialist MDT was interrogated. Patients selected for radical treatment without CTS evidence of M1 disease were identified. The influence of PET-CT on MDT decision making was examined by establishing whether the PET-CT confirmed CTS findings of M0 disease (and did not change the patient staging pathway) or whether the PET-CT changed the pathway by showing unsuspected M1 disease, refuting CTS suspicious metastases, or identifying another lesion (needing further investigation).

RESULTS

In 102 MDT meetings, 418 patients were discussed, of whom 240 were initially considered for radical treatment and 238 undergoing PET-CT. The PET-CT confirmed CTS findings for 147 (61.8%) and changed MDT recommendations in 91 patients (38.2%) by (i) identifying M1 disease (n=43), (ii) refuting CTS suspicions of M1 disease (n=25), and (iii) identifying new lesions required for investigations (n=23).

CONCLUSION

The addition of PET-CT to standard staging for oesophageal cancer led to changes in MDT recommendations in 93 (38.2%) patients, improving patient selection for radical treatment. The validity of the proposed methods for evaluating PET-CT on MDT decision making requires more work in other centres and teams.

PET imaging of proliferation with pyrimidines

Several new tracers are being developed for use with PET to assess pathways that are altered in cancers, including energy use, cellular signaling, transport, and proliferation. Because increased proliferation is a hallmark of many cancers, several tracers have been tested to track the DNA synthesis pathway. Thymidine, which is incorporated into DNA but not RNA, has been used in laboratory studies to measure tumor growth. Because thymidine labeled with (11)C undergoes rapid biologic degradation and has a short physical half-life, tracers labeled with (18)F have been preferred in PET imaging. One such tracer is (18)F-labeled 3'-deoxy-3'-fluorothymidine ((18)F-FLT). (18)F-FLT is trapped after phosphorylation by thymidine kinase 1, whose expression is increased in replicating cells. Several studies on breast, lung, and brain tumors have demonstrated that retention of (18)F-FLT correlated with tumor proliferation. Although (18)F-FLT has been used to image and stage several tumor types, the standardized uptake value is generally lower than that obtained with (18)F-FDG. (18)F-FLT can be used to image many areas of the body, but background uptake is high in the liver, marrow, and renal system, limiting use in these organs. (18)F-FLT PET imaging has primarily been studied in the assessment of treatment response. Rapid declines in (18)F-FLT retention within days to weeks have been demonstrated in several tumor types treated with cytotoxic drugs, targeted agents, and radiotherapy. Further work is ongoing to validate this approach and determine its utility in the development of new drugs and in the clinical evaluation of standard treatment approaches.