Potential impact of [18F]3'-deoxy-3'-fluorothymidine versus [18F]fluoro-2-deoxy-D-glucose in positron emission tomography for colorectal cancer

Molecular PET and PET/CT imaging of tumour cell proliferation using F-18 fluoro-L-thymidine: a comprehensive evaluation

Positron emission tomography (PET) using F-18 fluoro-3'-deoxy-3-L-fluorothymidine (FLT) offers noninvasive assessment of cell proliferation in vivo. The most important application refers to the evaluation of tumour proliferative activity, representing a key feature of malignancy. Most data to date suggest that FLT is not a suitable biomarker for staging of cancers. This is because of the rather low fraction of tumour cells that undergo replication at a given time with subsequently relatively low tumour FLT uptake. In addition, generally, the high FLT uptake in liver and bone marrow limits the diagnostic use. We describe the current status on preclinical and clinical applications of FLT-PET including our own experience in brain tumours. The future of FLT-PET probably lies in the evaluation of tumour response to therapy and more importantly, in the prediction of early response in the course of treatment. The level of FLT accumulation in tumours depends on thymidine kinase 1 activity and on the therapy-induced activation of the salvage pathway and expression of nucleoside transporters. Therefore, cytostatic agents that cause arrest of the cell cycle in the S-phase may initially increase FLT uptake rather than reducing the tumour cell accumulation. In addition, agents that block the endogenous thymidine pathway may lead to overactivity of the salvage pathway and increase tumour FLT uptake. In contrast, many therapeutic agents inhibit both pathways and subsequently reduce tumour FLT uptake. Further studies comparing FLT with F-18 fluorodeoxyglucose-PET will be important to determine the complementary advantage of FLT-PET in early cancer therapy response assessment. Further research should be facilitated by simplified synthesis of FLT with improved yields and an increasing commercial availability.

Positron emission tomography measurement of tumor metabolism and growth: its expanding role in oncology

This work highlights the explosion and evolution of positron emission tomography (PET) for use in oncology research and clinical practice. 2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-PET is important in the staging of cancer, estimation of prognosis, and for its ability to predict therapeutic outcome. A number of new imaging agents are under development and may find a place in oncology when studies prove their utility. This scientific overview includes a review of the development of a number of thymidine analogs, such as 18F-3'-deoxy-3'-fluorothymidine (FLT) and 18F-1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-thymine (FMAU), including chemical structure variations; their application in a variety of tumors; and the role of various kinetic models for understanding cellular proliferation. The greatest unmet need for PET is in further developing and validating its use in the measurement of treatment response.

Detection of gastric cancer using 18F-FLT PET: comparison with 18F-FDG PET

PURPOSE

We prospectively investigated the feasibility of 3'-deoxy-3'-(18)F-fluorothymidine (FLT) positron emission tomography (PET) for the detection of gastric cancer, in comparison with 2-deoxy-2-(18)F-fluoro-D-glucose (FDG) PET, and determined the degree of correlation between the two radiotracers and proliferative activity as indicated by Ki-67 index.

METHODS

A total of 21 patients with newly diagnosed advanced gastric cancer were examined with FLT PET and FDG PET. Tumour lesions were identified as areas of focally increased uptake, exceeding that of surrounding normal tissue. For semiquantitative analysis, the maximal standardized uptake value (SUV) was calculated.

RESULTS

For detection of advanced gastric cancer, the sensitivities of FLT PET and FDG PET were 95.2% and 95.0%, respectively. The mean (+/-SD) SUV for FLT (7.0 +/- 3.3) was significantly lower than that for FDG (9.4 +/- 6.3 p < 0.05). The mean FLT SUV and FDG SUV in nonintestinal tumours were higher than in intestinal tumours, although the difference was not statistically significant. The mean (+/-SD) FLT SUV in poorly differentiated tumours (8.5 +/- 3.5) was significantly higher than that in well and moderately differentiated tumours (5.3 +/- 2.1; p < 0.04). The mean FDG SUV in poorly differentiated tumours was higher than in well and moderately differentiated tumours, although the difference was not statistically significant. There was no significant correlation between Ki-67 index and either FLT SUV or FDG SUV.

CONCLUSION

FLT PET showed as high a sensitivity as FDG PET for the detection of gastric cancer, although uptake of FLT in gastric cancer was significantly lower than that of FDG.

Biological image-guided radiotherapy in rectal cancer: is there a role for FMISO or FLT, next to FDG?

PURPOSE

The purpose of this study is to investigate the use of PET/CT with fluorodeoxyglucose (FDG), fluorothymidine (FLT) and fluoromisonidazole (FMISO) for radiotherapy (RT) target definition and evolution in rectal cancer.

MATERIALS AND METHODS

PET/CT was performed before and during preoperative chemoradiotherapy (CRT) in 15 patients with resectable rectal cancer. PET signals were delineated and CT images on the different time points were non-rigidly registered. Mismatch analyses were carried out to quantify the overlap between FDG and FLT or FMISO tumour volumes (TV) and between PET TVs over time.

RESULTS

Ninety sequential PET/CT images were analyzed. The mean FDG, FLT and FMISO-PET TVs showed a tendency to shrink during preoperative CRT. On each time point, the mean FDG-PET TV was significantly larger than the FMISO-PET TV but not significantly larger than the mean FLT-PET TV. There was a mean 65% mismatch between the FMISO and FDG TVs obtained before and during CRT. FLT TVs corresponded better with the FDG TVs (25% mismatch before and 56% during CRT). During CRT, on average 61% of the mean FDG TV (7 cc) overlapped with the baseline mean TV (15.5 cc) (n=15). For FLT, the TV overlap was 49% (n=5) and for FMISO only 20% of the TV during CRT remained inside the contour at baseline (n=10).

CONCLUSION

FDG, FLT and FMISO-PET reflect different functional characteristics that change during CRT in rectal cancer. FLT and FDG show good spatial correspondence, while FMISO seems less reliable due to the non-specific FMISO uptake in normoxic tissue and tracer diffusion through the bowel wall. FDG and FLT-PET/CT imaging seem most appropriate to integrate in preoperative RT for rectal cancer.

Positron Emission Tomography (PET) radiotracers in oncology--utility of 18F-Fluoro-deoxy-glucose (FDG)-PET in the management of patients with non-small-cell lung cancer (NSCLC)

PET (Positron Emission Tomography) is a nuclear medicine imaging method, frequently used in oncology during the last years. It is a non-invasive technique that provides quantitative in vivo assessment of physiological and biological phenomena. PET has found its application in common practice for the management of various cancers.Lung cancer is the most common cause of death for cancer in western countries.This review focuses on radiotracers used for PET scan with particular attention to Non Small Cell Lung Cancer diagnosis, staging, response to treatment and follow-up.

Imaging of cell proliferation: status and prospects

Increased cellular proliferation is an integral part of the cancer phenotype. Several in vitro assays have been developed to measure the rate of tumor growth, but these require biopsies, which are particularly difficult to obtain over time and in different areas of the body in patients with multiple metastatic lesions. Most of the effort to develop imaging methods to noninvasively measure the rate of tumor cell proliferation has focused on the use of PET in conjunction with tracers for the thymidine salvage pathway of DNA synthesis, because thymidine contains the only pyrimidine or purine base that is unique to DNA. Imaging with 11C-thymidine has been tested for detecting tumors and tracking their response to therapy in animals and patients. Its major limitations are the short half-life of 11C and the rapid catabolism of thymidine after injection. These limitations led to the development of analogs that are resistant to degradation and can be labeled with radionuclides more conducive to routine clinical use, such as 18F. At this point, the thymidine analogs that have been studied the most are 3'-deoxy-3'-fluorothymidine (FLT) and 1-(2'-deoxy-2'-fluoro-1-beta-d-arabinofuranosyl)-thymine (FMAU). Both are resistant to degradation and track the DNA synthesis pathway. FLT is phosphorylated by thymidine kinase 1, thus being retained in proliferating cells. It is incorporated by the normal proliferating marrow and is glucuronidated in the liver. FMAU can be incorporated into DNA after phosphorylation but shows less marrow uptake. It shows high uptake in the normal heart, kidneys, and liver, in part because of the role of mitochondrial thymidine kinase 2. Early clinical data for 18F-FLT demonstrated that its uptake correlates well with in vitro measures of proliferation. Although 18F-FLT can be used to detect tumors, its tumor-to-normal tissue contrast is generally lower than that of 18F-FDG in most cancers outside the brain. The most promising use for thymidine and its analogs is in monitoring tumor treatment response, as demonstrated in animal studies and pilot human trials. Further work is needed to determine the optimal tracer(s) and timing of imaging after treatment.

Fundamentals of molecular imaging: rationale and applications with relevance for radiation oncology

Molecular imaging allows for the visualization and quantification biologic processes at cellular levels. This article focuses on positron emission tomography as one readily available tool for clinical molecular imaging. To prove its clinical utility in oncology, molecular imaging will ultimately have to provide valuable information in the following 4 pertinent areas: staging; assessment of extent of disease; target delineation for radiation therapy planning; response prediction and assessment and differentiation between treatment sequelae and recurrent disease. These issues are addressed in other contributions in this issue of Seminars in Nuclear Medicine. In contrast, this article will focus on the biochemical principles of cancer metabolism that provide the rationale for positron emission tomography imaging in radiation oncology.

Conventional and novel PET tracers for imaging in oncology in the era of molecular therapy

In the last ten years, the development of several novel targeted drugs and the refinement of state of the art technologies such as the genomics and proteomics and their introduction to clinical practice have revolutionized the management of patients affected by cancer. However, everyday practice points out several clinical questions: the difficulty of response assessment to new drugs especially using standard RECIST criteria that do not provide information on biological, vascular or metabolic variations; the inadequate selection of patients who are likely to benefit from a targeted therapy excluding those with breast cancer and gastrointestinal stromal tumours; the need to know the global biological background of diseases especially in metastatic setting using repeatable non-invasive procedures. Molecular imaging could provide information on in vivo distribution of biological markers in response to targeted therapy and could improve the selection of patients before therapies. The aim of this review is to analyze the current role of conventional and innovative positron emission tomography (PET) radiotracers in clinical practice and to explore the promising perspectives of molecular imaging in cancer research.

Non-[18F]FDG PET in clinical oncology

PET is an exquisitely sensitive molecular imaging technique using positron-emitting radioisotopes coupled to specific ligands. Many biological targets of great interest can be imaged with these radiolabelled ligands. This review describes the current status of non-18-fluorodeoxyglucose PET tracers that have a potential clinical effect in oncology. With the help of these tracers, knowledge is being acquired on the molecular characterisation of specific tumours, their biological signature, and postinterventional response. The potential role of these imaging probes for tumour detection and monitoring is progressively being recognised by clinical oncologists, biologists, and pharmacologists.

Measuring proliferation in breast cancer: practicalities and applications

Various methods are available for the measurement of proliferation rates in tumours, including mitotic counts, estimation of the fraction of cells in S-phase of the cell cycle and immunohistochemistry of proliferation-associated antigens. The evidence, advantages and disadvantages for each of these methods along with other novel approaches is reviewed in relation to breast cancer. The potential clinical applications of proliferative indices are discussed, including their use as prognostic indicators and predictors of response to systemic therapy.

Fluorine-18 fluorothymidine: a new positron emission radioisotope for renal tumors

Fluorine-18 fluorothymidine (F-18 FLT) is a radioisotope based on the nucleic acid thymidine and has emerged as an important tracer that mirrors cellular proliferation in positron emission tomography (PET) studies. Early studies in human tumors have been promising. However, imaging of renal tumors using F-18 FLT PET studies has not previously been described. In this report, a difficult case of renal transitional cell carcinoma in a longstanding cyst was clearly delineated using F-18 FLT. Importantly, the study was able to guide clinicians toward appropriate surgical management. The use of such tracers may herald a new era in renal tumor imaging.

Nuklearmedizinische Diagnostik von Lebertumoren

Standard nuclear medical procedures, such as functional, blood-pool and colloid scintigraphy, play a minor role in the routine workup of liver tumors. However, these techniques are capable of assessing specific organ functions and frequently allow the diagnosis of unclear liver lesions. The sensitivity of scintigraphic procedures can be increased using tomographic imaging (SPECT), the specificity with the introduction of hybrid scanners such as SPECT/CT. Whole body positron emission tomography with 18F-fluoro-deoxy-glucose (FDG) in combination with CT scanning (PET/CT) represents one of the most sensitive imaging modalities for the detection of hepatic metastases and extrahepatic tumor manifestations. For the staging and follow-up of colorectal cancer, FDG-PET/CT represents a standard imaging modality. Metastases from neuroendocrine tumors can be detected using PET and specific tracers such as [68Ga]DOTATOC and [18F]DOPA. Molecular imaging with PET allows the quantification of metabolic processes which can be used for the assessment of an early response to treatment.

PET imaging with [18F]3′-deoxy-3′-fluorothymidine for prediction of response to neoadjuvant treatment in patients with rectal cancer

PURPOSE

Positron emission tomography (PET) using 18F-labelled 3'-deoxy-3'-fluorothymidine (FLT) was assessed for therapy monitoring in patients with rectal cancer undergoing neoadjuvant chemoradiotherapy.

METHODS

Ten patients with locally advanced rectal cancer were included and underwent long-course preoperative chemoradiotherapy (total dose 45 Gy, 1.8 Gy/day, concomitant 250 mg/m2 5-fluorouracil) followed by surgery. FLT-PET was performed prior to chemoradiotherapy, 2 weeks after initiation of chemoradiotherapy and preoperatively (3-4 weeks post chemoradiotherapy). FLT uptake was correlated with histopathological tumour regression and changes in T stage.

RESULTS

Mean tumour FLT uptake was 4.2+/-1.0 SUV before therapy and decreased significantly to 2.9+/-0.6 SUV 14 days after initiation of chemoradiotherapy (-28.6%+/-10.7%, p=0.005). The preoperative scan showed a further decrease to 1.9+/-0.4 SUV (-54.7%+/-7.6%, p=0.005). However, the degree of change in FLT uptake 2 weeks after initiation and after completion of neoadjuvant therapy did not correlate with histopathological tumour regression.

CONCLUSION

FLT-PET did not seem to be a promising method for assessment of tumour response in the studied chemoradiotherapy regimen in patients with rectal cancer.

Evaluation of Primary Brain Tumors With FLT-PET: Usefulness and Limitations

PURPOSE OF THE REPORT

The purpose of this report was to investigate the potential of positron emission tomography using F-18 fluorodeoxythymidine (FLT-PET) in evaluating primary brain tumors.

MATERIALS AND METHODS

FLT-PET was performed in 25 patients with primary brain tumors. FLT uptake in the lesion was semiquantitatively evaluated by measuring the maximal standardized uptake value (SUVmax) and the tumor-to-normal tissue ratio (TNR). SUVmax and TNR were compared with the histologic grade and the expression of the proliferation marker (Ki-67).

RESULTS

FLT uptake in normal brain parenchyma was very low, resulting in the visualization of brain tumors with high contrast. Both SUVmax and TNR significantly correlated with the malignant grade of brain gliomas, in which high SUVmax/TNR was obtained for high-grade gliomas. Patients with primary lymphoma also showed SUVmax/TNR equivalent to glioblastoma. There was a positive correlation between SUVmax/TNR and the Ki-67 index. In contrast, spuriously high SUVmax and TNR were obtained in 3 of 6 patients with suspected recurrent tumors (2 patients with recurrent grade 2 glioma and one patient with postoperative granuloma), all of which showed lesion enhancement on MRI after Gd administration.

CONCLUSIONS

FLT-PET can be used to evaluate the malignant grade and proliferation activity of primary brain tumors, especially malignant brain tumors. However, the presence of benign lesions showing blood-brain barrier disruption cannot be distinguished from malignant tumors and needs to be carefully evaluated.

Molecular Imaging of Proliferation in Malignant Lymphoma

We have determined the ability of positron emission tomography (PET) with the thymidine analogue 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) to detect manifestation sites of malignant lymphoma, to assess proliferative activity, and to differentiate aggressive from indolent tumors. In this prospective study, FLT-PET was done additionally to routine staging procedures in 34 patients with malignant lymphoma. Sixty minutes after i.v. injection of approximately 330 MBq FLT, emission and transmission scanning was done. Tracer uptake in lymphoma was evaluated semiquantitatively by calculation of standardized uptake values (SUV) and correlated to tumor grading and proliferation fraction as determined by Ki-67 immunohistochemistry. FLT-PET detected a total of 490 lesions compared with 420 lesions revealed by routine staging. In 11 patients with indolent lymphoma, mean FLT-SUV in biopsied lesions was 2.3 (range, 1.2-4.5). In 21 patients with aggressive lymphoma, a significantly higher FLT uptake was observed (mean FLT-SUV, 5.9; range, 3.2-9.2; P < 0.0001) and a cutoff value of SUV = 3 accurately discriminated between indolent and aggressive lymphoma. Linear regression analysis indicated significant correlation of FLT uptake in biopsied lesions and proliferation fraction (r = 0.84; P < 0.0001). In this clinical study, FLT-PET was suitable for imaging malignant lymphoma and noninvasive assessment of tumor grading. Due to specific imaging of proliferation, FLT may be a superior PET tracer for detection of malignant lymphoma in organs with high physiologic fluorodeoxyglucose uptake and early detection of progression to a more aggressive histology or potential transformation.

3′-Deoxy-3′-[F-18]Fluorothymidine Positron Emission Tomography in Patients with Recurrent Glioblastoma Multiforme: Comparison with Gd-DTPA Enhanced Magnetic Resonance Imaging

INTRODUCTION

The accumulation of 3'-deoxy-3'-[F-18]fluorothymidine (FLT) on positron emission tomography (PET) images in patients with glioblastoma multiforme was evaluated and correlated with gadopentetate dimeglumine (Gd-DTPA) enhancement in magnetic resonance images (MRIs).

METHODS

FLT studies in 10 patients with recurrent glioblastoma multiforme were retrospectively investigated. Dynamic emission data were acquired for 60 minutes immediately after injection of FLT. The standardized uptake value (SUV) for tumor and reference tissue (contralateral hemisphere and ipsilateral cerebellum) was calculated. The volumes of the metabolically active part of the tumor (V (PET)) and that of the Gd-DTPA enhancing part of the tumor (V (MR)) were calculated.

RESULTS

FLT uptake in tumors peaked before 5 minutes and sometimes as early as 0.5 minutes, and reached a constant level at approximately 10 minutes after injection. The reference tissue time-activity curves had an early peak and reached a constant low background level. All tumors had increased FLT uptake and showed Gd-DTPA enhancement. The SUV in tumor was significantly higher than that in the reference tissue (P<0.0001). A significant correlation between V (PET) and V (MR) was found (P<0.0001) although there was a difference in the areas of Gd-DTPA enhancement and FLT uptake.

CONCLUSION

These preliminary results indicate that FLT-PET may be useful for the detection of recurrent glioblastoma multiforme. Our data in a relatively small patient population do not support a clear-cut relationship between FLT accumulation and Gd-DTPA enhancement. Further pathologic correlation will determine if it can be used for detecting recurrent tumoral disease.

In vivo biological activity of the histone deacetylase inhibitor LAQ824 is detectable with 3'-deoxy-3'-[18F]fluorothymidine positron emission tomography

Histone deacetylase inhibitors (HDACI) are emerging as growth inhibitory compounds that modulate gene expression and inhibit tumor cell proliferation. We assessed whether 3'-deoxy-3'-[(18)F]fluorothymidine-positron emission tomography ([18F]FLT-PET) could be used to noninvasively measure the biological activity of a novel HDACI LAQ824 in vivo. We initially showed that thymidine kinase 1 (TK1; EC2.7.1.21), the enzyme responsible for [18F]FLT retention in cells, was regulated by LAQ824 in a drug concentration-dependent manner in vitro. In HCT116 colon carcinoma xenograft-bearing mice, LAQ824 significantly decreased tumor [18F]FLT uptake in a dose-dependent manner. At day 4 of treatment, [18F]FLT tumor-to-heart ratios at 60 minutes (NUV60) were 2.16 +/- 0.15, 1.86 +/- 0.13, and 1.45 +/- 0.20 in vehicle, and 5 and 25 mg/kg LAQ824 treatment groups, respectively (P < or = 0.05). LAQ825 at 5 mg/kg also significantly reduced both TK1 levels and [18F]FLT uptake at day 10 but not at day 2 (P < or = 0.05). [18F]FLT NUV60 correlated significantly with cellular proliferation (r = 0.68; P = 0.0019) and was associated with drug-induced histone H4 hyperacetylation. Of interest to [18F]FLT-PET imaging, both TK1 mRNA copy numbers and protein levels decreased in the order vehicle >5 mg/kg LAQ824 > 25 mg/kg LAQ824, providing a rationale for the use of [18F]FLT-PET in this setting. We also observed increases in Rb hypophosphorylation and p21 levels, factors that could have contributed to the alteration in TK1 transcription in vivo. In conclusion, we have shown the utility of [18F]FLT-PET for monitoring the biological activity of the HDACI, LAQ824. Drug-induced changes in tumor [18F]FLT uptake were due, at least in part, to reductions in TK1 transcription and translation.

Targeted molecular imaging in oncology

Improvement of scintigraphic tumor imaging is extensively determined by the development of more tumor specific radiopharmaceuticals. Thus, to improve the differential diagnosis, prognosis, planning and monitoring of cancer treatment, several functional pharmaceuticals have been developed. Application of molecular targets for cancer imaging, therapy and prevention using generator-produced isotopes is the major focus of ongoing research projects. Radionuclide imaging modalities (positron emission tomography, PET; single photon emission computed tomography, SPECT) are diagnostic cross-sectional imaging techniques that map the location and concentration of radionuclide-labeled radiotracers. 99mTc- and 68Ga-labeled agents using ethylenedicysteine (EC) as a chelator were synthesized and their potential uses to assess tumor targets were evaluated. 99mTc (t1/2 = 6 hr, 140 keV) is used for SPECT and 68Ga (t1/2 = 68 min, 511 keV) for PET. Molecular targets labeled with Tc-99m and Ga-68 can be utilized for prediction of therapeutic response, monitoring tumor response to treatment and differential diagnosis. Molecular targets for oncological research in (1) cell apoptosis, (2) gene and nucleic acid-based approach, (3) angiogenesis (4) tumor hypoxia, and (5) metabolic imaging are discussed. Numerous imaging ligands in these categories have been developed and evaluated in animals and humans. Molecular targets were imaged and their potential to redirect optimal cancer diagnosis and therapeutics were demonstrated.

Positron emission tomography in patients with breast cancer using 18F-3′-deoxy-3′-fluoro-l-thymidine (18F-FLT)—a pilot study

BACKGROUND

This pilot study investigated the feasibility of (18)F-3'-deoxy-3'-fluoro-l-thymidine ((18)F-FLT) as a positron emission tomography (PET) tracer for the visualisation of breast cancer.

METHODS

Patients with breast cancer underwent (18)F-FLT-PET prior to surgery. The uptake of (18)F-FLT was determined in the primary tumour and in the axilla.

RESULTS

Eight tumours were visualized by (18)F-FLT-PET with a mean uptake value (SUV(mean)) of 1.7 and mean tumour-non-tumour ratio (TNT) of 5.0. In seven patients, axillary lymph-node metastases were found at pathological examinations, however, (18)F-FLT-PET showed uptake in only two large (and clinically evident) lymph-node metastases.

CONCLUSIONS

(18)F-FLT shows uptake in most primary breast tumours and in large axillary lymph-node metastases.

The contribution of PET/CT to improved patient management

With the introduction of both SPET/CT and PET/CT, multimodality imaging has truly entered routine clinical practice. Multiple slice spiral CT scanners have been incorporated with multiple detector gamma cameras or PET systems, such that the benefit of these modalities can be achieved in one patient sitting. The subject of this manuscript is PET/CT and its impact on patient management. Applications of PET/CT span the whole field of medical and surgical oncology since very few cancers do not take up the labelled glucose tracer, (18)F-FDG. Given the contrast achieved, high-quality data can be obtained with FDG PET/CT. This technology has now spread worldwide and has been the subject of intense interest, as witnessed by the vast body of published evidence. In this short overview, only a brief discussion of the main clinical applications is possible. Novel applications of PET/CT outside the field of oncology are expected in the near future.

Applications of PET in Liver Imaging

Fluorodeoxyglucose PET (FDG-PET) imaging has an important role in determining if there are metastases to the liver and whether disease has spread beyond the liver. Such information is critical for planning surgical resections of liver metastases. The ability of FDG-PET quantitatively to estimate metabolic rates makes it an important tool for monitoring. With increasingly broad indications for FDG-PET imaging, it is expected that FDG-PET (and PET-CT) of the liver will play a growing and increasingly important role in detecting and monitoring treatment of tumors involving the liver.

(99m)Tc-EC-guanine: synthesis, biodistribution, and tumor imaging in animals

PURPOSE

DNA markers are useful in assessing cell proliferation. The purpose of this study was to synthesize (99m)Tc-ethylenedicysteine-guanine (EC-Guan) for evaluation of cell proliferation.

METHODS

Tumor cells were incubated with (99m)Tc-EC-Guan for cell cycle analysis. Prostate tumor cells that were overexpressing the HSV thymidine kinase gene, or various tumor cells were incubated with (99m)Tc-EC-Guan at 0.5-2 h. Thymidine incorporation assays were performed in lung cancer cells incubated with EC-Guan at 0.1-1 mg/well. Tissue distribution, autoradiography, and planar scintigraphy of (99m)Tc-EC-Guan and (99m)Tc-EC (control) were determined in tumor-bearing rodents at 0.5-4 h.

RESULTS

Cell culture assays indicated that EC-Guan was incorporated in DNA, and there was no significant uptake difference between HSVTK overexpressed and normal groups. Biodistribution and scintigraphic imaging studies of (99m)Tc-EC-Guan showed increased tumor/tissue count density ratios as a function of time.

CONCLUSIONS

Our results indicate that (99m)Tc-EC-Guan may be useful as a tumor proliferation imaging agent.

[18F]FLT-PET in oncology: current status and opportunities

In recent years, [18F]-fluoro-3'-deoxy-3'-L: -fluorothymidine ([18F]FLT) has been developed as a proliferation tracer. Imaging and measurement of proliferation with PET could provide us with a non-invasive staging tool and a tool to monitor the response to anticancer treatment. In this review, the basis of [18F]FLT as a proliferation tracer is discussed. Furthermore, an overview of the current status of [18F]FLT-PET research is given. The results of this research show that although [18F]FLT is a tracer that visualises cellular proliferation, it also has certain limitations. In comparison with the most widely used PET tracer, [18F]FDG, [18F]FLT uptake is lower in most cases. Furthermore, [18F]FLT uptake does not always reflect the tumour cell proliferation rate, for example during or shortly after certain chemotherapy regimens. The opportunities provided by, and the limitations of, [18F]FLT as a proliferation tracer are addressed in this review, and directions are given for further research, taking into account the strong and weak points of the new tracer.

Molecular Nuclear Imaging: The Radiopharmaceuticals (Review)

Application of the nuclear approach for the detection of inherited diseases is an important goal for nuclear medicine and will likely result in an important breakthrough, which will, hopefully, lead to improved diagnoses of genetic defects and objective evaluations of the efficacy of therapeutic strategies. Although still largely in the research realm, molecular imaging is in the process of emerging as a vital component of the diagnosis of disease and monitoring of the therapy. The clinical research in nuclear medicine has made major advancements in the direction of molecular medicine and targeted therapy. In the past few years, exponential achievements have been accomplished in the development of molecular nuclear imaging agents, as described below.

Monitoring response to treatment in patients utilizing PET

Establishing new surrogate end points for monitoring response to treatment is needed for current therapy modalities and for new therapeutic strategies including molecular targeted cancer therapies. PET as a functional imaging technology provides rapid, reproducible, noninvasive in vivo assessment and quantification of several biologic processes targeted by these therapies. PET is useful in a variety of clinical relevant applications, including distinguishing between radiation necrosis and tumor recurrence, determining the resectability of recurrent tumor, and evaluating response to therapy. FDG-PET has demonstrated efficacy for monitoring therapeutic response in a wide range of cancers, including breast, esophageal, lung, head and neck, and lymphoma. FDG-PET can assess tumor glucose use with high reproducibility. Following therapy, the decrease of glucose use correlates with the reduction of viable tumor cells. FDG-PET allows the prediction of therapy response early in the course of therapy and determining the viability of residual masses after completion of treatment. The molecular basis for the success of FDG-PET is the rapid reduction of tumor glucose metabolism in effective therapies. Of even higher clinical relevance is the accurate identification of nonresponders in patients without a significant change in tumor glucose metabolism after initiation of therapy. PET imaging can easily visualize these changes in metabolic activity and indicate, sometimes within hours of the first treatment, whether or not a patient will respond to a particular therapy. In contrast to CT, MR imaging, or ultrasound, PET imaging allows identification of responding and nonresponding tumors early in the course of therapy. With this information, physicians can rapidly modify ineffective therapies for individual patients and thereby potentially improve patient outcomes and reduce cost. One of the major limitations for the routine application of FDG-PET imaging for therapy monitoring is that no generally accepted cutoff values have been established to differentiate optimally between responders and nonresponders. The patient series are still relatively small and frequently consist of different tumor types and different therapy regimens. Prospective studies including a sufficient number of patients are needed to define cutoff values to differentiate between responder and nonresponder for different tumors and different treatment regimes. In the future, PET imaging can also serve in the evaluation of new therapeutic agents, new experimental treatments, and specifically in monitoring clinical phase II studies.

[18F]3′-deoxy-3′-fluorothymidine PET for the diagnosis and grading of brain tumors

The aim of this study was to evaluate the feasibility of using [(18)F] 3'-deoxy-3'-fluorothymidine (FLT) positron emission tomography (PET) for the diagnosis and grading of brain tumors.

METHODS

The patient population comprised 26 patients (15 males, 11 females) with brain tumors (n=18) or nontumorous lesions (n=8). 2-[(18)F]fluoro-2-deoxy-D: -glucose (FDG) and FLT PET images were obtained using a dedicated PET scanner 1 h after the injection of 370 MBq of FDG or FLT. Uptake of FDG and FLT by the lesions was visually and semiquantitatively assessed in comparison with normal brain tissue.

RESULTS

Of 26 brain lesions, four showed increased FDG uptake compared with normal gray matter (grade 5). These four lesions showed intensely increased FLT uptake and were all high-grade tumors. Twenty-two lesions with similar (grade 4) or decreased (grades 1-3) FDG uptake compared with normal gray matter showed variable pathology. Among the 18 brain tumors, FLT PET showed increased uptake in all 12 high-grade tumors but FDG uptake was variable. In 22 brain lesions with similar or decreased uptake compared with normal gray matter on FDG PET, the sensitivity and specificity of FLT PET for the diagnosis of brain tumor were 79% (11/14) and 63% (5/8), respectively. The uptake ratios of 14 brain tumors on FLT PET were significantly higher than the lesion to gray matter ratios (p=0.012) and lesion to white matter ratios (p=0.036) of FDG uptake and differed significantly between high (5.1+/-2.6) and low (2.1+/-1.1) grade tumors (p=0.029). In nine gliomas, FLT uptake was significantly correlated with the Ki-67 proliferation index (rho=0.817, p=0.007).

CONCLUSION

These findings indicate that FLT PET is useful for evaluating tumor grade and cellular proliferation in brain tumors. It displayed high sensitivity and good contrast in evaluating brain lesions that showed similar or decreased uptake compared with normal gray matter on FDG PET. FLT PET, however, did not appear to be sufficiently useful for differentiating tumors from nontumorous lesions.

PET imaging of cellular proliferation

PET cellular proliferation imaging has its roots in a long history of in vitro cellular proliferation studies to characterize cancer and in the understanding of the biology of thymidine incorporation into DNA gained from these studies. PET imaging represents the logical translation of the in vitro work to measure in vivo tumor proliferation. Preclinical studies of [11C]-thymidine and other PET-labeled thymidine analogues set the stage for early clinical studies that provided very promising results. Recent progress in the application of [18F]-FLT, a clinically practical PET thymidine analogue, to patient studies sets the next stage for clinical PET cellular proliferation imaging. Further mechanistic studies of the imaging agents and well-designed clinical trials will be important in moving PET proliferation imaging into what is likely to be a significant role in the care of cancer patients by providing a quantitative measure of tumor response to cytotoxic or cytostatic therapy.

Clinical relevance of imaging proliferative activity in lung nodules

PURPOSE

Recently, the thymidine analogue 3'-deoxy-3'[18F]fluorothymidine (FLT) has been introduced for imaging proliferation with positron emission tomography (PET). In this prospective study, we examined the accuracy of FLT for differentiation of benign from malignant lung lesions and for tumour staging.

METHODS

A total of 47 patients with newly diagnosed pulmonary nodules on chest CT suspicious for malignancy were examined with FLT-PET in addition to routine staging procedures. A total of 43 patients also underwent 2-[18F]fluoro-2-deoxy-D-glucose (FDG) PET imaging. Within 2 weeks, patients underwent resective surgery or core biopsy of the pulmonary lesion.

RESULTS

Histopathology revealed malignant lung tumours in 32 patients (20 non-small cell lung cancer, 1 small cell lung cancer, 1 pulmonary carcinoid, 1 non-Hodgkin's lymphoma, nine metastases from extrapulmonary tumours) and benign lesions in 15 patients. Increased FLT uptake was exclusively related to malignant tumours. FLT-PET was false negative in two patients with non-small cell lung cancer, in the patient with a pulmonary carcinoid and in three patients with lung metastases. The sensitivity of FLT-PET for detection of lung cancer was 90%, the specificity 100% and the accuracy 94%. Fifteen out of 21 patients with lung cancer had mediastinal lymph node metastases. FLT-PET was true positive in 7/15 patients, resulting in a sensitivity of 53% for N-staging (specificity 100%, accuracy 67%). Clinical TNM stage was correctly identified in 67% (20/30) patients, compared to 85% (23/27) with FDG-PET.

CONCLUSION

FLT-PET has a high specificity for the detection of malignant lung tumours. Compared with FDG, FLT-PET is less accurate for N-staging in patients with lung cancer and for detection of lung metastases. FLT-PET therefore cannot be recommended for staging of lung cancer.

PET imaging in assessing gastrointestinal tumors

The clinical usefulness of FDG-PET imaging is now firmly established in various situations, such as the preoperative staging of esophageal cancer and recurrent colorectal carcinoma and the detection and staging of recurrent colorectal cancer when there is a clinical or biologic suspicion with inconclusive conventional findings. Encouraging results were obtained in the evaluation of the therapeutic response of various gastrointestinal malignancies, either during the treatment or after its completion. There is no firm consensus regarding its role in pancreatic cancer, either proved or suspected, but it may be valuable in selected clinical situations. Its role seems fairly limited in patients with hepatocellular carcinoma, although PET findings may have prognostic implications. Evaluation of cholangiocarcinoma is an emerging indication, albeit with limited data to date. Finally, PET/CT is very likely to enhance the role of FDG imaging further in the work-up of patients with gastrointestinal tumors.

PET and PET-CT for evaluation of colorectal carcinoma

The evaluation of patients with known or suspected recurrent colorectal carcinoma is now an accepted indication for positron emission tomography using (18)F-fluorodeoxyglucose (FDG-PET) imaging. FDG-PET does not replace imaging modalities such as computed tomography (CT) for preoperative anatomic evaluation but is indicated as the initial test for diagnosis and staging of recurrence and for preoperative staging (N and M) of known recurrence that is considered to be resectable. FDG-PET imaging is valuable for the differentiation of posttreatment changes from recurrent tumor, differentiation of benign from malignant lesions (indeterminate lymph nodes, hepatic and pulmonary lesions), and the evaluation of patients with rising tumor markers in the absence of a known source. The addition of FDG-PET to the evaluation of these patients reduces overall treatment costs by accurately identifying patients who will and will not benefit from surgical procedures. Although initial staging at the time of diagnosis is often performed during colectomy, FDG-PET imaging is recommended for a subgroup of patients at high risk (with elevated CEA levels) and normal CT and for whom surgery can be avoided if FDG-PET shows metastases. Screening for recurrence in patients at high risk has also been advocated. FDG-PET imaging seems promising for monitoring patient response to therapy but larger studies are necessary. The diagnostic implications of integrated PET-CT imaging include improved detection of lesions on both the CT and FDG-PET images, better differentiation of physiologic from pathologic foci of metabolism, and better localization of the pathologic foci. This new powerful technology provides more accurate interpretation of both CT and FDG-PET images and therefore more optimal patient care. PET-CT fusion images affect the clinical management by guiding further procedures (biopsy, surgery, radiation therapy), excluding the need for additional procedures, and changing both inter- and intramodality therapy.

Metabolic Imaging with FDG

The rapid advances in imaging technologies are a challenge for both radiologists and clinicians who must integrate these technologies for optimal patient care and outcomes at minimal cost. Multiple indications for functional imaging using fluorode-oxyglucose (FDG) are now well accepted in the fields of neurology, cardiology, and oncology, including differentiation of benign from malignant lesions, staging of malignant lesions, detection of malignant recurrence, and monitoring of therapy. The fusion of anatomic and molecular images (computed tomography [CT] and FDG) obtained with integrated positron emission tomography (PET)-CT systems, sequentially in time but without moving the patient from the imaging table, allows optimal co-registration of anatomic and molecular images, leading to accurate attenuation correction and precise anatomic localization of lesions with increased metabolism. This powerful technology provides a valuable new tool for diagnostic and therapeutic applications. The diagnostic accuracy is improved in approximately 50% of patients because of improvement of lesion detection on both CT and FDG PET images, better differentiation between physiologic and pathological foci of FDG uptake, and better localization of malignant foci of FDG uptake. This new technology affects the management of 10%-20% of cases by guiding further therapy. Promising clinical applications include guiding biopsy to the metabolically active sites of tumors, guiding surgery, and planning intensity-modulated radiation therapy. In addition, new PET radiopharmaceuticals are emerging for indications for which FDG has limitations. Some of the new PET tracers are labeled with (18)F, which has a practical half-life for commercial distribution. In the past few years, the clinical indications for FDG have broadened dramatically, and the rapid technical developments of integrated multimodality imaging systems and new PET tracers further extend the horizon.

Positron Emission Tomography Imaging of Cell Proliferation in Oncology

Tumour-cell proliferation is a hallmark of the malignant phenotype. Positron emission tomography (PET) offers a unique method of imaging biological and biochemical changes in vivo. Radiolabelled thymidine and thymidine analogues are currently in development as PET tracers. By studying the uptake and kinetics of such compounds using PET, a measure of DNA synthesis and hence cell proliferation can be obtained. Molecular imaging of cellular proliferation with PET is now possible, and has the potential to play an important role in the evaluation of efficacy of new anti-cancer agents.