Hypoxia imaging in brain tumors

Hypovascular Cellular Tumor in Primary Central Nervous System Lymphoma is Associated with Treatment Resistance: Tumor Habitat Analysis Using Physiologic MRI

BACKGROUND AND PURPOSE

The microenvironment of lymphomas is known to be highly variable and closely associated with treatment resistance and survival. We tried to develop a physiologic MR imaging-based spatial habitat analysis to identify regions associated with treatment resistance to facilitate the prediction of tumor response after initial chemotherapy in patients with primary central nervous system lymphoma.

MATERIALS AND METHODS

Eighty-one patients with pathologically confirmed primary central nervous system lymphoma were enrolled. Pretreatment physiologic MR imaging was performed, and K-means clustering was used to separate voxels into 3 spatial habitats according to ADC and CBV values. Associations of spatial habitats and clinical and conventional imaging predictors with time to progression were analyzed using Cox proportional hazards modeling. The performance of statistically significant predictors for time to progression was assessed using the concordance probability index.

RESULTS

The 3 spatial habitats of hypervascular cellular tumor, hypovascular cellular tumor, and hypovascular hypocellular tumor were identified. A large hypovascular cellular habitat was most significantly associated with short time to progression (hazard ratio, 2.83; P = . 017). The presence of an atypical finding (hazard ratio, 4.41; P = . 016), high performance score (hazard ratio, 5.82; P = . 04), and high serum lactate dehydrogenase level (hazard ratio, 1.01; P = .013) was significantly associated with time to progression. A predictive model constructed using the habitat score and other imaging parameters showed a concordance probability index for prediction of time to progression of 0.70 (95% CI, 0.54-0.87).

CONCLUSIONS

A hypovascular cellular tumor habitat is associated with treatment resistance in primary central nervous system lymphoma, and its assessment may refine prechemotherapy imaging-based response prediction for patients with primary central nervous system lymphoma.

BOLD Coupling between Lesioned and Healthy Brain Is Associated with Glioma Patients' Recovery

Predicting functional outcomes after surgery and early adjuvant treatment is difficult due to the complex, extended, interlocking brain networks that underpin cognition. The aim of this study was to test glioma functional interactions with the rest of the brain, thereby identifying the risk factors of cognitive recovery or deterioration. Seventeen patients with diffuse non-enhancing glioma (aged 22-56 years) were longitudinally MRI scanned and cognitively assessed before and after surgery and during a 12-month recovery period (55 MRI scans in total after exclusions). We initially found, and then replicated in an independent dataset, that the spatial correlation pattern between regional and global BOLD signals (also known as global signal topography) was associated with tumour occurrence. We then estimated the coupling between the BOLD signal from within the tumour and the signal extracted from different brain tissues. We observed that the normative global signal topography is reorganised in glioma patients during the recovery period. Moreover, we found that the BOLD signal within the tumour and lesioned brain was coupled with the global signal and that this coupling was associated with cognitive recovery. Nevertheless, patients did not show any apparent disruption of functional connectivity within canonical functional networks. Understanding how tumour infiltration and coupling are related to patients' recovery represents a major step forward in prognostic development.

Specific Preoperative Dynamic Contrast-Enhanced MRI Semi-quantitative Markers Can Correlate With Vascularity in Specific Areas of Glioblastoma Tissue and Predict Recurrence

Background: Glioblastoma is one of the most aggressive tumours despite all advanced therapies. We aimed to investigate the correlation between qualitative markers of dynamic contrast-enhanced magnetic resonance imaging and vascularity in different tumour regions and elucidate their potential in predicting recurrence.

Methods: Radiological markers of vascularity as wash-in rate, washout rate, and capillary time to peak in different single tumour regions were extracted for all glioblastoma patients before being surgically resected using preoperative dynamic contrast-enhanced MRI (DCE-MRI). Tissue samples were obtained from different intratumoral regions and peritumoral oedema and evaluated for the vascular endothelial growth factor (VEGF).

Results: Two hundred sixty individuals were included in the final analysis, with 180 dead ones and 80 survivors. Radio- and chemo-therapy were received by all surviving patients and 77.8% (n= 140) of the dead ones. The mean time to peak, in seconds, was longest at the peritumoral oedema region (71.7±23.5), followed by the tumour's necrotic centre (50.0±28.5) and its periphery (2.9±1.8). The expression of VEGF at the peritumoral edema region was inversely correlated to the washout rate at the periphery (r= -0.66; P-value= 0.014) and positively correlated to peritumoral TTP (r= 0.94; P-value< 0.001).

Conclusion: Using DCE-MRI, VEGF expression may be used as a non-invasive marker to estimate tumour grade for clinical diagnosis and treatment. Moreover, the risk of glioblastoma recurrence could be determined by evaluating the washout rate at the tumour's periphery. Further large-scale studies are needed to validate the results and to have concrete evidence.

Hyperbaric oxygen therapy as adjunctive strategy in treatment of glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common type of malignant intracranial tumor in adults. Tumor tissue hypoxia, high mitotic rate, and rapid tumor spread account for its poor prognosis. Hyperbaric oxygen therapy (HBOT) may improve the sensitivity of radio-chemotherapy by increasing oxygen tension within the hypoxic regions of the neoplastic tissue. This review summarizes the research of HBOT applications within the context of experimental and clinical GBM. Limited clinical trials and preclinical studies suggest that radiotherapy immediately after HBOT enhances the effects of radiotherapy in some aspects. HBOT also is able to strengthen the anti-tumor effect of chemotherapy when applied together. Overall, HBOT is well tolerated in the GBM patients and does not significantly increase toxicity. However, HBOT applied by itself as curative strategy against GBM is controversial in preclinical studies and has not been evaluated rigorously in GBM patients. In addition to HBOT favorably managing the therapeutic resistance of GBM, future research needs to focus on the multimodal or cocktail approaches to treatment, as well as molecular strategies targeting GBM stem cells.

Leakage correction improves prognosis prediction of dynamic susceptibility contrast perfusion MRI in primary central nervous system lymphoma

To evaluate whether the cerebral blood volume (CBV) measurement with leakage correction from dynamic susceptibility contrast perfusion weighted imaging can be useful in predicting prognosis for primary central nervous system lymphoma (PCNSL). 46 PCNSL patients were included and classified by radiation therapy (RT) stratification into RT (n = 30) and non-RT (n = 16) groups. The corresponding histogram parameters of normalized CBV (nCBV) maps with or without leakage correction were calculated on contrast-enhanced T1 weighted image (CE T1WI) or on fluid attenuated inversion recovery image. The 75^th percentile nCBV with leakage correction based on CE T1WI (T1 nCBVL_75%) had a significant difference between the short and long progression free survival (PFS) subgroups of the RT group and the non-RT group, respectively. Based on the survival analysis, patients in the RT group with high T1 nCBVL_75% had earlier progression than the others with a low T1 nCBVL_75%. However, patients in the non-RT group with a high T1 nCBVL_75% had slower progression than the others with a low T1 nCBVL_75%. Based on RT stratification, the CBV with leakage correction has potential as a noninvasive biomarker for the prognosis prediction of PCNSL to identify high risk patients and it has a different correlation with the PFS based on the presence of combined RT.

Respiratory challenge MRI: Practical aspects

Respiratory challenge MRI is the modification of arterial oxygen (PaO₂) and/or carbon dioxide (PaCO₂) concentration to induce a change in cerebral function or metabolism which is then measured by MRI. Alterations in arterial gas concentrations can lead to profound changes in cerebral haemodynamics which can be studied using a variety of MRI sequences. Whilst such experiments may provide a wealth of information, conducting them can be complex and challenging. In this paper we review the rationale for respiratory challenge MRI including the effects of oxygen and carbon dioxide on the cerebral circulation. We also discuss the planning, equipment, monitoring and techniques that have been used to undertake these experiments. We finally propose some recommendations in this evolving area for conducting these experiments to enhance data quality and comparison between techniques.

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Oxygen and carbon dioxide affect cerebral blood flow and metabolism.

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This can be imaged with various MRI sequences.

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The practicalities of these techniques are reviewed.

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Examples of how this has been used to understand disease mechanisms.

Methodology for Registration of Shrinkage Tumors in Head-and-Neck CT Studies

Tumor shrinkage occurs in many patients undergoing radiotherapy for head-and-neck (H&N) cancer. However, one-to-one correspondence is not always available between voxels of two image sets. This makes intensity-based deformable registration difficult and inaccurate. In this paper, we describe a novel method to increase the performance of the registration in presence of tumor shrinkage. The method combines an image modification procedure and a fast symmetric Demons algorithm to register CT images acquired at planning and posttreatment fractions. The image modification procedure modifies the image intensities of the primary tumor by calculating tumor cell survival rate using the linear quadratic (LQ) model according to the dose delivered to the tumor. A scale operation is used to deal with uncertainties in biological parameters. The method was tested in 10 patients with nasopharyngeal cancer (NPC). Registration accuracy was improved compared with that achieved using the symmetric Demons algorithm. The average Dice similarity coefficient (DSC) increased by 21%. This novel method is suitable for H&N adaptive radiation therapy.

MRS water resonance frequency in childhood brain tumours: a novel potential biomarker of temperature and tumour environment

(1)H MRS thermometry has been investigated for brain trauma and hypothermia monitoring applications but has not been explored in brain tumours. The proton resonance frequency (PRF) of water is dependent on temperature but is also influenced by microenvironment factors, such as fast proton exchange with macromolecules, ionic concentration and magnetic susceptibility. (1)H MRS has been utilized for brain tumour diagnostic and prognostic purposes in children; however, the water PRF measure may provide complementary information to further improve characterization. Water PRF values were investigated from a repository of MRS data acquired from childhood brain tumours and children with apparently normal brains. The cohort consisted of histologically proven glioma (22), medulloblastoma (19) and control groups (28, MRS in both the basal ganglia and parietal white matter regions). All data were acquired at 1.5 T using a short TE (30 ms) single voxel spectroscopy (PRESS) protocol. Water PRF values were calculated using methyl creatine and total choline. Spectral peak amplitude weighted averaging was used to improve the accuracy of the measurements. Mean PRF values were significantly larger for medulloblastoma compared with glioma, with a difference in the means of 0.0147 ppm (p < 0.05), while the mean PRF for glioma was significantly lower than for the healthy cohort, with a difference in the means of 0.0061 ppm (p < 0.05). This would suggest the apparent temperature of the glioma group was ~1.5 °C higher than the medulloblastomas and ~0.7 °C higher than a healthy brain. However, the PRF shift may not reflect a change in temperature, given that alterations in protein content, microstructure and ionic concentration contribute to PRF shifts. Measurement of these effects could also be used as a supplementary biomarker, and further investigation is required. This study has shown that the water PRF value has the potential to be used for characterizing childhood brain tumours, which has not been reported previously.

Preoperative dynamic contrast-enhanced MRI correlates with molecular markers of hypoxia and vascularity in specific areas of intratumoral microenvironment and is predictive of patient outcome

BACKGROUND

Measures of tumor vascularity and hypoxia have been correlated with glioma grade and outcome. Dynamic contrast-enhanced (DCE) MRI can noninvasively map tumor blood flow, vascularity, and permeability. In this prospective observational cohort pilot study, preoperative imaging was correlated with molecular markers of hypoxia, vascularity, proliferation, and progression-free and overall patient survival.

METHODS

Pharmacokinetic modeling methods were used to generate maps of tumor blood flow, extraction fraction, permeability-surface area product, transfer constant, washout rate, interstitial volume, blood volume, capillary transit time, and capillary heterogeneity from preoperative DCE-MRI data in human glioma patients. Tissue was obtained from areas of peritumoral edema, active tumor, hypoxic penumbra, and necrotic core and evaluated for vascularity, proliferation, and expression of hypoxia-regulated molecules. DCE-MRI parameter values were correlated with hypoxia-regulated protein expression at tissue sample sites.

RESULTS

Patient survival correlated with DCE parameters in 2 cases: capillary heterogeneity in active tumor and interstitial volume in areas of peritumoral edema. Statistically significant correlations were observed between several DCE parameters and tissue markers. In addition, MIB-1 index was predictive of overall survival (P = .044) and correlated with vascular endothelial growth factor expression in hypoxic penumbra (r = 0.7933, P = .0071) and peritumoral edema (r = 0.4546). Increased microvessel density correlated with worse patient outcome (P = .026).

CONCLUSIONS

Our findings suggest that DCE-MRI may facilitate noninvasive preoperative predictions of areas of tumor with increased hypoxia and proliferation. Both imaging and hypoxia biomarkers are predictive of patient outcome. This has the potential to allow unprecedented prognostic decisions and to guide therapies to specific tumor areas.

Combined diffusion and perfusion MR imaging as biomarkers of prognosis in immunocompetent patients with primary central nervous system lymphoma

BACKGROUND AND PURPOSE

ADC derived from DWI has been shown to correlate with PFS and OS in immunocompetent patients with PCNSL. The purpose of our study was to confirm the validity of ADC measurements as a prognostic biomarker and to determine whether rCBV measurements derived from DSC perfusion MR imaging provide prognostic information.

MATERIALS AND METHODS

Pretherapy baseline DWI and DSC perfusion MR imaging in 25 patients with PCNSL was analyzed before methotrexate-based induction chemotherapy. Contrast-enhancing tumor was segmented and coregistered with ADC and rCBV maps, and mean and minimum values were measured. Patients were separated into high or low ADC groups on the basis of previously published threshold values of ADC(min) < 384 × 10(-6) mm(2)/s. High and low rCBV groups were defined on the basis of receiver operating curve analysis. High and low ADC and rCBV groups were analyzed independently and in combination. Multivariate Cox survival analysis was performed.

RESULTS

Patients with ADC(min) values < 384 × 10(-6) mm(2)/s or rCBV(mean) values < 1.43 had worse PFS and OS. The patient cohort with combined low ADC(min)-low rCBV(mean) had the worst prognosis. No other variables besides ADC and rCBV significantly affected survival.

CONCLUSIONS

Our study reinforces the validity of ADC values as a prognostic biomarker and provides the first evidence of low tumor rCBV as a novel risk factor for adverse prognosis in immunocompetent patients with PCNSL.

Dynamic and simultaneous MR measurement of R1 and R2* changes during respiratory challenges for the assessment of blood and tissue oxygenation

This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady-state spoiled multigradient-echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid-rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen-enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply.

Using magnetic resonance imaging and spectroscopy in cancer diagnostics and monitoring: preclinical and clinical approaches

Nuclear Magnetic Resonance (MR) based imaging has become an integrated domain in today's oncology research and clinical management of cancer patients. MR is a unique imaging modality among numerous other imaging modalities by providing access to anatomical, physiological, biochemical and molecular details of tumour with excellent spatial and temporal resolutions. In this review we will cover established and investigational MR imaging (MRI) and MR spectroscopy (MRS) techniques used for cancer imaging and demonstrate wealth of information on tumour biology and clinical applications MR techniques offer for oncology research both in preclinical and clinical settings. Emphasis is given not only to the variety of information which may be obtained but also the complementary nature of the techniques. This ability to determine tumour type, grade, invasiveness, degree of hypoxia, microvacular characteristics, and metabolite phenotype, has already profoundly transformed oncology research and patient management. It is evident from the data reviewed that MR techniques will play a key role in uncovering molecular fingerprints of cancer, developing targeted treatment strategies and assessing responsiveness to treatment for personalized patient management, thereby allowing rapid translation of imaging research conclusions into the benefit of clinical oncology.

MR imaging features of high-grade gliomas in murine models: how they compare with human disease, reflect tumor biology, and play a role in preclinical trials

Murine models are the most commonly used and best investigated among the animal models of HGG. They constitute an important weapon in the development and testing of new anticancer drugs and have long been used in preclinical trials. Neuroimaging methods, particularly MR imaging, offer important advantages for the evaluation of treatment response: shorter and more reliable treatment end points and insight on tumor biology and physiology through the use of functional imaging DWI, PWI, BOLD, and MR spectroscopy. This functional information has been progressively consolidated as a surrogate marker of tumor biology and genetics and may play a pivotal role in the assessment of specifically targeted drugs, both in clinical and preclinical trials. The purpose of this Research Perspectives was to compile, summarize, and critically assess the available information on the neuroimaging features of different murine models of HGGs, and explain how these correlate with human disease and reflect tumor biology.

The development of copper radiopharmaceuticals for imaging and therapy

The increasing use of positron emission tomography in preclinical and clinical settings has widened the demand for radiopharmaceuticals with high specificity that can image biological phenomena in vivo. While many PET tracers have been developed from small organic molecules labeled with carbon-11 or fluorine-18, the short half-lives of these radionuclides preclude their incorporation into radiotracers, which can be used to image biological processes that are not induced immediately after system perturbation. Additionally, the continuing development of targeted agents, such as antibodies and nanoparticles, which undergo extended circulation, require that radionuclides with half-lives that are complimentary to the biological half-lives of these molecules be developed. Copper radionuclides have received considerable attention since they offer a variety of half-lives and decay energies and because the coordination chemistry of cooper and its role in biology is well understood. However, in addition to the radiometal chelate, a successful copper based radiopharmaceutical depends upon the chemical structure of the entire radiotracer, which may include a biologically important molecule and a chemical linker that can be used to deliver the copper radionuclide to a specific target and modulate its in vivo properties, respectively. This review discusses the development of copper radiopharmaceuticals and the importance of factors such as chemical structure on their pharmacokinetics in vivo.

Changes in the MR relaxation rate R(2)* induced by respiratory challenges at 3.0 T: a comparison of two quantification methods

The consistent determination of changes in the transverse relaxation rate R(2)* (ΔR(2)*) is essential for the mapping of the effect of hyperoxic and hypercapnic respiratory challenges, which enables the noninvasive assessment of blood oxygenation changes and vasoreactivity by MRI. The purpose of this study was to compare the performance of two different methods of ΔR(2)* quantification from dynamic multigradient-echo data: (A) subtraction of R(2)* values calculated from monoexponential decay functions; and (B) computation of ΔR(2)* echo-wise from signal intensity ratios. A group of healthy volunteers (n = 12) was investigated at 3.0 T, and the brain tissue response to carbogen and CO(2)-air inhalation was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. Results of the ΔR(2)* quantification obtained by the two methods were compared with respect to the quality of the voxel-wise ΔR(2)* response, the number of responding voxels and the behaviour of the 'global' response of all voxels with significant R(2)* changes. For the two ΔR(2)* quantification methods, we found no differences in the temporal variation of the voxel-wise ΔR(2)* responses or in the detection sensitivity. The maximum change in the 'global' response was slightly smaller when ΔR(2)* was derived from signal intensity ratios. In conclusion, this first methodological comparison shows that both ΔR(2)* quantifications, from monoexponential approximation as well as from signal intensity ratios, are applicable for the monitoring of R(2)* changes during respiratory challenges.

Intracranial tumor response to respiratory challenges at 3.0 T: impact of different methods to quantify changes in the MR relaxation rate R2*

PURPOSE

To compare two DeltaR2* quantification methods for analyzing the response of intracranial tumors to different breathing gases. The determination of changes in the magnetic resonance imaging (MRI) relaxation rate R2* (DeltaR2*), induced by hyperoxic and hypercapnic respiratory challenges, enables the noninvasive assessment of blood oxygenation changes and vasoreactivity.

MATERIALS AND METHODS

Sixteen patients with various intracranial tumors were examined at 3.0 T. The response to respiratory challenges was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. At each dynamic step, DeltaR2* was derived in two different ways: 1) by subtraction of R2* values obtained from monoexponential decay functions, 2) by computing DeltaR2* echo-wise from signal intensity ratios. The sensitivity for detection of responding voxels and the behavior of the "global" response were investigated.

RESULTS

Significantly more responding voxels (about 4%) were found for method (1). The "global" response was independent from the chosen quantification method but showed slightly larger changes (about 6%) when DeltaR2* was derived from method (1).

CONCLUSION

Similar results were observed for the two methods, with a slightly higher detection sensitivity of responding voxels when DeltaR2* was obtained from monoexponential approximation.

Quantification of the magnetic resonance signal response to dynamic (C)O(2)-enhanced imaging in the brain at 3 T: R*(2) BOLD vs. balanced SSFP

PURPOSE

To compare two magnetic resonance (MR) contrast mechanisms, R*(2) BOLD and balanced SSFP, for the dynamic monitoring of the cerebral response to (C)O(2) respiratory challenges.

MATERIALS AND METHODS

Carbogen and CO(2)-enriched air were delivered to 9 healthy volunteers and 1 glioblastoma patient. The cerebral response was recorded by two-dimensional (2D) dynamic multi-gradient-echo and passband-balanced steady-state free precession (bSSFP) sequences, and local changes of R*(2) and signal intensity were investigated. Detection sensitivity was analyzed by statistical tests. An exponential signal model was fitted to the global response function delivered by each sequence, enabling quantitative comparison of the amplitude and temporal behavior.

RESULTS

The bSSFP signal changes during carbogen and CO(2)/air inhalation were lower compared with R*(2) BOLD (ca. 5% as opposed to 8-13%). The blood-oxygen-level-dependent (BOLD) response amplitude enabled differentiation between carbogen and CO(2)/air by a factor of 1.4-1.6, in contrast to bSSFP, where differentiation was not possible. Furthermore, motion robustness and detection sensitivity were higher for R*(2) BOLD.

CONCLUSION

Both contrast mechanisms are well suited to dynamic (C)O(2)-enhanced MR imaging, although the R*(2) BOLD mechanism was demonstrated to be superior in several respects for the chosen application. This study suggests that the R*(2) BOLD and bSSFP-response characteristics are related to different physiologic mechanisms.

Analysing the response in R2* relaxation rate of intracranial tumours to hyperoxic and hypercapnic respiratory challenges: initial results

OBJECTIVE

To investigate the response in R2* relaxation rate of human intracranial tumours during hyperoxic and hypercapnic respiratory challenges.

METHODS

In seven patients with different intracranial tumours, cerebral R2* changes during carbogen and CO(2)/air inhalation were monitored at 3 T using a dynamic multigradient-echo sequence of high temporal and spatial resolution. The R2* time series of each voxel was tested for significant change. Regions of interest were analysed with respect to response amplitude and velocity.

RESULTS

The tumours showed heterogeneous R2* responses with large interindividual variability. In the 'contrast-enhancing' area of five patients and in the 'non-tumoral' tissue most voxels showed a decrease in R2* for carbogen. For the 'contrast-enhancing' area of two patients hardly any responses were found. In areas of 'necrosis' and perifocal 'oedema' typically voxels with R2* increase and no response were found for both gases. For tissue responding to CO(2)/air, the R2* changes were of the same order of magnitude as those for carbogen. The response kinetic was generally attenuated in tumoral tissue.

CONCLUSION

The spatially resolved determination of R2* changes reveals the individual heterogeneous response characteristic of intracranial human tumours during hyperoxic and hypercapnic respiratory challenges.

High resolution ultra high field magnetic resonance imaging of glioma microvascularity and hypoxia using ultra-small particles of iron oxide

OBJECTIVES

This study assessed whether ultra-small particles of iron oxide (USPIO) intravascular contrast agent could enhance visualization of tumor microvascularity in F98 glioma bearing rats by means of ultra high field (UHF) high-resolution gradient echo (GRE) magnetic resonance imaging (MRI). In an effort to explain differences in visualization of microvascularity before and after USPIO administration, hypoxia and vessel diameters were assessed on corresponding histopathologic sections.

MATERIALS AND METHODS

F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats. Based on clinical criteria, rats were imaged 1 to 2 days before their death with and without USPIO contrast on an 8 Tesla MRI. To identify hypoxic regions of the brain tumor by immunohistochemical staining, a subset of animals also received a nitroimidazole-based hypoxia marker, EF5, before euthanasia. These sections then were compared with noncontrast enhanced MR images. The relative caliber of tumor microvasculature, compared with that of normal brain, was analyzed in a third group of animals.

RESULTS

After USPIO administration, UHF high-resolution GRE MRI consistently predicted increased microvascular density relative to normal gray matter when correlated with histopathology. The in-plane visibility of glioma microvascularity in 22 rats increased by an average of 115% and signal intensity within the tumor decreased by 13% relative to normal brain. Tumor microvascularity identified on noncontrast MR images matched hypoxic regions identified by immunohistochemical staining with a sensitivity of 83% and specificity of 89%. UHF GRE MRI was able to resolve microvessels less than 20 micro in diameter, although differences in tumor vessel size did not consistently account for differences in visualization of microvascularity.

CONCLUSIONS

USPIO administration significantly enhanced visualization of tumor microvascularity on gradient echo 8 T MRI and significantly improved visualization of tumor microvascularity. Microvascularity identified on precontrast images is suspected to be partly associated with hypoxia.

The blood oxygen level-dependent functional MR imaging signal can be used to identify brain tumors and distinguish them from normal tissue

BACKGROUND AND PURPOSE

In neuro-oncology, a major problem is clear identification of tumor from the surrounding normal tissue. We hypothesized that we could use the blood oxygen level-dependent functional MR imaging (BOLD fMRI) signals from tumors and normal brain to identify the tumors and distinguish them from the surrounding brain.

MATERIALS AND METHODS

Fourteen patients with meningiomas, gliomas, and metastatic tumors were scanned before surgery. All subjects performed a motor task; 2 subjects were also scanned while in a resting state. The BOLD signals were taken from selected points within the tumor and from the surrounding normal brain and were analyzed by using correlation analysis to determine how closely they were related.

RESULTS

The BOLD signals from all of the tumors were significantly different from those in the surrounding normal tissue. In meningiomas and gliomas, selection of a voxel in the tumor for signal-intensity analysis highlighted the entire tumor mass while excluding the normal tissue. The BOLD signal intensity was the same whether the subjects were motionless or finger tapping.

CONCLUSIONS

Analysis of the BOLD signal intensity provides a relatively simple and straightforward method for identifying brain tumors and distinguishing them from normal tissue. This approach may be of use in neurosurgery.

Brain tumor hypoxia: tumorigenesis, angiogenesis, imaging, pseudoprogression, and as a therapeutic target

Hypoxia is implicated in many aspects of tumor development, angiogenesis, and growth in many different tumors. Brain tumors, particularly the highly aggressive glioblastoma multiforme (GBM) with its necrotic tissues, are likely affected similarly by hypoxia, although this involvement has not been closely studied. Invasion, apoptosis, chemoresistance, resistance to antiangiogenic therapy, and radiation resistance may all have hypoxic mechanisms. The extent of the influence of hypoxia in these processes makes it an attractive therapeutic target for GBM. Because of their relationship to glioma and meningioma growth and angiogenesis, hypoxia-regulated molecules, including hypoxia inducible factor-1, carbonic anhydrase IX, glucose transporter 1, and vascular endothelial growth factor, may be suitable subjects for therapies. Furthermore, other novel hypoxia-regulated molecules that may play a role in GBM may provide further options. Emerging imaging techniques may allow for improved determination of hypoxia in human brain tumors to better focus therapeutic treatments; however, tumor pseudoprogression, which may be prompted by hypoxia, poses further challenges. An understanding of the role of hypoxia in tumor development and growth is important for physicians involved in the care of patients with brain tumors.

Hypoxia-regulated protein expression, patient characteristics, and preoperative imaging as predictors of survival in adults with glioblastoma multiforme

BACKGROUND

Regions of hypoxia within glioblastoma multiforme (GBM) are common and may influence a tumor's aggressiveness, response to treatment, and the patient's overall survival. In this study, the authors examined 4 markers of hypoxia (hypoxia-inducible factor 1 [HIF-1alpha], glucose transporter 1 [GLUT-1], vascular endothelial growth factor [VEGF], and carbonic anhydrase 9 [CA IX]), cellular proliferation and microvascular density (MVD) indices, extent of surgical resection, and preoperative imaging characteristics and compared them with the overall survival rates of adults with GBM.

METHODS

In this retrospective cohort study, patients who had lower grade astrocytomas were compared with patients who had GBM to verify that the methods used could establish differences between tumor grades. By using preoperative imaging, the amount of necrosis was established versus the overall tumor area. The authors also compared preoperative images with postoperative images to define the amount of tumor resected; and they compared molecular markers, proliferation, MVD, and imaging studies with survival among patients who had GBM.

RESULTS

The hypoxia-regulated molecules (HRMs) and indices for MVD and cellular proliferation were associated significantly with tumor grade. Survival was improved when >or=95% of the tumor was resected. Although the total tumor area was associated with overall survival, no differences were observed when the amount of necrosis or a tumor necrosis index (area of necrosis/area of tumor) was compared with survival. The findings indicated that GLUT-1 and VEGF were correlated with survival after controlling for age.

CONCLUSIONS

Tumor grade was differentiated with HRMs, MVD, and proliferation, but only GLUT-1 predicted survival in this group of patients with GBM. The results suggested that GLUT-1 may be an important independent prognostic indicator.

Identification of noninvasive imaging surrogates for brain tumor gene-expression modules

Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor in adults. We combined neuroimaging and DNA microarray analysis to create a multidimensional map of gene-expression patterns in GBM that provided clinically relevant insights into tumor biology. Tumor contrast enhancement and mass effect predicted activation of specific hypoxia and proliferation gene-expression programs, respectively. Overexpression of EGFR, a receptor tyrosine kinase and potential therapeutic target, was also directly inferred by neuroimaging and was validated in an independent set of tumors by immunohistochemistry. Furthermore, imaging provided insights into the intratumoral distribution of gene-expression patterns within GBM. Most notably, an "infiltrative" imaging phenotype was identified that predicted patient outcome. Patients with this imaging phenotype had a greater tendency toward having multiple tumor foci and demonstrated significantly shorter survival than their counterparts. Our findings provide an in vivo portrait of genome-wide gene expression in GBM and offer a potential strategy for noninvasively selecting patients who may be candidates for individualized therapies.

BGRT: biologically guided radiation therapy-the future is fast approaching!

Rapid advances in functional and biological imaging, predictive assays, and our understanding of the molecular and cellular responses underpinning treatment outcomes herald the coming of the long-sought goal of implementing patient-specific biologically guided radiation therapy (BGRT) in the clinic. Biological imaging and predictive assays have the potential to provide patient-specific, three-dimensional information to characterize the radiation response characteristics of tumor and normal structures. Within the next decade, it will be possible to combine such information with advanced delivery technologies to design and deliver biologically conformed, individualized therapies in the clinic. The full implementation of BGRT in the clinic will require new technologies and additional research. However, even the partial implementation of BGRT treatment planning may have the potential to substantially impact clinical outcomes.

Measuring oxygenation in vivo with MRS/MRI--from gas exchange to the cell

Oxygen plays a major role as a substrate in metabolic processes in numerous signaling pathways, in redox metabolism, and in free radical metabolism. To study the role of oxygen in normal and pathophysiological states, methods that can be used noninvasively are required. This review examines the potential of nuclear magnetic resonance techniques to study tissue oxygenation. It is written from a systems perspective, looking at detection methods with respect to the path that oxygen takes in the mammalian system-from the lungs, through the vascular system, into the interstitial space, and finally into the cell. Methods discussed range from those that are quantifiable, such as the assessment of spin lattice relaxation time in fluorocarbon solutions, to those that are more correlative, such as assessment of lactate and high energy phosphates. Since the methods vary in their site of application, sensitivity, and specificity to the quantification of oxygen, this review provides examples of how each method has been applied. This may facilitate the reader's understanding of how to optimally apply different methods to study specific biomedical problems.

Radiation Oncology Physicists Will Need to Better Understand Medical Imaging

Imaging is affecting radiation oncology at a dramatically advancing pace and scale and is likely to create a transformation to individualized, biologically conformal radiation therapy. Deploying and improving imaging technologies and ensuring their correct uses in treatment planning and delivery are the responsibilities of radiation oncology physicists. The potential magnitude of errors arising from the incorrect use of imaging may be far greater than that resulting from typical errors in dose calibration. A major effort is required for radiation oncology physicists to raise the quality assurance of image guidance to a level comparable with that achieved in the maintenance of dosimetric performance. Most radiation oncology physicists lack adequate knowledge to assume this emerging responsibility. Their knowledge of imaging must be enhanced, in most cases through on-the-job training and self-learning. Effective learning strategies include routine interactions with diagnostic radiology and nuclear medicine physicists and physicians and the use of educational opportunities provided by professional organizations and vendors.

Effects of hyperoxygenation on FDG-uptake in head-and-neck cancer

PURPOSE

Tumor hyperoxygenation results in high response rates to ARCON (accelerated radiotherapy with carbogen and nicotinamide). The effect of hyperoxygenation on tumor metabolism using [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET) was investigated.

METHODS

Within one week, FDG-PET was performed without and with hyperoxygenation by carbogen breathing and/or nicotinamide administration in 22 patients, eligible for ARCON for head-and-neck cancer. Maximum standardized uptake values (SUV(max)) in both scans and the relative change were calculated in the primary tumor and in normal muscle.

RESULTS

Alteration of the tumor oxygenation state induced profound, but variable, metabolic changes (median DeltaSUV(max) -4%; range -61% to +30%). Metabolism in normal muscle was not affected. In three patients who did not achieve local tumor control, the SUV(max) after hyperoxygenation differed less than 5% change as compared to baseline, whereas 13 of the 16 patients with local tumor control showed a larger difference (p<0.05).

CONCLUSION

Given the heterogeneous response pattern of nicotinamide and carbogen on FDG-uptake in head-and-neck carcinoma, the prognostic significance of semiquantitative FDG-PET before and after hyperoxygenation remains uncertain and requires confirmation in larger clinical studies before introducing the procedure as a predictive tool for oxygenation modifying treatments.

A four-dimensional computer simulation model of the in vivo response to radiotherapy of glioblastoma multiforme: studies on the effect of clonogenic cell density

Tumours behave as complex, self-organizing, opportunistic dynamic systems. In an attempt to better understand and describe the highly complicated tumour behaviour, a novel four-dimensional simulation model of in vivo tumour growth and response to radiotherapy has been developed. This paper presents the latest improvements to the model as well as a parametric validation of it. Improvements include an advanced algorithm leading to conformal tumour shrinkage, a quantitative consideration of the influence of oxygenation on radiosensitivity and a more realistic, imaging based description of the neovasculature distribution. The tumours selected for the validation of the model are a wild type and a mutated p53 gene glioblastomas multiforme. According to the model predictions, a whole tumour with larger cell cycle duration tends to repopulate more slowly. A lower oxygen enhancement ratio value leads to a more radiosensitive whole tumour. Higher clonogenic cell density (CCD) produces a higher number of proliferating tumour cells and, therefore, a more difficult tumour to treat. Simulation predictions agree at least semi-quantitatively with clinical experience, and particularly with the outcome of the Radiation Therapy Oncology Group (RTOG) Study 83-02. It is stressed that the model allows a quantitative study of the interrelationship between the competing influences in a complex, dynamic tumour environment. Therefore, the model can already be useful as an educational tool with which to study, understand and demonstrate the role of various parameters in tumour growth and response to irradiation. A long term quantitative clinical adaptation and validation of the model aiming at its integration into the treatment planning procedure is in progress.

Hypoxia in the tumorigenesis of gliomas and as a potential target for therapeutic measures

✓ In this article, the author provides a brief description of the role of hypoxia in the tumorigenesis of gliomas and suggests potential ways of exploiting this role to design treatment modalities. Tumor hypoxia predicts the likelihood of metastases, tumor recurrence, resistance to chemotherapy and radiation therapy, invasive potential, and decreased patient survival for many human malignancies. Various methods of measurement of tumor hypoxia are discussed, including direct measurement and imaging methods.

The role of hypoxia-responsive molecules, especially hypoxia-inducible factor-1 (HIF-1), in glioma tumorigenesis is explored. Treatment modalities regulated by hypoxia are proposed and some potential strategies reviewed. The progression of a low-grade astrocytoma to a glioblastoma multiforme may be mediated by hypoxia-induced phenotypic changes and subsequent clonal selection of cells that overexpress hypoxia-responsive molecules, such as HIF-1. In this model, intratumoral hypoxia causes genetic changes that produce a microenvironment that selects for cells of a more aggressive phenotype.

Barriers to carrier mediated drug and gene delivery to brain tumors

Brain tumor patients face a poor prognosis despite significant advances in tumor imaging, neurosurgery and radiation therapy. Potent chemotherapeutic drugs fail when used to treat brain tumors because biochemical and physiological barriers limit drug delivery into the brain. In the past decade a number of strategies have been introduced to increase drug delivery into the brain parenchyma. In particular, direct drug administration into the brain tumor has shown promising results in both animal models and clinical trials. This technique is well suited for the delivery of liposome and polymer drug carriers, which have the potential to provide a sustained level of drug and to reach cellular targets with improved specificity. We will discuss the current approaches that have been used to increase drug delivery into the brain parenchyma in the context of fluid and solute transport into, through and from the brain, with a focus on liposome and polymer drug carriers.

Imaging angiogenesis: applications and potential for drug development

Recognition of the importance of angiogenesis to tumor growth and metastasis has led to efforts to develop new drugs that are targeted to angiogenic vasculature. Clinical trials of these agents are challenging, both because there is no agreed upon method of establishing the correct dosage for drugs whose mechanism of action is not primarily cytotoxic and because of the long time it takes to determine whether such drugs have a clinical effect. Therefore, there is a need for rapid and effective biomarkers to establish drug dosage and monitor clinical response. This review addresses the potential of imaging as a way to accurately and reliably assess changes in angiogenic vasculature in response to therapy. We describe the advantages and disadvantages of several imaging modalities, including positron emission tomography, x-ray computed tomography, magnetic resonance imaging, ultrasound, and optical imaging, for imaging angiogenic vasculature. We also discuss the analytic methods used to derive blood flow, blood volume, empirical semiquantitative hemodynamic parameters, and quantitative hemodynamic parameters from pharmacokinetic modeling. We examine the validity of these methods, citing studies that test correlations between data derived from imaging and data derived from other established methods, their reproducibility, and correlations between imaging-derived hemodynamic parameters and other pathologic indicators, such as microvessel density, pathology score, and disease outcome. Finally, we discuss which imaging methods are most likely to have the sensitivity and reliability required for monitoring responses to cancer therapy and describe ways in which imaging has been used in clinical trials to date.

[Susceptibility weighted imaging. Theory and applications]

Susceptibility Weighted Imaging (SWI) is a new MR imaging technique using the BOLD effect (Blood Oxygen Level Dependent) and the differences of susceptibility between tissues. It is a 3D gradient echo, fully velocity compensated sequence. The echo time is chosen to maximize the signal cancellation in veins and a specific post-processing is applied using the phase images as a complementary source of contrast. It is very useful for the visualization of veins either normal or abnormal. It shows hemorrhage, even of small quantity, better than conventional gradient echo sequences. Its use is still limited by a long acquisition time and some remaining artifacts.

A spatio-temporal simulation model of the response of solid tumours to radiotherapyin vivo: parametric validation concerning oxygen enhancement ratio and cell cycle duration

Advanced bio-simulation methods are expected to substantially improve radiotherapy treatment planning. To this end a novel spatio-temporal patient-specific simulation model of the in vivo response of malignant tumours to radiotherapy schemes has been recently developed by our group. This paper discusses recent improvements to the model: an optimized algorithm leading to conformal shrinkage of the tumour as a response to radiotherapy, the introduction of the oxygen enhancement ratio (OER), a realistic initial cell phase distribution and finally an advanced imaging-based algorithm simulating the neovascularization field. A parametric study of the influence of the cell cycle duration Tc, OER, OERbeta for the beta LQ parameter on tumour growth. shrinkage and response to irradiation under two different fractionation schemes has been made. The model has been applied to two glioblastoma multiforme (GBM) cases, one with wild type (wt) and another one with mutated (mt) p53 gene. Furthermore, the model has been applied to a hypothetical GBM tumour with alpha and beta values corresponding to those of generic radiosensitive tumours. According to the model predictions, a whole tumour with shorter Tc tends to repopulate faster, as is to be expected. Furthermore, a higher OER value for the dormant cells leads to a more radioresistant whole tumour. A small variation of the OERbeta value does not seem to play a major role in the tumour response. Accelerated fractionation proved to be superior to the standard scheme for the whole range of the OER values considered. Finally, the tumour with mt p53 was shown to be more radioresistant compared to the tumour with wt p53. Although all simulation predictions agree at least qualitatively with the clinical experience and literature, a long-term clinical adaptation and quantitative validation procedure is in progress.