Measuring tumor blood flow with H(2)(15)O: practical considerations

Unconventional non-amino acidic PET radiotracers for molecular imaging in gliomas

PURPOSE

The objective of this review was to explore the potential clinical application of unconventional non-amino acid PET radiopharmaceuticals in patients with gliomas.

METHODS

A comprehensive search strategy was used based on SCOPUS and PubMed databases using the following string: ("perfusion" OR "angiogenesis" OR "hypoxia" OR "neuroinflammation" OR proliferation OR invasiveness) AND ("brain tumor" OR "glioma") AND ("Positron Emission Tomography" OR PET). From all studies published in English, the most relevant articles were selected for this review, evaluating the mostly used PET radiopharmaceuticals in research centers, beyond amino acid radiotracers and 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG), for the assessment of different biological features, such as perfusion, angiogenesis, hypoxia, neuroinflammation, cell proliferation, tumor invasiveness, and other biological characteristics in patients with glioma.

RESULTS

At present, the use of non-amino acid PET radiopharmaceuticals specifically designed to assess perfusion, angiogenesis, hypoxia, neuroinflammation, cell proliferation, tumor invasiveness, and other biological features in glioma is still limited.

CONCLUSION

The use of investigational PET radiopharmaceuticals should be further explored considering their promising potential and studies specifically designed to validate these preliminary findings are needed. In the clinical scenario, advancements in the development of new PET radiopharmaceuticals and new imaging technologies (e.g., PET/MR and the application of the artificial intelligence to medical images) might contribute to improve the clinical translation of these novel radiotracers in the assessment of gliomas.

Management of the Cavity After Removal of Giant Cell Tumor of the Bone

Purpose: To find out the most appropriate management scheme through the analysis and comparison of different inactivation methods and filling materials. Method: A systematic literature search was performed using the terms, anhydrous ethanol, phenol, hypertonic saline, cryotherapy, thermal therapy, bone reconstruction, GCTB, and etc., Selected articles were studied and summarized. The mechanism, clinical effects, and influence on bone repair of various methods are presented. Recent developments and perspectives are also demonstrated. Recent Findings: Compared to curettage alone, management of the residual cavity can effectively reduce the recurrence of giant cell tumours of bone. It is a complex and multidisciplinary process that includes three steps: local control, cavity filling, and osteogenic induction. In terms of local control, High-speed burring can enlarge the area of curettage but may cause the spread and planting of tumour tissues. Among the inactivation methods, Anhydrous ethanol, and hyperthermia therapy are relatively safe and efficient. The combination of the two may achieve a better inactivation effect. When inactivating the cavity, we need to adjust the approach according to the invasion of the tumour. Filling materials and bone repair should also be considered in management.

Whole-body PET/CT evaluation of tumor perfusion using generator-based 62Cu-ethylglyoxal bis(thiosemicarbazonato)copper(II): validation by direct comparison to 15O-water in metastatic renal cell carcinoma

This study was undertaken to demonstrate the feasibility of whole-body (62)Cu-ethylglyoxal bis(thiosemicarbazonato)copper(II) ((62)Cu-ETS) PET/CT tumor perfusion imaging in patients with metastatic renal carcinoma and to validate (62)Cu-ETS as a quantitative marker of tumor perfusion by direct comparison with (15)O-water perfusion imaging.

METHODS

PET/CT imaging of 10 subjects with stage IV renal cell cancer was performed after intravenous administration of (15)O-water (10-min dynamic list-mode study) with the heart and at least 1 tumor in the PET field of view, followed 10 min later by intravenous (62)Cu-ETS (6-min list-mode study). Whole-body (62)Cu imaging was then performed from 6 to 20 min at 2-3 min/bed position. Blood flow (K1) was quantified with both agents for normal and malignant tissues in the 21.7-cm dynamic field of view. The required arterial input functions were derived from the left atrium and, in the case of (62)Cu-ETS, corrected for partial decomposition of the agent by blood with data from an in vitro analysis using a sample of each patient's blood. This imaging protocol was repeated at an interval of 3-4 wk after initiation of a standard clinical treatment course of the antiangiogenic agent sunitinib.

RESULTS

All subjects received the scheduled (62)Cu-ETS doses for the dynamic and subsequent whole-body PET/CT scans, but technical issues resulted in no baseline (15)O-water data for 2 subjects. Direct comparisons of the perfusion estimates for normal tissues and tumor metastases were made in 18 paired baseline and treatment studies (10 subjects; 8 baseline studies, 10 repeated studies during treatment). There was an excellent correlation between the blood flow estimates made with (62)Cu-ETS and (15)O-water for normal tissues (muscle, thyroid, myocardium) and malignant lesions (pulmonary nodules, bone lesions); the regression line was y = 0.85x + 0.15, R(2) = 0.83, for the 88 regions analyzed.

CONCLUSION

(62)Cu-ETS provided high-quality whole-body PET/CT images, and (62)Cu-ETS measures of blood flow were highly and linearly correlated with (15)O-water-derived K1 values (mL(-1) ⋅ min(-1) ⋅ g). This tracer is suitable for use as a PET tracer of tumor perfusion in patients with metastatic renal cell carcinoma.

Pharmacodynamic analysis of magnetic resonance imaging-monitored focused ultrasound-induced blood-brain barrier opening for drug delivery to brain tumors

Microbubble-enhanced focused ultrasound (FUS) can enhance the delivery of therapeutic agents into the brain for brain tumor treatment. The purpose of this study was to investigate the influence of brain tumor conditions on the distribution and dynamics of small molecule leakage into targeted regions of the brain after FUS-BBB opening. A total of 34 animals were used, and the process was monitored by 7T-MRI. Evans blue (EB) dye as well as Gd-DTPA served as small molecule substitutes for evaluation of drug behavior. EB was quantified spectrophotometrically. Spin-spin (R₁) relaxometry and area under curve (AUC) were measured by MRI to quantify Gd-DTPA. We found that FUS-BBB opening provided a more significant increase in permeability with small tumors. In contrast, accumulation was much higher in large tumors, independent of FUS. The AUC values of Gd-DTPA were well correlated with EB delivery, suggesting that Gd-DTPA was a good indicator of total small-molecule accumulation in the target region. The peripheral regions of large tumors exhibited similar dynamics of small-molecule leakage after FUS-BBB opening as small tumors, suggesting that FUS-BBB opening may have the most significant permeability-enhancing effect on tumor peripheral. This study provides useful information toward designing an optimized FUS-BBB opening strategy to deliver small-molecule therapeutic agents into brain tumors.

Noninvasive diffuse optical monitoring of head and neck tumor blood flow and oxygenation during radiation delivery

This study explored using a novel diffuse correlation spectroscopy (DCS) flow-oximeter to noninvasively monitor blood flow and oxygenation changes in head and neck tumors during radiation delivery. A fiber-optic probe connected to the DCS flow-oximeter was placed on the surface of the radiologically/clinically involved cervical lymph node. The DCS flow-oximeter in the treatment room was remotely operated by a computer in the control room. From the early measurements, abnormal signals were observed when the optical device was placed in close proximity to the radiation beams. Through phantom tests, the artifacts were shown to be caused by scattered x rays and consequentially avoided by moving the optical device away from the x-ray beams. Eleven patients with head and neck tumors were continually measured once a week over a treatment period of seven weeks, although there were some missing data due to the patient related events. Large inter-patient variations in tumor hemodynamic responses were observed during radiation delivery. A significant increase in tumor blood flow was observed at the first week of treatment, which may be a physiologic response to hypoxia created by radiation oxygen consumption. Only small and insignificant changes were found in tumor blood oxygenation, suggesting that oxygen utilizations in tumors during the short period of fractional radiation deliveries were either minimal or balanced by other effects such as blood flow regulation. Further investigations in a large patient population are needed to correlate the individual hemodynamic responses with the clinical outcomes for determining the prognostic value of optical measurements.

Molecular imaging of gliomas with PET: opportunities and limitations

Neuroimaging enables the noninvasive evaluation of glioma and is considered to be one of the key factors for individualized therapy and patient management, since accurate diagnosis and demarcation of viable tumor tissue is required for treatment planning as well as assessment of treatment response. Conventional imaging techniques like MRI and CT reveal morphological information but are of limited value for the assessment of more specific and reproducible information about biology and activity of the tumor. Molecular imaging with PET is increasingly implemented in neuro-oncology, since it provides additional metabolic information of the tumor, both for patient management as well as for evaluation of newly developed therapeutics. Different molecular processes have been proposed to be useful, like glucose consumption, expression of amino acid transporters, proliferation rate, membrane biosynthesis, and hypoxia. Thus, PET might help neuro-oncologists gain further insights into tumor biology by "true molecular imaging" as well as understand treatment-related phenomena. This review describes the method of PET acquisition as well as the tracers used to image biological processes in gliomas. Furthermore, it considers the clinical impact of PET on the use of currently available radiotracers, which were shown to be potentially valuable for discrimination between neoplastic and nonneoplastic tissue, as well as on tumor grading, determinination of treatment response, and providing an outlook toward further developments.

Molecular imaging with dynamic contrast-enhanced computed tomography

Dynamic contrast-enhanced computed tomography (DCE-CT) is a quantitative technique that employs rapid sequences of CT images after bolus administration of intravenous contrast material to measure a range of physiological processes related to the microvasculature of tissues. By combining knowledge of the molecular processes underlying changes in vascular physiology with an understanding of the relationship between vascular physiology and CT contrast enhancement, DCE-CT can be redefined as a molecular imaging technique. Some DCE-CT derived parameters reflect tissue hypoxia and can, therefore, provide information about the cellular microenvironment. DCE-CT can also depict physiological processes, such as vasodilatation, that represent the physiological consequences of molecular responses to tissue hypoxia. To date the main applications have been in stroke and oncology. Unlike some other molecular imaging approaches, DCE-CT benefits from wide availability and ease of application along with the use of contrast materials and software packages that have achieved full regulatory approval. Hence, DCE-CT represents a molecular imaging technique that is applicable in clinical practice today.

Quantitative organ time activity curve extraction from rodent PET images without anatomical prior

PURPOSE

Numerous new drug candidates fail because of inadequate pharmacokinetics. Positron emission tomography (PET) enables the noninvasive characterization of the drug in humans and animals. The aim of the present work was the comparison of methods for the extraction of organ time activity curves from rodent PET images without requiring resort to anatomical information.

METHODS

The rodent organs were segmented using the local means analysis method and the accuracy of the time activity curve (TAC) estimated using four methods was compared: The mean TAC (Mean), the TAC computed in a selection of organ voxels (ROIopt), and the TAC corrected for partial volume effect using the geometric transfer matrix (GTM) method. The accuracy of the TAC estimated using the three methods was compared on phantom simulations and on experimental data sets on mice injected with fluorothymidine.

RESULTS

The segmentation quality measured on phantom simulation was 80% of overlap between segmented and gold standard organs. On the phantom simulations, the error on the TAC estimation on phantom simulations was lower for ROIopt (8%) than using the GTM (18%) and the Mean (27%) methods. Similar results were achieved on the experimental data sets: ROIopt (5.8%), GTM (9.7%), and Mean (12%).

CONCLUSIONS

The new ROI optimization method was fast and precise for all homogeneous organs, while mean organ TAC computation led as expected to important errors. GTM improved the quantification accuracy but showed instabilities due to segmentation errors and to small organ sizes. Partial volume effect correction or limitation is thus possible for the extraction of precise organ TACs without requiring either manual delineation or an anatomical modality.

Imaging angiogenesis of genitourinary tumors

Angiogenesis is a key process in the growth and metastasis of cancer, and genitourinary tumors are no exception. The evolution of angiogenesis as an important target for novel anticancer therapeutics has brought with it new challenges for in vivo imaging. Most imaging techniques quantify physiological parameters, such as blood volume and capillary endothelial permeability. Although CT, PET and ultrasonography have shown promise, MRI is the most common method used to evaluate angiogenesis in clinical trials of genitourinary tumors. Pilot studies of MRI, CT and ultrasonography in patients with renal cancer have produced promising results; reductions in vascular permeability and blood flow have been correlated with progression-free survival. The vascular characteristics of prostate cancer have been evaluated by MRI, and this has been suggested as a means of assessing tumor response to hormone deprivation therapy. Current evidence highlights the potential of angiogenesis imaging in the diagnosis, staging and possibly response monitoring of bladder cancer. In the future, assessment of the angiogenic process at the structural, functional and molecular levels, before, during and after antiangiogenic therapy will undoubtedly be integrated into wider clinical practice.

Imaging angiogenesis

There is a need for direct imaging of effects on tumor vasculature in assessment of response to antiangiogenic drugs and vascular disrupting agents. Imaging tumor vasculature depends on differences in permeability of vasculature of tumor and normal tissue, which cause changes in penetration of contrast agents. Angiogenesis imaging may be defined in terms of measurement of tumor perfusion and direct imaging of the molecules involved in angiogenesis. In addition, assessment of tumor hypoxia will give an indication of tumor vasculature. The range of imaging techniques available for these processes includes positron emission tomography (PET), dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), perfusion computed tomography (CT), and ultrasound (US).

PET-CT in oncology: making the most of CT

Combined positron emission tomography–computed tomography (PET-CT) has made a significant impact on cancer imaging. The use of CT to map tissue attenuation for correction of PET images and the ability to co-register the functional information provided by PET with the anatomical data afforded by CT, has resulted in demonstrable improvements in diagnostic accuracy. However, attenuation correction and anatomical localisation may not represent the full benefits of integrating CT with PET. The use of CT acquisition techniques for patient positioning and the use of contrast media can improve diagnostic performance, and incorporation of CT image processing techniques such as perfusion CT, 3D imaging and computer-assisted diagnosis offers new applications. The interpretation of PET-CT images can be improved by fully integrating the morphological appearances on CT into image analysis. Better utilisation of the CT component of PET-CT could further enhance the benefits of PET-CT in oncology but will have implications for manufacturers and purchasers of PET-CT equipment and analysis software. Furthermore, specialists working in PET-CT will need CT competencies beyond knowledge of cross-sectional anatomy. CT continues to exhibit rapid evolution and these advances will inevitably impact on the practice of PET-CT.

Warburg revisited: imaging tumour blood flow and metabolism

In the 1930s, Otto Warburg reported that anaerobic metabolism of glucose is a fundamental property of all tumours, even in the presence of an adequate oxygen supply. He also demonstrated a relationship between the degree of anaerobic metabolism and tumour growth rate. Today, this phenomenon forms the basis of tumour imaging with fluorodeoxyglucose positron emission tomography (FDG-PET). More recently, Folkman has demonstrated that malignant growth and survival are also dependent on tumour vascularity which is increasingly evaluated in vivo using techniques such as contrast enhanced computed tomography or magnetic resonance imaging (MRI). Although it is reasonable to hypothesise that the metabolic requirements of tumours are mirrored by alterations in tumour haemodynamics, the relationship between tumour blood flow and metabolism is in fact complex. A well-developed tumour vascular supply is required to ensure a sufficient delivery of glucose and oxygen to support the metabolism essential for tumour growth. However, an inadequate vascularisation of tumour will result in hypoxia, a factor that is known to stimulate anaerobic metabolism of glucose. Thus, the balance between tumour blood flow and metabolism will be an important indicator of the biological status of a tumour and hence the tumour's likely progression and response to treatment. This article reviews the molecular biology of tumour vascularisation and metabolism, relating these processes to currently available imaging techniques while summarising the imaging studies that have compared tumour blood flow and metabolism. The potential for vascular metabolic imaging to assess tumour aggression and sub-classify treatment response is highlighted.

Analysis of blood flow and glucose metabolism in mammary carcinomas and normal breast: a H2(15)O PET and 18F-FDG PET study

OBJECTIVE

To determine parameters of perfusion, distribution coefficient, and glucose metabolism as part of the tumour-specific micromilieu of breast cancer and compare them with corresponding values in normal breast tissue.

METHODS

H2(15)O PET and 18F-FDG PET were performed on 10 patients with advanced invasive ductal carcinomas of the breast. Perfusion, distribution coefficient, and glucose metabolism and standardized uptake were quantified and analysed.

RESULTS

Mean values based on the regions of interest were 59.2+/-43.9 ml x min(-1) x 100 g(-1) (perfusion), 0.58+/-0.26 ml x g(-1) (distribution coefficient), 7.76+/-6.10 (standardized uptake), and 5.4+/-2.5 mg x min(-1) x 100 g(-1) (glucose metabolism). The corresponding values for normal breast tissue were 22.1+/-13.2 ml x min x 100 g(-1) (perfusion), 0.16+/-0.05 ml x g(-1) (distribution coefficient), 0.33+/-0.07 (standardized uptake), and 0.18+/-0.08 mg x min x 100 g(-1) (glucose metabolism). For each tumour-normal tissue parameter pair, the mean values were significantly higher in tumours than normal breast tissue. Region-of-interest and pixel-wise correlation analysis revealed a positive association between glucose metabolism and distribution coefficient and glucose metabolism and perfusion for 7/10 tumours investigated.

CONCLUSIONS

H2(15)O PET and 18F-FDG PET were able to differentiate breast cancer and normal breast tissue. The pixel-wise analysis revealed information about the heterogeneity of tumour fine structure in perfusion, distribution coefficient, and glucose metabolism, which may provide important guidelines for improving individual treatment.

Monitoring thermal-induced changes in tumor blood flow and microvessels with laser speckle contrast imaging

Laser speckle contrast imaging (LSCI) was used to monitor thermal-induced changes in the blood flow and the diameter of mesenteric microvessels of normal and tumor bearing mice under 60 min treatment at different constant temperatures between 41 degrees C and 45 degrees C. The results show that the blood flow and the diameter increase at the beginning and then reach a plateau and finally start to decrease. The lower the temperature, the longer the plateau stays. A t-test indicates that there is no significant difference in plateau values of relative blood flow and relative diameter for the same group. For normal mice, the relative increases in the blood flow and the diameter are 1.26 and 1.41, respectively, while for tumor-bearing mice they are 1.08 and 1.13, respectively. At higher treatment temperature or under longer heat treatment, there are decreases in the blood flow and the diameter, such changes in tumor-bearing mice are more obvious than those in normal mice, which means tumor microvessels are more sensitive to heat than normal. Moreover, thermal induced shrink of microvessel usually occurs sooner than the decrease in blood flow, and the relative change in diameter is larger than that in blood flow. Therefore we may conclude that deformation of vessel is a main factor for changing the blood perfusion of a microvessel.

Noninvasive diffuse optical measurement of blood flow and blood oxygenation for monitoring radiation therapy in patients with head and neck tumors: a pilot study

This pilot study explores the potential of noninvasive diffuse correlation spectroscopy (DCS) and diffuse reflectance spectroscopy (DRS) for monitoring early relative blood flow (rBF), tissue oxygen saturation (StO(2)), and total hemoglobin concentration (THC) responses to chemo-radiation therapy in patients with head and neck tumors. rBF, StO(2), and THC in superficial neck tumor nodes of eight patients are measured before and during the chemo-radiation therapy period. The weekly rBF, StO(2), and THC kinetics exhibit different patterns for different individuals, including significant early blood flow changes during the first two weeks. Averaged blood flow increases (52.7+/-9.7)% in the first week and decreases (42.4+/-7.0)% in the second week. Averaged StO(2) increases from (62.9+/-3.4)% baseline value to (70.4+/-3.2)% at the end of the second week, and averaged THC exhibits a continuous decrease from pretreatment value of (80.7+/-7.0) [microM] to (73.3+/-8.3) [microM] at the end of the second week and to (63.0+/-8.1) [microM] at the end of the fourth week of therapy. These preliminary results suggest daily diffuse-optics-based therapy monitoring is feasible during the first two weeks and may have clinical promise.

New technologies and directed agents for applications of cancer imaging

Molecular imaging represents tissue-specific imaging and quantification of physiologic (functional) and molecular events in tumors utilizing new noninvasive imaging modalities, radioligands, and contrast agents. It combines anatomic, physiologic, and metabolic information in a single imaging session. Molecular imaging relies on the ability to target genes and proteins that are linked directly or indirectly to human disease. New imaging biomarkers are being developed. In addition, functional and molecular imaging can potentially replace anatomic longitudinal studies by assessing treatment response earlier. Vascular targeting agents can be evaluated by imaging of tumor angiogenesis using magnetic resonance imaging (MRI), computed tomography and ultrasound, and positron emission tomography (PET). Targeted contrast agents can accomplish site-directed imaging or therapy by a variety of active and passive mechanisms. Furthermore, there is the possibility of combining different modalities such as ultrasonic imaging and MRI or MRI and PET to increase the flexibility unachievable with either modality alone. However, there is a need to standardize these techniques so that longitudinal evaluation of tumor response to treatment is feasible.

Evaluation of Basis Function and Linear Least Squares Methods for Generating Parametric Blood Flow Images Using 15O-Water and Positron Emission Tomography

PURPOSE

Parametric analysis of (15)O-water positron emission tomography (PET) studies allows determination of blood flow (BF), perfusable tissue fraction (PTF), and volume of distribution (V (d)) with high spatial resolution. In this paper the performance of basis function and linear least squares methods for generating parametric flow data were evaluated.

PROCEDURES

Monte Carlo simulations were performed using typical perfusion values for brain, tumor, and heart. Clinical evaluation was performed using seven cerebral and 10 myocardial (15)O-water PET studies. Basis function (BFM), linear least squares (LLS), and generalized linear least squares (GLLS) methods were used to calculate BF, PTF, or V(d).

RESULTS

Monte Carlo simulations and human studies showed that, for low BF values (<1 ml/min(-1)ml(-1), BF, PTF, and V(d) were calculated with accuracies better than 5% for all methods tested. For high BF (>2 ml/min(-1)ml(-1)), use of BFM provided more accurate V(d) compared with (G)LLS.

CONCLUSIONS

In general, BFM provided the most accurate estimates of BF, PTF, and V(d).

Molecular imaging of antiangiogenic agents

Many novel antiangiogenic agents are currently in various phases of clinical testing. These agents tend to be cytostatic, and therefore few responses are observed with conventional imaging by computerized tomography. Furthermore, toxicity with these agents is seen when the maximum-tolerated dose is combined with chemotherapy. Hence, there is a need to develop imaging strategies that can determine the minimum and optimum biologically active doses. There is increasing awareness of the need to obtain evidence of drug activity through the use of surrogate markers of the biologic mechanism of action during early clinical trials, in addition to determining the pharmacokinetics, toxicity profile, and maximum-tolerated dose. One of the major impediments to the rapid development of antiangiogenic agents in the past has been the lack of validated assays capable of measuring an antiangiogenic effect directly in patients. Recently, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) has emerged as a useful technique for noninvasive imaging of tumor vasculature in preclinical and clinical models. The problem of tumor heterogeneity remains to be addressed. The major challenge is the standardization of the technique worldwide for the purpose of early clinical studies that are likely to be multicenter. Convincing data on correlations between changes observed through molecular imaging and changes in tumor angiogenesis, and hence tumor biology, are still lacking. Whether this would translate into a survival advantage remains to be seen. The ultimate test of the surrogate biological end points determined by molecular imaging will occur in randomized phase III trials. Results of the first randomized trial that showed a survival advantage in favor of antiangiogenic agents were released at the American Society of Clinical Oncology meeting in 2003. There it was reported that the combination of 5-fluorouracil, leucovorin, and irinotecan (Camptosar; Pfizer Pharmaceuticals; New York, NY) with anti-vascular endothelial growth factor antibody (bevacizumab-Avastin; Genentech, Inc.; South San Francisco, CA) was superior to the chemotherapy regimen alone when used to treat patients with metastatic colorectal cancer. However, until further phase III clinical trials confirm these results, surrogate end points of clinical efficacy of the newer agents are urgently needed so that development of ineffective drugs can be halted early. This review briefly discusses the role of molecular imaging in general, and DCE-MRI in particular, in relation to treatment with antiangiogenic agents and highlights some of the difficulties encountered in this area.

The role of imaging in the clinical development of antiangiogenic agents

Early clinical development of novel antiangiogenesis agents is hampered by the fact that classic response end points are unlikely to be relevant and there is a lack of validated surrogate markers of efficacy. Toxicity-based decisions for dose setting and tumor size measurements by standard imaging probably are not be applicable. Because these agents modify a multitude of biologic processes that may cause early measurable effects, there is great interest in developing imaging tests that are sensitive to changes in tissue function. This article discusses the development of such "functional" clinical imaging and attempts to address the questions that are being asked of imaging departments by oncologists and pharmaceutical companies.

Drug delivery and transport to solid tumors

PURPOSE

The purpose of this review is to provide an overview of the principles of and barriers to drug transport and delivery to solid tumors.

METHODS

This review consists of four parts. Part I provides an overview of the differences in the vasculature in normal and tumor tissues, and the relationship between tumor vasculature and drug transport. Part II describes the determinants of transport of drugs and particles across tumor vasculature into surrounding tumor tissues. Part III discusses the determinants and barriers of drug transport, accumulation, and retention in tumors. Part IV summarizes the experimental approaches used to enhance drug delivery and transport in solid tumors.

RESULTS

Drug delivery to solid tumors consists of multiple processes, including transport via blood vessels, transvascular transport, and transport through interstitial spaces. These processes are dynamic and change with time and tumor properties and are affected by multiple physicochemical factors of a drug, multiple tumor biologic factors, and as a consequence of drug treatments. The biologic factors, in turn, have opposing effects on one or more processes in the delivery of drugs to solid tumors.

CONCLUSION

The effectiveness of cancer therapy depends in part on adequate delivery of the therapeutic agents to tumor cells. A better understanding of the processes and contribution of these factors governing drug delivery may lead to new cancer therapeutic strategies.

Quantitative tissue perfusion measurements in head and neck carcinoma patients before and during radiation therapy with a non-invasive MR imaging spin-labeling technique

PURPOSE

Tumor blood flow, tumor tissue perfusion and oxygen supply have substantial influence on the responsiveness of tumors to radiotherapy. This study was aimed at implementing and evaluating a non-invasive functional magnetic resonance (MR) imaging spin-labeling technique at a main magnetic field strength of 2T for measuring tissue perfusion changes in head and neck carcinoma patients before and during radiotherapy.

METHODS

Tissue perfusion was determined quantitatively in ten patients with head and neck cancer. Five patients were investigated twice during radiation therapy. For perfusion measurements, a non-invasive MR spin-labeling technique was employed: The longitudinal relaxation time T(1) was measured with segmented Snapshot-FLASH imaging after either slice-selective or non-selective spin inversion. Perfusion values were calculated pixelwise employing a two-compartment tissue model. With this technique no contrast agents are required so that repetitive measurements are possible. Perfusion images with a slice thickness of 10mm and an in-plane resolution of 1.9x2.8mm(2) were acquired at a total scan time of 8:30min per scan.

RESULTS

With the non-invasive MR imaging technique it was possible to visualize tumor and normal tissue perfusion as well as perfusion changes in the course of radiotherapy with a spatial resolution of less than 3mm. Among the investigated subjects measured tumor perfusion and changes in perfusion were heterogenous. In 4/5 patients studied at the start and end of radiotherapy, perfusion decreased, while in one patient there was an increase.

CONCLUSIONS

A method is presented that allows non-invasive and repetitive characterization of tissue perfusion. This parameter may be used for treatment stratification, especially in treatments that use vasomodulation or anti-angiogenic agents.

Development of biologic markers of response and assessment of antiangiogenic activity in a clinical trial of human recombinant endostatin

PURPOSE

Angiogenesis is a target for the treatment of cancer and other diseases, and its complex biology suggests that establishing the appropriate dose and schedule for antiangiogenic treatment will require extensive study. We present the initial results of a dose-finding clinical trial of recombinant human endostatin (rh-Endo) that examined potential surrogates for response to antiangiogenic therapy.

PATIENTS AND METHODS

Twenty-five patients were treated with escalating doses of rh-Endo. Positron emission tomography (PET) was used to assess tumor blood flow (with [15O]H2O) and metabolism (with [18F]fluorodeoxyglucose) before the start of therapy and then every 4 weeks. To directly assess the effects of rh-Endo on endothelial cells within the tumors, biopsy specimens of tumor tissue were obtained before therapy and again at 8 weeks and evaluated for endothelial cell and tumor cell apoptosis.

RESULTS

Tumor blood flow and metabolism as measured by PET scans generally decreased with increasing doses of rh-Endo; however, the effects were complex and in some analyses nonlinear. Tumor biopsy analysis revealed a significant increase in tumor cell apoptosis (P =.027) and endothelial cell apoptosis (P =.027) after 8 weeks of therapy. However, there was no statistically significant relationship between rh-Endo dose and induction of tumor cell or endothelial cell apoptosis.

CONCLUSION

These initial data suggest that rh-Endo has measurable effects on tumor blood flow and metabolism and induces endothelial and tumor cell apoptosis even in the absence of demonstrable anticancer effects. Further study and validation of these biomarkers in the context of antiangiogenic therapy will be required.

Molecular imaging of cancer with positron emission tomography

The imaging of specific molecular targets that are associated with cancer should allow earlier diagnosis and better management of oncology patients. Positron emission tomography (PET) is a highly sensitive non-invasive technology that is ideally suited for pre-clinical and clinical imaging of cancer biology, in contrast to anatomical approaches. By using radiolabelled tracers, which are injected in non-pharmacological doses, three-dimensional images can be reconstructed by a computer to show the concentration and location(s) of the tracer of interest. PET should become increasingly important in cancer imaging in the next decade.

Practical aspects of in situ 16O (gamma,n) 15O activation using a conventional medical accelerator for the purpose of perfusion imaging

We report investigations into the feasibility of generating radioactive oxygen (15O, a positron emitter, with half-life 2.05 min) using a tuned Elekta SL25 accelerator, for the end purpose of imaging tumor perfusion. 15O is produced by the "gamma, neutron," (gamma,n) reaction between high-energy photons and normal oxygen (16O) in the body. As most in vivo 16O is bound in water molecules the 15O radio-marker is produced in proportion to water content in tissue. Imaging the washout of the 15O distribution using sensitive positron-emission-tomography (PET) technology can yield spatial information about blood perfusion in the tissue. The aim of this article was to determine the amount of 15O activity that could be produced by the tuned medical accelerator. A further aim was to model the activation process using Monte Carlo and to investigate ways to optimize the amount of 15O that could be generated. Increased activation was achieved by (i) tuning the beam to give higher-energy electrons incident on the target of the accelerator, (ii) increasing dose rate by removing the conventional filtration in the beam and reducing the source to object distance, and (iii) reducing low-energy photons by means of a carbon block absorber. The activity per-unit-dose produced by the tuned beam was measured by irradiating spheres of water to known doses and placing the spheres in a calibrated coincidence-counting apparatus. Peak energy of the tuned bremsstrahlung beam was estimated at 29 MeV, and generated activity up to 0.24/microCi/cc/3Gy in water. The measured amount of 15O agreed to within 10% of the prediction from the Monte-Carlo-computed spectrum, indicating reasonable ability to model the activation process. The optimal thickness of the carbon absorber was found to be about 25 cm. The insertion of a carbon absorber improved spectral quality for activation purposes but at the cost of reduced dose rate. In conclusion, the viability of generating 15O with an Elekta SL25 has been demonstrated. In conjunction with recent advances in high-sensitivity portable PET imaging devices, real potential exists for imaging in situ activated 15O washout as a surrogate measurement of macroscopic tumor perfusion.