Kinetic filtering of [18F]Fluorothymidine in positron emission tomography studies

Nonmalignant Thoracic Disorders: An Appraisal of Fluorodeoxyglucose and Non-fluorodeoxyglucose PET/Computed Tomography Applications

PET/computed tomography (CT) with fluorodeoxyglucose and nonfluorodeoxyglucose PET tracers has established itself in the management of malignant disorders. Its role in the assessment of nonmalignant conditions, such as infectious and noninfectious inflammatory diseases and other benign conditions, has emerged independently and alongside its role being evaluated in malignancy and continues to evolve. It is evident that PET/CT has the potential to play a significant role in various nonmalignant disorders of the thorax. This review highlights current developments and areas where PET/CT has a potential to impact the clinical management of nonmalignant thoracic conditions with special focus on nonfluorodeoxyglucose tracers.

Supervised clustering for TSPO PET imaging

PURPOSE

This technical note seeks to act as a practical guide for implementing a supervised clustering algorithm (SVCA) reference region approach and to explain the main strengths and limitations of the technique in the context of 18-kilodalton translocator protein (TSPO) positron emission tomography (PET) studies in experimental medicine.

BACKGROUND

TSPO PET is the most widely used imaging technique for studying neuroinflammation in vivo in humans. Quantifying neuroinflammation with PET can be a challenging and invasive procedure, especially in frail patients, because it often requires blood sampling from an arterial catheter. A widely used alternative to arterial sampling is SVCA, which identifies the voxels with minimal specific binding in the PET images, thus extracting a pseudo-reference region for non-invasive quantification. Unlike other reference region approaches, SVCA does not require specification of an anatomical reference region a priori, which alleviates the limitation of TSPO contamination in anatomically-defined reference regions in individuals with underlying inflammatory processes. Furthermore, SVCA can be applied to any TSPO PET tracer across different neurological and neuropsychiatric conditions, providing noninvasivequantification of TSPO expression.

METHODS

We provide an overview of the development of SVCA as well as step-by-step instructions for implementing SVCA with suggestions for specific settings. We review the literature on SVCAapplications using first- and second- generation TSPO PET tracers and discuss potential clinically relevant limitations and applications.

CONCLUSIONS

The correct implementation of SVCA can provide robust and reproducible estimates of brain TSPO expression. This review encourages the standardisation of SVCA methodology in TSPO PET analysis, ultimately aiming to improve replicability and comparability across study sites.

Monitoring Response to Transarterial Chemoembolization in Hepatocellular Carcinoma Using 18F-Fluorothymidine PET

Accurate disease monitoring is essential after transarterial chemoembolization (TACE) in hepatocellular carcinoma (HCC) because of the potential for profound adverse events and large variations in survival outcome. Posttreatment changes on conventional imaging can confound determination of residual or recurrent disease, magnifying the clinical challenge. On the basis of increased expression of thymidylate synthase (TYMS), thymidine kinase 1 (TK-1), and equilibrative nucleoside transporter 1 (SLC29A1) in HCC compared with liver tissue, we conducted a proof-of-concept study evaluating the efficacy of 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) PET to assess response to TACE. Because previous PET studies in HCC have been hampered by high background liver signal, we investigated whether a temporal-intensity voxel clustering (kinetic spatial filtering, or KSF) improved lesion detection. Methods: A tissue microarray was built from 36 HCC samples and from matching surrounding cirrhotic tissue and was stained for TK-1 A prospective study was conducted; 18 patients with a diagnosis of HCC by the criteria of the American Association for the Study of Liver Diseases who were eligible for treatment with TACE were enrolled. The patients underwent baseline conventional imaging and dynamic 18F-FLT PET with KSF followed by TACE. Imaging was repeated 6-8 wk after TACE. The PET parameters were compared with modified enhancement-based RECIST. Results: Cancer Genome Atlas analysis revealed increased RNA expression of TYMS, TK-1, and SLC29A1 in HCC. TK-1 protein expression was significantly higher in HCC (P < 0.05). The sensitivity of 18F-FLT PET for baseline HCC detection was 73% (SUVmax, 9.7 ± 3.0; tumor to liver ratio, 1.2 ± 0.3). Application of KSF did not improve lesion detection. Lesion response after TACE by modified RECIST was 58% (14 patients with 24 lesions). A 30% reduction in mean 18F-FLT PET uptake was observed after TACE, correlating with an observed PET response of 60% (15/25). A significant and profound reduction in the radiotracer delivery parameter K1 after TACE was observed. Conclusion:18F-FLT PET can differentiate HCC from surrounding cirrhotic tissue, with PET parameters correlating with TACE response. KSF did not improve visualization of tumor lesions. These findings warrant further investigation.

Evaluation of 18F-fluorothymidine positron emission tomography ([18F]FLT-PET/CT) methodology in assessing early response to chemotherapy in patients with gastro-oesophageal cancer

BACKGROUND

3'-Deoxy-3'-[18F]fluorothymidine ([18F]FLT) PET has limited utility in abdominal imaging due to high physiological hepatic uptake of a tracer. We evaluated [18F]FLT-PET/CT combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (KSF) to improve tumour visualisation in patients with locally advanced and metastatic gastro-oesophageal cancer and as a marker of early response to chemotherapy. Dynamic [18F]FLT-PET/CT data were collected before and 3 weeks post first cycle of chemotherapy. Changes in tumour [18F]FLT-PET/CT variables were determined. Response was determined on contrast-enhanced CT after three cycles of therapy using RECIST 1.1.

RESULTS

Ten patients were included. Following application of the KSF, visual distinction of all oesophageal and/or gastric tumours was observed in [18F]FLT-PET images. Among the nine patients available for response evaluation (RECIST 1.1), three patients had responded (partial response) and six patients were non-responders (stable disease). There was a significant association between Ki-67 and all baseline [18F]FLT-PET parameters. Area under the curve (AUC) from 0 to 1 min was associated with treatment response.

CONCLUSIONS

The results of this study indicate that application of the KSF allowed accurate visualisation of both primary and metastatic lesions following imaging with the proliferation marker, [18F]FLT-PET/CT. However, [18F]FLT-PET uptake parameters did not correlate with response. Instead, we observe significant changes in tracer delivery following chemotherapy suggesting that further [18F]FLT-PET/CT studies in this tumour type should be undertaken with caution.

Functional imaging of hepatocellular carcinoma

Imaging plays a key role in the clinical management of hepatocellular carcinoma (HCC), but conventional imaging techniques have limited sensitivity in visualizing small tumors and assessing response to locoregional treatments and sorafenib. Functional imaging techniques allow visualization of organ and tumor physiology. Assessment of functional characteristics of tissue, such as metabolism, proliferation and stiffness, may overcome some of the limitations of structural imaging. In particular, novel molecular imaging agents offer a potential tool for early diagnosis of HCC, and radiomics may aid in response assessment and generate prognostic models. Further prospective research is warranted to evaluate emerging techniques and their cost-effectiveness in the context of HCC in order to improve detection and response assessment.

A Phase II Study of 3'-Deoxy-3'-18F-Fluorothymidine PET in the Assessment of Early Response of Breast Cancer to Neoadjuvant Chemotherapy: Results from ACRIN 6688

Our objective was to determine whether early change in standardized uptake values (SUVs) of 3'deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) using PET with CT could predict pathologic complete response (pCR) of primary breast cancer to neoadjuvant chemotherapy (NAC). The key secondary objective was to correlate SUV with the proliferation marker Ki-67 at baseline and after NAC.

METHODS

This prospective, multicenter phase II study did not specify the therapeutic regimen, thus, NAC varied among centers. All evaluable patients underwent (18)F-FLT PET/CT at baseline (FLT1) and after 1 cycle of NAC (FLT2); 43 patients were imaged at FLT1, FLT2, and after NAC completion (FLT3). The percentage change in maximum SUV (%ΔSUVmax) between FLT1 and FLT2 and FLT3 was calculated for the primary tumors. The predictive value of ΔSUVmax for pCR was determined using receiver-operating-characteristic curve analysis. The correlation between SUVmax and Ki-67 was also assessed.

RESULTS

Fifty-one of 90 recruited patients (median age, 54 y; stage IIA-IIIC) met the eligibility criteria for the primary objective analysis, with an additional 22 patients totaling 73 patients for secondary analyses. A pCR in the primary breast cancer was achieved in 9 of 51 patients. NAC resulted in a significant reduction in %SUVmax (mean Δ, 39%; 95% confidence interval, 31-46). There was a marginal difference in %ΔSUVmax_FLT1-FLT2 between pCR and no-pCR patient groups (Wilcoxon 1-sided P = 0.050). The area under the curve for ΔSUVmax in the prediction of pCR was 0.68 (90% confidence interval, 0.50-0.83; Delong 1-sided P = 0.05), with slightly better predictive value for percentage mean SUV (P = 0.02) and similar prediction for peak SUV (P = 0.04). There was a weak correlation with pretherapy SUVmax and Ki-67 (r = 0.29, P = 0.04), but the correlation between SUVmax and Ki-67 after completion of NAC was stronger (r = 0.68, P < 0.0001).

CONCLUSION

(18)F-FLT PET imaging of breast cancer after 1 cycle of NAC weakly predicted pCR in the setting of variable NAC regimens. Posttherapy (18)F-FLT uptake correlated with Ki-67 on surgical specimens. These results suggest some efficacy of (18)F-FLT as an indicator of early therapeutic response of breast cancer to NAC and support future multicenter studies to test (18)F-FLT PET in a more uniformly treated patient population.

3'-Deoxy-3'-¹⁸F-fluorothymidine positron emission tomography as an early predictor of disease progression in patients with advanced and metastatic pancreatic cancer

PURPOSE

3'-Deoxy-3'-(18)F-fluorothymidine (FLT) positron emission tomography (PET) has limited utility in abdominal imaging due to high physiological hepatic uptake of tracer. We evaluated FLT PET/CT combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (FLT PET/CTKSF) for early prediction of response and survival outcomes in locally advanced and metastatic pancreatic cancer patients receiving gemcitabine-based chemotherapy.

METHODS

Dynamic FLT PET/CT data were collected before and 3 weeks after the first cycle of chemotherapy. Changes in tumour FLT PET/CT variables were determined. The primary end point was RECIST 1.1 response on contrast-enhanced CT after 3 months of therapy.

RESULTS

Twenty patients were included. Visual distinction between tumours and normal pancreas was seen in FLT PETKSF images. All target lesions (>2 cm), including all primary pancreatic tumours, were visualised. Of the 11 liver metastases, 3 (<2 cm) were not visible after kinetic filtering. Of the 20 patients, 7 progressed (35%). Maximum standardised uptake value at 60 min post-injection (SUV60,max) significantly increased in patients with disease progression (p = 0.04). Receiver-operating characteristic curve analysis indicated that a threshold of SUV60,max increase of ≥ 12% resulted in sensitivity, specificity and positive predictive value (PPV) of 71, 100 and 100%, respectively [area under the curve (AUC) 0.90, p = 0.0001], to predict patients with disease progression. Changes in SUV60,max were not predictive of survival.

CONCLUSION

FLT PET/CT detected changes in proliferation, with early increase in SUV60,max predicting progressive disease with a high specificity and PPV. Therefore, FLT PET/CT could be used as an early response biomarker for gemcitabine-based chemotherapy, to select a poor prognostic group who may benefit from novel therapeutic agents in advanced and metastatic pancreatic cancer.

Metabolically active tumour volume segmentation from dynamic [(18)F]FLT PET studies in non-small cell lung cancer

BACKGROUND

Positron emission tomography (PET) with (18)F-3'-deoxy-3'-fluorothymidine ([(18)F]FLT) can be used to assess tumour proliferation. A kinetic-filtering (KF) classification algorithm has been suggested for segmentation of tumours in dynamic [(18)F]FLT PET data. The aim of the present study was to evaluate KF segmentation and its test-retest performance in [(18)F]FLT PET in non-small cell lung cancer (NSCLC) patients.

METHODS

Nine NSCLC patients underwent two 60-min dynamic [(18)F]FLT PET scans within 7 days prior to treatment. Dynamic scans were reconstructed with filtered back projection (FBP) as well as with ordered subsets expectation maximisation (OSEM). Twenty-eight lesions were identified by an experienced physician. Segmentation was performed using KF applied to the dynamic data set and a source-to-background corrected 50% threshold (A50%) was applied to the sum image of the last three frames (45- to 60-min p.i.). Furthermore, several adaptations of KF were tested. Both for KF and A50% test-retest (TRT) variability of metabolically active tumour volume and standard uptake value (SUV) were evaluated.

RESULTS

KF performed better on OSEM- than on FBP-reconstructed PET images. The original KF implementation segmented 15 out of 28 lesions, whereas A50% segmented each lesion. Adapted KF versions, however, were able to segment 26 out of 28 lesions. In the best performing adapted versions, metabolically active tumour volume and SUV TRT variability was similar to those of A50%. KF misclassified certain tumour areas as vertebrae or liver tissue, which was shown to be related to heterogeneous [(18)F]FLT uptake areas within the tumour.

CONCLUSIONS

For [(18)F]FLT PET studies in NSCLC patients, KF and A50% show comparable tumour volume segmentation performance. The KF method needs, however, a site-specific optimisation. The A50% is therefore a good alternative for tumour segmentation in NSCLC [(18)F]FLT PET studies in multicentre studies. Yet, it was observed that KF has the potential to subsegment lesions in high and low proliferative areas.

Alterations of choline phospholipid metabolism in endometrial cancer are caused by choline kinase alpha overexpression and a hyperactivated deacylation pathway

Metabolic rearrangements subsequent to malignant transformation are not well characterized in endometrial cancer. Identification of altered metabolites could facilitate imaging-guided diagnosis, treatment surveillance, and help to identify new therapeutic options. Here, we used high-resolution magic angle spinning magnetic resonance mass spectroscopy on endometrial cancer surgical specimens and normal endometrial tissue to investigate the key modulators that might explain metabolic changes, incorporating additional investigations using qRT-PCR, Western blotting, tissue microarrays (TMA), and uptake assays of [(3)H]-labeled choline. Lipid metabolism was severely dysregulated in endometrial cancer with various amino acids, inositols, nucleobases, and glutathione also altered. Among the most important lipid-related alterations were increased phosphocholine levels (increased 70% in endometrial cancer). Mechanistic investigations revealed that changes were not due to altered choline transporter expression, but rather due to increased expression of choline kinase α (CHKA) and an activated deacylation pathway, as indicated by upregulated expression of the catabolic enzymes LYPLA1, LYPLA2, and GPCPD1. We confirmed the significance of CHKA overexpression on a TMA, including a large series of endometrial hyperplasia, atypical hyperplasia, and adenocarcinoma tissues, supporting a role for CHKA in malignant transformation. Finally, we documented several-fold increases in the uptake of [(3)H]choline in endometrial cancer cell lines compared with normal endometrial stromal cells. Our results validate deregulated choline biochemistry as an important source of noninvasive imaging biomarkers for endometrial cancer.

Methodological considerations in quantification of 3'-deoxy-3'-[18F]fluorothymidine uptake measured with positron emission tomography in patients with non-small cell lung cancer

PURPOSE

To investigate the effect of image-derived input functions (IDIF), input function corrections and volume of interest (VOI) size in quantification of [(18)F]FLT uptake in non-small cell lung cancer (NSCLC) patients.

PROCEDURES

Twenty-three NSCLC patients were scanned on a HR+ scanner. IDIFs were defined over the aorta and left ventricle. Activity concentration and metabolite fraction were measured in venous blood samples. Venous blood samples at 30, 40 and 60 min after injection were used to calibrate the IDIF time-activity curves. Adaptive thresholds were used for VOI definition. Full kinetic analysis and simplified measures were performed.

RESULTS

Non-linear regression analysis showed better fits for the irreversible model compared to the reversible model in the majority. Calibrated and metabolite corrected plus plasma-to-blood ratio corrected input function resulted in high correlations between SUV and Patlak K i (Pearson correlation coefficients 0.86-0.96, p value < 0.001). No significant differences in correlation between SUV and Patlak K i were observed with variation of IDIF structure or VOI size.

CONCLUSIONS

Plasma-to-blood ratio correction, metabolite correction and calibration improved the correlation between SUV and Patlak K i significantly, indicating the need for these corrections when K i is used to validate semi-quantitative measures, such as SUV.

Proliferation imaging with ¹⁸F-fluorothymidine PET/computed tomography: physiologic uptake, variants, and pitfalls

For noninvasive in vivo imaging of proliferation, 18F-FLT PET/CT remains a promising tool, owing to its correlation with proliferation indexes in many tumor entities. Future clinical applications will focus on monitoring response to cancer therapy, whereas tumor detection will be limited to organs with high physiologic 18F-FDG uptake. Use and interpretation of 18F-FLT requires knowledge of the physiologic tracer distribution and how it will be affected by anticancer treatment. Further studies are needed to determine the optimal timing of 18F-FLT PET/CT imaging in the course of cancer therapies or at the conclusion of therapy.

Novel methods and tracers for breast cancer imaging

Although positron emission tomography (PET) using [(18)F]fluorodeoxyglucose (FDG) has an established role in breast cancer staging and monitoring response to therapy, more specifically novel targeted tracers are under investigation and hold promise toward identification of critical molecular targets of therapy. We review herein novel tracers in breast cancer including steroidal endocrine tracers, 16α-[(18)F]fluoro-17β-estradiol (FES) to measure tumor estrogen receptor density and function and 21-(18)F-fluoro-16α,17α-[(R)-(1'-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione (FFNP) to assay tumor progesterone receptor (PgR) expression, and to asses nuclear proliferation using 3'-deoxy-3'-fluorothymidine (FLT), membrane lipids using (11)C- or (18)F-labeled choline and amino acid transport using (11)C-methionine. These investigational tracers are moving closer to clinical use, and are likely to affect clinical care by aiding in characterization of breast cancer biology, which can have an important effect in the selection of targeted therapy and monitoring responsiveness to such therapy.

Clinical translation of molecular imaging agents used in PET studies of cancer

Over recent years, there has been a rapid expansion in our knowledge of the factors that regulate tumor growth; this has resulted in the identification of new therapeutic targets and improvements in the long-term survival of cancer patients. New noninvasive biomarkers of drug targets and pathway modulation in vivo are needed to guide therapy selection and detect drug resistance early so that alternative, more effective treatments can be offered. The translation of new therapeutics into the clinic is disappointingly slow, expensive, and subject to high rates of attrition often occurring at late stages (phase 3) of development. In an attempt to mitigate these delays and failures, there has been resurgence in the development of new molecular imaging probes for studies with positron emission tomography (PET) to characterize tumor biology. In the assessment of therapeutic effects, PET allows imaging of entire tumor burden in a noninvasive repeatable manner. This chapter focuses on the clinical translation of PET imaging agents from bench to bedside. New probes are being used to study a diverse range of processes such as angiogenesis, apoptosis, fatty acid metabolism, and growth factor receptor expression. In the future, validation of these novel imaging probes could allow more innovative therapies to be adapted earlier in the clinic leading to improved patient outcomes.

3'-Deoxy-3'-18F-fluorothymidine PET for the early prediction of response to leucovorin, 5-fluorouracil, and oxaliplatin therapy in patients with metastatic colorectal cancer

The aim of this study was to evaluate 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET for early prediction of the standard anatomic response and survival outcomes in patients with metastatic colorectal cancer (mCRC) receiving leucovorin, 5-fluorouracil (5-FU), and oxaliplatin (FOLFOX).

METHODS

The main eligibility criteria included histologically confirmed mCRC, ≥ 1 extrahepatic measurable lesions, and no prior chemotherapy in a metastatic setting. Chemotherapy consisted of leucovorin on day 1, followed by the continuous infusion of 5-FU on days 1 and 2, and oxaliplatin on day 3. In the second and subsequent cycles of chemotherapy, oxaliplatin was administered simultaneously with leucovorin on day 1. (18)F-FLT PET scans were obtained 3 times during the first cycle of chemotherapy: before chemotherapy, 24 h after infusion of 5-FU (day 2), and 48 h after completion of chemotherapy (day 5). The maximum standardized uptake value (SUVMAX) of (18)F-FLT was measured. Treatment responses were assessed by CT after 3 cycles of FOLFOX.

RESULTS

Eighteen patients were included in the study. The response rate after 3 cycles of FOLFOX was 27.8% (5/18). The SUVMAX was increased in responders (P = 0.043) and nonresponders (P < 0.001) on day 2 and was decreased, compared with baseline values, on day 5 in responders only (P = 0.043). Receiver-operating-characteristic curve analysis indicated that the use of a threshold of an SUVMAX increase on day 2 of ≤ 45.8% resulted in a sensitivity of 100%, specificity of 69.2%, and relative risk of 2.250 (P = 0.029) for the diagnosis of responders. Use of a threshold of an SUVMAX decrease on day 5 of ≥ 10.6% resulted in a sensitivity of 100%, specificity of 76.9%, and relative risk of 2.667 (P = 0.007). Patients with low (18)F-FLT flare tended to have longer survivals than patients with high flare (2-y overall survival rate, 77.8% vs. 44.4%; P = 0.051).

CONCLUSION

The (18)F-FLT flare observed during 5-FU infusion was associated with poor treatment response in patients with mCRC. The degree of (18)F-FLT flare might be used to predict the outcome of patients who receive infusional 5-FU-based chemotherapy.

Quantification of intra-tumour cell proliferation heterogeneity using imaging descriptors of 18F fluorothymidine-positron emission tomography

Intra-tumour heterogeneity is a characteristic shared by all cancers. We explored the use of texture variables derived from images of [(18)F]fluorothymidine-positron emission tomography (FLT-PET), thus notionally assessing the heterogeneity of proliferation in individual tumours. Our aims were to study the range of textural feature values across tissue types, verify the repeatability of these image descriptors and further, to explore associations with clinical response to chemotherapy in breast cancer patients. The repeatability of 28 textural descriptors was assessed in patients who had two FLT-PET scans prior to therapy using relative differences and the intra-class correlation coefficient (ICC). We tested associations between features at baseline and clinical response measured in 11 patients after three cycles of chemotherapy, and explored changes in FLT-PET at one week after the start of therapy. A subset of eight features was characterized by low variations at baseline (<±30%) and high repeatability (0.7 ≤ ICC ≤ 1). The intensity distribution profile suggested fewer highly proliferating cells in lesions of non-responders compared to responders at baseline. A true increase in CV and homogeneity was measured in four out of six responders one week after the start of therapy. A number of textural features derived from FLT-PET are altered following chemotherapy in breast cancer, and should be evaluated in larger clinical trials for clinical relevance.

Importance of quantification for the analysis of PET data in oncology: review of current methods and trends for the future

In oncology, positron emission tomography (PET) is an important tool for tumour diagnosis and staging, assessment of response to treatment and evaluation of the pharmacokinetic properties and efficacy of new drugs. Despite its quantitative potential, however, in daily clinical practice PET is used almost exclusively with 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) and, in addition, [(18)F]FDG data are normally assessed visually or using simple indices as the standardised uptake value (SUV). After explaining why more sophisticated quantification methods can be useful in oncology, the paper reviews the approaches that are commonly used and those available but not routinely employed. Particular emphasis is addressed to the SUV, for its importance in clinical practice. Issues specific to PET quantification in oncology and related examples are then discussed. Finally, some ideas for the development of new quantitative methods for analysing PET data in oncology and for the application of approaches already existing but not commonly employed are presented.

Gaussian process models of dynamic PET for functional volume definition in radiation oncology

In routine oncologic positron emission tomography (PET), dynamic information is discarded by time-averaging the signal to produce static images of the "standardised uptake value" (SUV). Defining functional volumes of interest (VOIs) in terms of SUV is flawed, as values are affected by confounding factors and the chosen time window, and SUV images are not sensitive to functional heterogeneity of pathological tissues. Also, SUV iso-contours are highly affected by the choice of threshold and no threshold, or other SUV-based segmentation method, is universally accepted for a given VOI type. Gaussian Process (GP) time series models describe macro-scale dynamic behavior arising from countless interacting micro-scale processes, as is the case for PET signals from heterogeneous tissue. We use GPs to model time-activity curves (TACs) from dynamic PET and to define functional volumes for PET oncology. Probabilistic methods of tissue discrimination are presented along with novel contouring methods for functional VOI segmentation. We demonstrate the value of GP models for voxel classification and VOI contouring of diseased and metastatic tissues with functional heterogeneity in prostate PET. Classification experiments reveal superior sensitivity and specificity over SUV calculation and a TAC-based method proposed in recent literature. Contouring experiments reveal differences in shape between gold-standard and GP VOIs and correlation with kinetic models shows that the novel VOIs contain extra clinically relevant information compared to SUVs alone. We conclude that the proposed models offer a principled data analysis technique that improves on SUVs for oncologic VOI definition. Continuing research will generalize GP models for different oncology tracers and imaging protocols with the ultimate goal of clinical use including treatment planning.

Optimization of supervised cluster analysis for extracting reference tissue input curves in (R)-[(11)C]PK11195 brain PET studies

Performance of two supervised cluster analysis (SVCA) algorithms for extracting reference tissue curves was evaluated to improve quantification of dynamic (R)-[(11)C]PK11195 brain positron emission tomography (PET) studies. Reference tissues were extracted from images using both a manually defined cerebellum and SVCA algorithms based on either four (SVCA4) or six (SVCA6) kinetic classes. Data from controls, mild cognitive impairment patients, and patients with Alzheimer's disease were analyzed using various kinetic models including plasma input, the simplified reference tissue model (RPM) and RPM with vascular correction (RPMV(b)). In all subject groups, SVCA-based reference tissue curves showed lower blood volume fractions (V(b)) and volume of distributions than those based on cerebellum time-activity curve. Probably resulting from the presence of specific signal from the vessel walls that contains in normal condition a significant concentration of the 18 kDa translocation protein. Best contrast between subject groups was seen using SVCA4-based reference tissues as the result of a lower number of kinetic classes and the prior removal of extracerebral tissues. In addition, incorporation of V(b) in RPM improved both parametric images and binding potential contrast between groups. Incorporation of V(b) within RPM, together with SVCA4, appears to be the method of choice for analyzing cerebral (R)-[(11)C]PK11195 neurodegeneration studies.

Imaging of cellular proliferation in liver metastasis by [18F]fluorothymidine positron emission tomography: effect of therapy

Although [(18)F]fluorothymidine positron emission tomography (FLT-PET) permits estimation of tumor thymidine kinase-1 expression, and thus, cell proliferation, high physiological uptake of tracer in liver tissue can limit its utility. We evaluated FLT-PET combined with a temporal-intensity information-based voxel-clustering approach termed kinetic spatial filtering (FLT-PET(KSF)) for detecting drug response in liver metastases. FLT-PET and computed tomography data were collected from patients with confirmed breast or colorectal liver metastases before, and two weeks after the first cycle of chemotherapy. Changes in tumor FLT-PET and FLT-PET(KSF) variables were determined. Visual distinction between tumor and normal liver was seen in FLT-PET(KSF) images. Of the 33 metastases from 20 patients studied, 26 were visible after kinetic filtering. The net irreversible retention of the tracer (Ki; from unfiltered data) in the tumor, correlated strongly with tracer uptake when the imaging variable was an unfiltered average or maximal standardized uptake value, 60 min post-injection (SUV(60,av): r = 0.9, SUV(60,max): r = 0.7; p < 0.0001 for both) and occurrence of high intensity voxels derived from FLT-PET(KSF) (r = 0.7, p < 0.0001). Overall, a significant reduction in the imaging variables was seen in responders compared to non-responders; however, the two week time point selected for imaging was too early to allow prediction of long term clinical benefit from chemotherapy. FLT-PET and FLT-PET(KSF) detected changes in proliferation in liver metastases.

[(18)F]FLT: an imaging biomarker of tumour proliferation for assessment of tumour response to treatment

The paradigm of drug development is shifting towards early use of imaging biomarkers as surrogate end-points in clinical trials. Quantitative Imaging in Cancer: Connecting Cellular Processes (QuIC-ConCePT) is an initiative to qualify complementary imaging biomarkers (IB) of proliferation, cell death and tumour heterogeneity as possible tools in early phase clinical trials to help pharmaceutical developers in 'go, no-go' decisions early in the process of drug development. One of the IBs is [(18)F]3'-deoxy-3'-fluorothymidine with Positron Emission Tomography (FLT-PET). We review results of recent clinical trials using FLT-PET for monitoring tumour response to drug treatment and discuss the potential and the possible pitfalls of using this IB as a surrogate end-point in early phase clinical trials for assessing tumour response to drug treatment. From first human trial results it seems that the degree of FLT accumulation in tumours is governed not only by the tumour proliferation rate but also by other factors. Nevertheless FLT-PET could potentially be used as a negative predictor of tumour response to chemotherapy, and hence evaluation of this IB is granted in multi-centre clinical trials.

Development of radiotracers for oncology--the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

Quantification of receptor-ligand binding with [¹⁸F]fluciclatide in metastatic breast cancer patients

PURPOSE

The purpose of the study was to estimate the receptor-ligand binding of an arginine-glycine-aspartic acid (RGD) peptide in somatic tumours. To this aim, we employed dynamic positron emission tomography (PET) data obtained from breast cancer patients with metastases, studied with the α(v)β(3/5) integrin receptor radioligand [(18)F]fluciclatide.

METHODS

First, compartmental modelling and spectral analysis with arterial input function were performed at the region of interest (ROI) level in healthy lung and liver, and in lung and liver metastases; compartmental modelling was also carried out at the pixel level. The selection of the most appropriate indexes for tumour/healthy tissue differentiation and for estimation of specific binding was then assessed.

RESULTS

The two-tissue reversible model emerged as the best according to the Akaike Information Criterion. Spectral analysis confirmed the reversibility of tracer kinetics. Values of kinetic parameters, estimated as mean from parametric maps, correlated well with those computed from ROI analysis. The volume of distribution V(T) was on average higher in lung metastases than in the healthy lung, but lower in liver metastases than in the healthy liver. In agreement with the expected higher α(v)β(3/5) expression in pathology, k(3) and k(3)/k(4) were both remarkably higher in metastases, which makes them more suitable than V(T) for tumour/healthy tissue differentiation. The ratio k(3)/k(4), in particular, appeared a reasonable measure of specific binding.

CONCLUSION

Besides establishing the best quantitative approaches for the analysis of [(18)F]fluciclatide data, this study indicated that the k(3)/k(4) ratio is a reasonable measure of specific binding, suggesting that this index can be used to estimate α(v)β(3/5) receptor expression in oncology, although further studies are necessary to validate this hypothesis.