Comparison of tumor blood perfusion assessed by dynamic contrast-enhanced MRI with tumor blood supply assessed by invasive imaging

Assessment of Intratumor Heterogeneity in Parametric Dynamic Contrast-Enhanced MR Images: A Comparative Study of Novel and Established Methods

Intratumor heterogeneity is associated with aggressive disease and poor survival rates in several types of cancer. A novel method for assessing intratumor heterogeneity in medical images, named the spatial gradient method, has been developed in our laboratory. In this study, we measure intratumor heterogeneity in K^trans maps derived by dynamic contrast-enhanced magnetic resonance imaging using the spatial gradient method, and we compare the performance of the novel method with that of histogram analyses and texture analyses using the Haralick method. K^trans maps of 58 untreated and sunitinib-treated pancreatic ductal adenocaricoma (PDAC) xenografts from two PDAC models were investigated. Intratumor heterogeneity parameters derived by the spatial gradient method were sensitive to tumor line differences as well as sunitinib-induced changes in intratumor heterogeneity. Furthermore, the parameters provided additional information to the median value and were not severely affected by imaging noise. The parameters derived by histogram analyses were insensitive to spatial heterogeneity and were strongly correlated to the median value, and the Haralick features were severely influenced by imaging noise and did not differentiate between untreated and sunitinib-treated tumors. The spatial gradient method was superior to histogram analyses and Haralick features for assessing intratumor heterogeneity in K^trans maps of untreated and sunitinib-treated PDAC xenografts, and can possibly be used to assess intratumor heterogeneity in other medical images and to evaluate effects of other treatments as well.

Assessment of Hypoxic Tissue Fraction and Prediction of Survival in Cervical Carcinoma by Dynamic Contrast-Enhanced MRI

Tumor hypoxia is a major cause of treatment resistance and poor survival in locally-advanced cervical carcinoma (LACC). It has been suggested that K^trans and v_e maps derived by dynamic contrast-enhanced magnetic resonance imaging can provide information on the oxygen supply and oxygen consumption of tumors, but it is not clear whether and how these maps can be combined to identify tumor hypoxia. The aim of the current study was to find the optimal strategy for calculating hypoxic fraction and predicting survival from K^trans and v_e maps in cervical carcinoma. K^trans and v_e maps of 98 tumors of four patient-derived xenograft models of cervical carcinoma as well as 80 patients with LACC were investigated. Hypoxic fraction calculated by using K^trans maps correlated strongly (P < 0.0001) to hypoxic fraction assessed with immunohistochemistry using pimonidazole as a hypoxia marker and was associated with disease-free and overall survival in LACC patients. Maps of v_e did not provide information on hypoxic fraction and patient outcome, and combinations of K^trans and v_e were not superior to K^trans alone for calculating hypoxic fraction. These observations imply that K^trans maps reflect oxygen supply and may be used to identify hypoxia and predict outcome in cervical carcinoma, whereas v_e is a poor parameter of oxygen consumption and does not provide information on tumor oxygenation status.

Intravital microscopy of tumor vessel morphology and function using a standard fluorescence microscope

Purpose

The purpose of the study was to demonstrate the performance and possible applications of an intravital microscopy assay using a standard fluorescence microscope.

Methods

Melanoma and pancreatic ductal adenocarcinoma xenografts were initiated in dorsal window chambers and subjected to repeated intravital microscopy. The entire tumor vasculature as well as the normal tissue surrounding the tumor was imaged simultaneously with high spatial and temporal resolution. Vascular morphology images were recorded by using transillumination, and vascular masks were produced to quantify vessel density, vessel diameter, vessel segment length, and vessel tortuosity. First-pass imaging movies were recorded after an intervenous injection of a fluorescent marker and were used to investigate vascular function. Lymphatics were visualized by intradermal injections of a fluorescent marker.

Results

The intravital microscopy assay was used to study tumor growth and vascularization, tumor vessel morphology and function, tumor-associated lymphatics, and vascular effects of acute cyclic hypoxia and antiangiogenic treatment. The assay was sensitive to tumor-line differences in vascular morphology and function and detected tumor-induced lymphatic dilation. Acute cyclic hypoxia induced angiogenesis and increased the density of small diameter vessels and blood supply times, whereas antiangiogenic treatment selectively removed small-diameter vessels, reduced blood supply times, and induced hypoxia. Moreover, the window chamber was compatible with magnetic resonance imaging (MRI), and parametric images derived by dynamic contrast-enhanced MRI were shown to reflect vascular morphology and function.

Conclusions

The presented assay represents a useful and affordable alternative to intravital microscopy assays using confocal and multi-photon microscopes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05243-0.

How Do Surface Properties of Nanoparticles Influence Their Diffusion in the Extracellular Matrix? A Model Study in Matrigel Using Polymer-Grafted Nanoparticles

Diffusion of nanomedicines inside the extracellular matrix (ECM) has been identified as a key factor to achieve homogeneous distribution and therefore therapeutic efficacy. Here, we sought to determine the impact of nanoparticles' (NPs) surface properties on their ability to diffuse in the ECM. As model nano-objects, we used a library of gold nanoparticles grafted with a versatile polymethacrylate corona, which enabled the surface properties to be modified. To accurately recreate the features of the native ECM, diffusion studies were carried out in a tumor-derived gel (Matrigel). We developed two methods to evaluate the diffusion ability of NPs inside this model gel: an easy-to-implement one based on optical monitoring and another one using small-angle X-ray scattering (SAXS) measurements. Both enabled the determination of the diffusion coefficients of NPs and comparison of the influence of their various surface properties, while the SAXS technique also allowed to monitor the NPs' structure as they diffused inside the gel. Positive charges and hydrophobicity were found to particularly hinder diffusion, and the different results suggested on the whole the presence of NPs-matrix interactions, therefore underlying the importance of the ECM model. The accuracy of the tumor-derived gels used in this study was evidenced by in vivo experiments involving intratumoral injections of NPs on mice, which showed that diffusion patterns in the peripheral tumor tissues were quite similar to the ones obtained within the chosen ECM model.

Patient-Centric Head and Neck Cancer Radiation Therapy: Role of Advanced Imaging

The traditional 'one-size-fits-all' approach to H&N cancer therapy is archaic. Advanced imaging can identify radioresistant areas by using biomarkers that detect tumor hypoxia, hypercellularity etc. Highly conformal radiotherapy can target resistant areas with precision. The critical information that can be gleaned about tumor biology from these advanced imaging modalities facilitates individualized radiotherapy. The tumor imaging world is pushing its boundaries. Molecular imaging can now detect protein expression and genotypic variations across tumors that can be exploited for tailoring treatment. The exploding field of radiomics and radiogenomics extracts quantitative, biologic and genetic information and further expands the scope of personalized therapy.

Pharmacokinetic analysis of DCE-MRI data of locally advanced cervical carcinoma with the Brix model

Background: There is significant evidence that DCE-MRI may have the potential to provide clinically useful biomarkers of the outcome of locally advanced cervical carcinoma. However, there is no consensus on how to analyze DCE-MRI data to arrive at the most powerful biomarkers. The purpose of this study was to analyze DCE-MRI data of cervical cancer patients by using the Brix pharmacokinetic model and to compare the biomarkers derived from the Brix analysis with biomarkers determined by non-model-based analysis [i.e., low-enhancing tumor volume (LETV) and tumor volume with increasing signal (TVIS)] of the same patient cohort. Material and methods: DCE-MRI recordings of 80 patients (FIGO stage IB-IVA) treated with concurrent cisplatin-based chemoradiotherapy were analyzed voxel-by-voxel, and frequency distributions of the three parameters of the Brix model (A_Brix, k_ep, and k_el) were determined. Moreover, risk volumes were calculated from the Brix parameters and termed RV-A_Brix, RV-k_ep, and RV-k_el, where the RVs represent the tumor volume with voxel values below a threshold value determined by ROC analysis. Disease-free survival (DFS) and overall survival (OS) were used as measures of treatment outcome. Results: Significant associations between the median value or any other percentile value of A_Brix, k_ep, or k_el and treatment outcome were not found. However, RV-A_Brix, RV-k_ep, and RV-k_el correlated with DFS and OS. Multivariate analysis revealed that the prognostic power of RV-A_Brix, RV-k_ep, and RV-k_el was independent of well-established clinical prognostic factors. RV-A_Brix, RV-k_ep, and RV-k_el correlated with each other as well as with LETV and TVIS. Conclusion: Strong biomarkers of the outcome of locally advanced cervical carcinoma can be provided by subjecting DCE-MRI series to pharmacokinetic analysis using the Brix model. The prognostic power of these biomarkers is not necessarily superior to that of biomarkers identified by non-model-based analyses.

DCE-MRI and Quantitative Histology Reveal Enhanced Vessel Maturation but Impaired Perfusion and Increased Hypoxia in Bevacizumab-Treated Cervical Carcinoma

PURPOSE

This study had a dual purpose: to investigate (1) whether bevacizumab can change the microvasculature and oxygenation of cervical carcinomas and (2) whether any changes can be detected with dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

METHODS AND MATERIALS

Two patient-derived xenograft models of cervical cancer (BK-12 and HL-16) were included in the study. Immunostained histologic preparations from untreated and bevacizumab-treated tumors were analyzed with respect to microvascular density, vessel pericyte coverage, and tumor hypoxia using CD31, α-SMA, and pimonidazole as markers, respectively. DCE-MRI was performed at 7.05 T, and parametric images of K^trans and v_e were derived from the data using the Tofts pharmacokinetic model.

RESULTS

The tumors of both models showed decreased microvascular density, increased vessel pericyte coverage, and increased vessel maturation after bevacizumab treatment. Bevacizumab-treated tumors were more hypoxic and had lower K^trans values than untreated tumors in the BK-12 model, whereas bevacizumab-treated and untreated HL-16 tumors had similar hypoxic fractions and similar K^trans values. Significant correlations were found between median K^trans and hypoxic fraction, and the data for untreated and bevacizumab-treated tumors were well fitted by the same curve in both tumor models.

CONCLUSIONS

Bevacizumab-treated tumors show less abnormal microvessels than untreated tumors do, but because of treatment-induced vessel pruning, the overall function of the microvasculature might be impaired after bevacizumab treatment, resulting in increased tumor hypoxia. DCE-MRI has great potential for monitoring bevacizumab-induced changes in tumor hypoxia in cervical carcinoma.

Inhibiting Solid Tumor Growth In Vivo by Non-Tumor-Penetrating Nanomedicine

Nanomedicine (NM) cannot penetrate deeply into solid tumors, which is partly attributed to the heterogeneous microenvironment and high interstitial fluid pressure of solid tumors. To improve NM efficacy, there has been tremendous effort developing tumor-penetrating NMs by miniaturizing NM sizes or controlling NM surface properties. But progress along the direction of developing tumor penetrating nanoparticle has been slow and improvement of the overall antitumor efficacy has been limited. Herein, a novel strategy of inhibiting solid tumor with high efficiency by dual-functional, nontumor-penetrating NM is demonstrated. The intended NM contains 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a vascular-disrupting agent, and doxorubicin (DOX), a cytotoxic drug. Upon arriving at the target tumor site, sustained release of DMXAA from NMs results in disruption of tumor vessel functions, greatly inhibiting the interior tumor cells by cutting off nutritional supply. Meanwhile, the released DOX kills the residual cells at the tumor exterior regions. The in vivo studies demonstrate that this dual-functional, nontumor penetrating NM exhibits superior anticancer activity, revealing an alternative strategy of effective tumor growth inhibition.

Early effects of low dose bevacizumab treatment assessed by magnetic resonance imaging

Background

Antiangiogenic treatments have been shown to increase blood perfusion and oxygenation in some experimental tumors, and to reduce blood perfusion and induce hypoxia in others. The purpose of this preclinical study was to investigate the potential of dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) and diffusion weighted MRI (DW-MRI) in assessing early effects of low dose bevacizumab treatment, and to investigate intratumor heterogeneity in this effect.

Methods

A-07 and R-18 human melanoma xenografts, showing high and low expression of VEGF-A, respectively, were used as tumor models. Untreated and bevacizumab-treated tumors were subjected to DCE-MRI and DW-MRI before treatment, and twice during a 7-days treatment period. Tumor images of K^trans (the volume transfer constant of Gd-DOTA) and v_e (the fractional distribution volume of Gd-DOTA) were produced by pharmacokinetic analysis of the DCE-MRI data, and tumor images of ADC (the apparent diffusion coefficient) were produced from DW-MRI data.

Results

Untreated A-07 tumors showed higher K^trans, v_e, and ADC values than untreated R-18 tumors. Untreated tumors showed radial heterogeneity in K^trans, i.e., K^trans was low in central tumor regions and increased gradually towards the tumor periphery. After the treatment, bevacizumab-treated A-07 tumors showed lower K^trans values than untreated A-07 tumors. Peripherial tumor regions showed substantial reductions in K^trans, whereas little or no effect was seen in central regions. Consequently, the treatment altered the radial heterogeneity in K^trans. In R-18 tumors, significant changes in K^trans were not observed. Treatment induced changes in tumor size, v_e, and ADC were not seen in any of the tumor lines.

Conclusions

Early effects of low dose bevacizumab treatment may be highly heterogeneous within tumors and can be detected with DCE-MRI.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1918-1) contains supplementary material, which is available to authorized users.

Coadministration of Vascular Disrupting Agents and Nanomedicines to Eradicate Tumors from Peripheral and Central Regions

A strategy for enhancing the treatment efficacy of nanomedicines within the central region of solid tumors is developed by combining nanomedicines and free small-molecule vascular disrupting agents (VDAs). The nanomedicines (cis-diamminedichloroplatinum-loaded nanoparticles) primarily target cells at the tumor periphery whereas the free small-molecule VDA (combretastatin A4 disodium phosphate) efficiently kills the cancer cells within the central regions of the tumor.

Assessment of the interstitial fluid pressure of tumors by dynamic contrast-enhanced magnetic resonance imaging with contrast agents of different molecular weights

BACKGROUND

Cancer patients showing highly elevated interstitial fluid pressure (IFP) in the primary tumor may benefit from particularly aggressive treatment. There is some evidence that gadolinium diethylene-triamine penta-acetic acid (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) may be a useful non-invasive method for providing information on the IFP of tumors. The purpose of this preclinical study was to investigate whether any association between DCE-MRI-derived parametric images and tumor IFP can be strengthened by using MR contrast agents with higher molecular weights than that of Gd-DTPA.

MATERIAL AND METHODS

A-07 human melanoma xenografts were used as preclinical models of human cancer. Three contrast agents were compared: Gd-DTPA (0.55 kDa), P846 (3.5 kDa), and gadomelitol (6.5 kDa). A total of 46 tumors were subjected to DCE-MRI and subsequent measurement of IFP. Parametric images of K(trans) (the volume transfer constant of the contrast agent) and v(e) (the fractional distribution volume of the contrast agent) were produced by pharmacokinetic analysis of the DCE-MRI series.

RESULTS

Significant inverse correlations were found between median K(trans) and IFP for Gd-DTPA (p = 0.0076; R(2) = 0.46; n = 14) and P846 (p = 0.0042; R(2) = 0.45; n = 16), whereas there was no correlation between median K(trans) and IFP for gadomelitol (p > 0.05; n = 16). Significant correlation between median v(e) and IFP was not found for any of the contrast agents (p > 0.05 for Gd-DTPA, P846, and gadomelitol).

CONCLUSION

K(trans) images, but not v(e) images, derived by pharmacokinetic analysis of DCE-MRI data for low-molecular-weight contrast agents may provide information on the IFP of tumors. Any association between K(trans) and IFP cannot be expected to be improved by using contrast agents with higher molecular weights than those of Gd-DTPA and P846.

Odyssey of a cancer nanoparticle: from injection site to site of action

No chemotherapeutic drug can be effective until it is delivered to its target site. Nano-sized drug carriers are designed to transport therapeutic or diagnostic materials from the point of administration to the drug's site of action. This task requires the nanoparticle carrying the drug to complete a journey from the injection site to the site of action. The journey begins with the injection of the drug carrier into the bloodstream and continues through stages of circulation, extravasation, accumulation, distribution, endocytosis, endosomal escape, intracellular localization and-finally-action. Effective nanoparticle design should consider all of these stages to maximize drug delivery to the entire tumor and effectiveness of the treatment.

Quantifying heterogeneity in human tumours using MRI and PET

Most tumours, even those of the same histological type and grade, demonstrate considerable biological heterogeneity. Variations in genomic subtype, growth factor expression and local microenvironmental factors can result in regional variations within individual tumours. For example, localised variations in tumour cell proliferation, cell death, metabolic activity and vascular structure will be accompanied by variations in oxygenation status, pH and drug delivery that may directly affect therapeutic response. Documenting and quantifying regional heterogeneity within the tumour requires histological or imaging techniques. There is increasing evidence that quantitative imaging biomarkers can be used in vivo to provide important, reproducible and repeatable estimates of tumoural heterogeneity. In this article we review the imaging methods available to provide appropriate biomarkers of tumour structure and function. We also discuss the significant technical issues involved in the quantitative estimation of heterogeneity and the range of descriptive metrics that can be derived. Finally, we have reviewed the existing clinical evidence that heterogeneity metrics provide additional useful information in drug discovery and development and in clinical practice.

Head and neck cancer as a model for advances in imaging prognosis, early assessment, and posttherapy evaluation

Novel noninvasive functional imaging methods are necessary to predict therapeutic outcome and thereby improve the ability to properly select patients for treatment with both conventional and targeted therapies, to better evaluate therapeutic effectiveness during the early phases of treatment, and to enhance a priori risk assessment for treatment induced toxicity. Functional metabolic imaging typically involves pretreatment baseline magnetic resonance imaging (MRI) and/or positron emission tomographic (PET) scans and performance of subsequent scans during and/or after treatment. Imaging parameter changes are routinely attributed to the intervening therapy and clinical outcomes subsequently correlated with these changes. The physiologic parameter(s) that best correlate with clinical outcome and the relative utility of MRI versus PET are unknown, however. Furthermore, tumor vascular physiology and metabolic parameters are heterogeneous and dynamic processes. Large daily fluctuations often occur in the absence of treatment. The magnitude of this temporal variability is not established for MRI or for PET. Routine and meaningful clinical application of functional imaging requires understanding and quantification of the intrinsic variability of the underlying biologic processes and a demonstration that treatment-induced changes exceed intrinsic temporal variation.

Quantifying tumor vascular heterogeneity with dynamic contrast-enhanced magnetic resonance imaging: a review

Tumor microvasculature possesses a high degree of heterogeneity in its structure and function. These features have been demonstrated to be important for disease diagnosis, response assessment, and treatment planning. The exploratory efforts of quantifying tumor vascular heterogeneity with DCE-MRI have led to promising results in a number of studies. However, the methodological implementation in those studies has been highly variable, leading to multiple challenges in data quality and comparability. This paper reviews several heterogeneity quantification methods, with an emphasis on their applications on DCE-MRI pharmacokinetic parametric maps. Important methodological and technological issues in experimental design, data acquisition, and analysis are also discussed, with the current opportunities and efforts for standardization highlighted.

Magnetic resonance imaging of tumor necrosis

BACKGROUND

The prognostic and predictive value of magnetic resonance (MR) investigations in clinical oncology may be improved by implementing strategies for discriminating between viable and necrotic tissue in tumors. The purpose of this preclinical study was to investigate whether the extent of necrosis in tumors can be assessed by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and/or T(2)-weighted MR imaging.

MATERIAL AND METHODS

Three amelanotic human melanoma xenograft lines differing substantially in tumor necrotic fraction, necrotic pattern, extracellular volume fraction, and blood perfusion were used as experimental models of human cancer. MRI was performed at 1.5 T and a spatial resolution of 0.23 × 0.47 × 2.0 mm(3). Gadolinium diethylene-triamine penta-acetic acid (Gd-DTPA) was used as contrast agent. Plots of Gd-DTPA concentration versus time were generated for each voxel, and three parameters were calculated for each curve: the extracellular volume fraction (ν(e)), the final slope (a), and the Gd-DTPA concentration at one minute after the contrast administration (C(1min)). Parametric images of ν(e), a, C(1min), and the signal intensity in T(2)-weighted images (SI(T2W)) were compared with the histology of the imaged tissue.

RESULTS

The ν(e), a, and C(1min) frequency distributions were significantly different for necrotic and viable tissue in all three tumor lines. By using adequate values of ν(e), a, and C(1min) to discriminate between necrotic and viable tissue, significant correlations were found between the fraction of necrotic tissue assessed by MRI and the fraction of necrotic tissue assessed by image analysis of histological preparations. On the other hand, the SI(T2W) frequency distributions did not differ significantly between necrotic and viable tissue in two of the three tumor lines.

CONCLUSION

Necrotic regions in tumor tissue can be identified in parametric images derived from DCE-MRI series, whereas T(2)-weighted images are unsuitable for detection of tumor necrosis.

Developing DCE-CT to quantify intra-tumor heterogeneity in breast tumors with differing angiogenic phenotype

The objective of this study is to evaluate the ability of dynamic contrast enhanced computed tomography (DCE-CT) to assess intratumor physiological heterogeneity in tumors with different angiogenic phenotypes. DCE-CT imaging was performed on athymic nude mice bearing xenograft wild type (MCF-7(neo)) and VEGF-transfected (MCF-7(VEGF)) tumors by using a clinical multislice CT, and compared to skeletal muscle. Parametrical maps of tumor physiology--perfusion (F), permeability-surface area (PS), fractional intravascular plasma (f(p)), and interstitial space (f(is))--were obtained by fitting the time-dependent contrast-enhanced curves to a two-compartmental kinetic model for each voxel (0.3 x 0.3 x 0.75 mm(3)). Mean physiological measurements were compared with (positron emission tomography (PET) imaging, and the spatial distribution of tumor vasculature was compared with histology. No statistically significant difference was found in mean physiological values of F, PS, and f(p) in MCF-7(neo) and muscle, while f(is) of MCF-7(neo) was a factor of two higher ( p < 0.04). MCF-7(neo) tumors also showed a radial heterogeneity with significant higher physiological values in periphery than those in middle and core regions ( p < 0.01 for all physiological parameters). MCF-7(VEGF) tumors demonstrated significant increases in all physiological parameters compared with MCF-7(neo) tumors, and a distinct saccular heterogeneous pattern compared with MCF-7(neo) and muscle. Both PET imaging and histological results showed good correlation with the above results for this same mouse model. No statistically significant difference was found in the mean perfusion and intravascular volume measured by PET imaging and DCE-CT. Increases in cross-sectional area of blood vessels ( p < 0.002) were observed in MCF-7(VEGF) tumors than MCF-7(neo), and their spatial distribution correlated well with the spatial distribution of f(p) obtained by DCE-CT. The results of this study demonstrated the feasibility of DCE-CT in quantification of spatial heterogeneity in tumor physiology in small animal models. Monitoring variations in the tumor environment using DCE-CT offers an in vivo tool for the evaluation and optimization of new therapeutic strategies.

Limitations of dynamic contrast-enhanced MRI in monitoring radiation-induced changes in the fraction of radiobiologically hypoxic cells in human melanoma xenografts

PURPOSE

To investigate the potential of gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in detecting radiation-induced changes in the fraction of radiobiologically hypoxic cells in A-07 human melanoma xenografts.

MATERIALS AND METHODS

A-07 tumors were randomly assigned to an unirradiated control group or a group given a single radiation dose of 20 Gy. DCE-MRI and measurement of fraction of hypoxic cells were performed immediately before and 24 h after the radiation exposure. Tumor images of E . F (E is the initial extraction fraction of Gd-DTPA and F is blood perfusion) and lambda (lambda is proportional to extracellular volume fraction) were produced by subjecting DCE-MRI series to Kety analysis. Fraction of hypoxic cells was measured by using a radiobiological assay based on the paired survival curve method.

RESULTS

Fraction of radiobiologically hypoxic cells was higher in irradiated tumors (26.2+/-5.8%) than in unirradiated tumors (7.5+/-2.7%) by a factor of 3.5+/-1.5 (P=0.0093), whereas only minor radiation-induced changes in E . F and lambda could be detected.

CONCLUSION

DCE-MRI does not seem to offer insight into the changes in fraction of radiobiologically hypoxic cells occurring in A-07 tumors within 24 h after irradiation with 20 Gy.

Tumor vascularity assessed by magnetic resonance imaging and intravital microscopy imaging

Gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is considered to be a useful method for characterizing the vascularity of tumors. However, detailed studies of experimental tumors comparing DCE-MRI-derived parametric images with images of the morphology and function of the microvascular network have not been reported. In this communication, we describe a novel MR-compatible mouse dorsal window chamber and report comparative DCE-MRI and intravital microscopy studies of A-07-GFP tumors xenografted to BALB/c nu/nu mice. Blood supply time (BST) images (i.e., images of the time from when arterial blood enters a tumor through the supplying artery until it reaches a vessel segment within the tumor) and morphologic images of the microvascular network were produced by intravital microscopy. Images of E.F (E is the initial extraction fraction of Gd-DTPA and F is perfusion) were produced by subjecting DCE-MRI series to Kety analysis. The E.F images mirrored the morphology (microvascular density) and the function (BST) of the microvascular networks well. Tumor regions showing high E.F values colocalized with tumor regions showing high microvascular density and low BST values. Significant correlations were found between E.F and microvascular density and between E.F and BST, both within and among tumors.

Dynamic contrast-enhanced magnetic resonance imaging of human melanoma xenografts with necrotic regions

PURPOSE

To investigate whether high-resolution images of necrotic regions in tumors can be derived from gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) series.

MATERIALS AND METHODS

E-13 human melanoma xenografts were used as preclinical models of human cancer. DCE-MRI was performed at a voxel size of 0.23 x 0.47 x 2.0 mm3 with the use of spoiled gradient recalled sequences. Tumor images of E . F (E is the initial extraction fraction of Gd-DTPA and F is blood perfusion) and lambda (the partition coefficient of Gd-DTPA, which is proportional to extracellular volume fraction) were produced by subjecting DCE-MRI series to Kety analysis, and these images were compared with histological preparations from the imaged slices.

RESULTS

Strong correlations were found between fraction of necrotic tissue and fraction of voxels with lambda > lambdaL for lambdaL values of 0.4 to 0.6. Binary lambda images differentiating between lambda values > lambdaL and lambda values < lambdaL were found to mirror necrotic regions well in tumors with large necroses. However, necrotic foci that were small compared with the voxel size were not detectable.

CONCLUSION

Clinically relevant images of necrotic tumor regions can be obtained for E-13 melanomas by subjecting Gd-DTPA-based DCE-MRI series to Kety analysis.

Assessment of extravascular extracellular space fraction in human melanoma xenografts by DCE-MRI and kinetic modeling

Tumor aggressiveness and response to therapy are influenced by the extravascular extracellular space fraction (EESF) of the malignant tissue. The EESF may, therefore, be an important prognostic parameter for cancer patients. The aim of this study was to investigate whether gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) can be used to assess the EESF of tumors. Amelanotic human melanoma xenografts (A-07, R-18) were used as preclinical models of human cancer. Images of E.F (E is the initial extraction fraction of Gd-DTPA and F is perfusion) and lambda (the partition coefficient of Gd-DTPA) were obtained by Kety analysis of DCE-MRI data. Our study was based on the hypothesis that lambda is governed by the EESF and is not influenced significantly by microvascular density (MVD) or blood perfusion. To test this hypothesis, we searched for correlations between lambda and E.F, MVD or EESF by comparing lambda images with E.F images, histological preparations from the imaged tissue and the radial heterogeneity in EESF obtained by invasive imaging. Positive correlations were found between lambda and EESF. Thus, median lambda was larger in A-07 tumors than in R-18 tumors by a factor of 4.2 (P<.00001), consistent with the histological observation that EESF is approximately fourfold larger in A-07 tumors than in R-18 tumors. The radial heterogeneity in lambda in A-07 and R-18 tumors was almost identical to the radial heterogeneity in EESF. Moreover, lambda was larger in tissue regions with high EESF than in tissue regions with low EESF in A-07 tumors (P=.048). On the other hand, significant correlations between lambda and MVD or E.F could not be detected. Consequently, Kety analysis of Gd-DTPA-based DCE-MRI series of xenografted tumors provides lambda images that primarily reflect the EESF of the tissue.

Dynamic MRI for imaging tumor microvasculature: comparison of susceptibility and relaxivity techniques in pelvic tumors

PURPOSE

To assess the reproducibility of intrinsic relaxivity and both relaxivity- and susceptibility-based dynamic contrast enhanced (DCE) MRI in pelvic tumors; to correlate kinetic parameters obtained and to assess whether acute antivascular effects are seen in response to cisplatin- or taxane-based chemotherapy.

MATERIALS AND METHODS

T1-weighted and T2*-weighted DCE-MRI and basal R2* measurements were performed on three consecutive days in women with gynecological tumors. The third scan was 21.0 (range 17.3-23.5) hours after the first cycle of chemotherapy. Kinetic parameter estimates were obtained and correlated between techniques. Test-retest reproducibility and response to treatment were assessed.

RESULTS

Relative blood volume (rBV) and relative blood flow (rBF) correlated strongly with transfer constant (Ktrans), kep, and the initial area under the gadopentetate dimeglumine (Gd-DTPA) concentration-time curve (IAUGC) (all P<0.01). The group 95% confidence interval (CI) for change was -10.8 to +12.1%; +/-5.1%; -9.5 to +10.5%; +/-7.5%; for Ktrans, ve, kep, and IAUGC, respectively, and +/-13.6%, +/-2.4%, +/-11.6%, and +/-11.0%, for rBV, mean transit time (MTT), rBF, and R2*, respectively. There were no significant acute changes in kinetic parameter estimates in response to treatment on group analysis, apart from a small decrease in ve.

CONCLUSION

The results confirm the dominant influence of flow on Ktrans in untreated gynecological tumors. There is no evidence of an acute, large magnitude antivascular effect caused by cisplatin- or taxane-based chemotherapy.

Assessment of microvascular density, extracellular volume fraction, and radiobiological hypoxia in human melanoma xenografts by dynamic contrast-enhanced MRI

PURPOSE

To investigate whether gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) may be a useful method for assessing fraction of radiobiologically hypoxic cells in tumors.

MATERIALS AND METHODS

A-07 and R-18 human melanoma xenografts were used as preclinical tumor models. DCE-MRI was performed at a voxel size of 0.23 x 0.47 x 2.0 mm(3). Tumor images of E . F (E is the initial extraction fraction of Gd-DTPA and F is blood perfusion) and lambda (the partition coefficient of Gd-DTPA) were produced by subjecting DCE-MRI series to Kety analysis. Microvascular density and extracellular volume fraction (ECVF) were determined by analysis of histological preparations. The fraction of radiobiologically hypoxic cells was measured by the paired survival curve method.

RESULTS

E . F correlated with microvascular density, and lambda correlated with ECVF. The fraction of hypoxic cells was approximately 6.5-fold higher in R-18 tumors than in A-07 tumors, consistent with the observation that A-07 tumors showed higher values for E . F and microvascular density and lower cell density (i.e., higher values for lambda and ECVF) than R-18 tumors.

CONCLUSION

E . F and lambda images obtained by Kety analysis of DCE-MRI series contain information that may be utilized to estimate the extent of radiobiological hypoxia in tumors.

Fluctuations in tumor blood perfusion assessed by dynamic contrast-enhanced MRI

Temporal heterogeneity in blood perfusion is a common phenomenon in tumors, but data characterizing the nature of the blood flow fluctuations are sparse. This study investigated the occurrence of blood flow fluctuations in A-07 melanoma xenografts by using gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced MRI (DCE-MRI). Each tumor was subjected to two DCE-MRI acquisitions separated by 1 hour. The data were processed by Kety analysis and resulted in two E.F images (E is the initial extraction fraction of Gd-DTPA and F is the perfusion) and two lambda images (lambda is the partition coefficient of Gd-DTPA) for each tumor. The E . F images were used to determine the changes in blood perfusion arising in the time between the two imaging sequences. The lambda images were used to control the reproducibility of the experimental procedure. The study showed that DCE-MRI with subsequent Kety analysis is a useful method for detection of blood flow fluctuations in A-07 tumors, and strongly suggested that the peripheral regions of A-07 tumors are more exposed to temporal changes in blood perfusion than are the central regions.

Effect of heterogeneous vasculature on interstitial transport within a solid tumor

Novel strategies for cancer treatment involving macromolecular therapeutic agents have been recently developed and show promising results. Inadequate and heterogeneous uptake in tumor tissue has been shown to be a major obstacle for these compounds in clinical cancer therapy. Such distributions have been difficult to account for in predictive models. A three-dimensional computational model was developed to investigate the role of heterogeneous vasculature on interstitial transport within a murine sarcoma. The model accounts for extravasation and extracellular transport in a porous media. Spatial variation of fluid filtration rate per unit volume of tissue and vascular permeability were estimated from a dynamic contrast-enhanced (DCE)-MRI data set. Fluid filtration (L(p)S/V) and permeability (PS/V) maps were embedded in a model of tumor tissue and used to predict interstitial fluid pressure (IFP) and fluid flow. As in previous studies, pressure profiles were predicted to be elevated within the tumor. The model predicted boundary-dependent variation in outwardly directed interstitial velocity with lower velocities predicted near the skin boundary. Simulated tissue distribution of a macromolecular albumin tracer (MW approximately 60 kDa) was found to be heterogeneous with lower concentrations predicted in certain central regions. Simulated distributions of Gd-DTPA tracer (MW approximately 0.57 kDa) were less heterogeneous than albumin tracer. In sensitivity analysis, predicted tracer uptake was enhanced by increasing vascular leakiness. Increasing the interstitial hydraulic conductivity relative to the surrounding tissue reduced the overall drug uptake.

Fluctuations in pO2 in irradiated human melanoma xenografts

Several studies have demonstrated that untreated tumors may show significant fluctuations in tissue oxygen tension (pO(2)). Radiation treatment may induce changes in the tumor microenvironment that alter the pO(2) fluctuation pattern. The purpose of the present study was to investigate whether pO(2) fluctuations may also occur in irradiated tumors. A-07 human melanoma xenografts were irradiated with single doses of 0, 5 or 10 Gy. Fluctuations in pO(2) were recorded with OxyLite probes prior to irradiation and 24 and 72 h after the radiation exposure. Radiation-induced changes in the tumor microenvironment (i.e. blood perfusion and extracellular volume fraction) were assessed by dynamic contrast-enhanced magnetic resonance imaging. Seventy-two hours after 10 Gy, tumor blood perfusion had decreased to approximately 40% of that prior to irradiation, whereas the extracellular volume fraction had increased by approximately 25%. Fluctuations in pO(2) were seen in most tumors, irrespective of radiation dose and time after irradiation. The mean pO(2), the number of fluctuations around the mean pO(2), the number of fluctuations around threshold pO(2) values of 1, 2, 3, 5, 7 and 10 mmHg, and the amplitude of the fluctuations were determined for each pO(2) trace. No significant differences were detected between irradiated and unirradiated tumors. The results showed that pO(2) fluctuations may occur in irradiated tumors and that the pO(2) fluctuation pattern in A-07 tumors exposed to 5 or 10 Gy is similar to that in untreated tumors. Consequently, these doses did not induce changes in the tumor microenvironment that were sufficient to cause detectable alterations in the pO(2) fluctuation pattern.

Assessment of fraction of radiobiologically hypoxic cells in human melanoma xenografts by dynamic contrast-enhanced MRI

A noninvasive method for assessment of the extent of hypoxia in experimental and human tumors is highly needed. In this study, the potential usefulness of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was investigated, using gadopentetate dimeglumine (Gd-DTPA) as contrast agent and A-07 human melanoma xenografts as tumor model. DCE-MRI was performed at a voxel size of 0.3 x 0.6 x 2.0 mm3 with spoiled gradient-recalled sequences. Images of E . F (E is the initial extraction fraction of Gd-DTPA and F is perfusion) and lambda (the partition coefficient of Gd-DTPA, which is proportional to extracellular volume fraction) were obtained by Kety analysis of DCE-MRI data. The study was based on the hypothesis that hypoxic tissue would have low E . F (i.e., poor oxygen supply) and/or low lambda (i.e., high cell density and, hence, high oxygen consumption rate). Twenty-two tumors were first subjected to DCE-MRI and then to measurement of fraction of hypoxic cells, using a radiobiological assay. E . F was found to be strongly correlated to fraction of hypoxic cells (P < 0.000001), whereas significant correlation between lambda and fraction of hypoxic cells could not be detected. It is thus possible that E . F may be a useful parameter for the extent of hypoxia in experimental and human tumors with physiologic properties similar to those of A-07 tumors. This possibility warrants further studies involving experimental tumors of several lines, as well as human tumors.

Changes in intratumor heterogeneity in blood perfusion in intradermal human melanoma xenografts during tumor growth assessed by DCE-MRI

The purpose of this study was to use dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) to search for systematic intratumor heterogeneity in blood perfusion in human melanoma xenografts growing intradermally in BALB/c-nu/nu mice. Six xenografted tumors of an amelanotic human melanoma line (A-07) were included in the study. DCE-MRI was performed daily for 5 days by using spoiled-gradient recalled sequences. Tumor images of E.F (E is initial extraction fraction and F is perfusion) were produced by subjecting DCE-MRI data to Kety analysis. E.F was used as a measure of tumor blood perfusion, since comparative studies have shown that E.F is closely related to blood perfusion in A-07 tumors. The E.F images indicated that the intratumor heterogeneity in blood perfusion was similar in all investigated tumors. The blood perfusion was low in the center of the tumors and increased toward the tumor periphery in the dorsal and ventral direction by a factor of 3-4, but not in the lateral and medial direction. The magnitude of the heterogeneity increased by a factor of approximately 2 during tumor growth. In conclusion, intradermal human melanoma xenografts show significant systematic intratumor heterogeneity in blood perfusion.

Intratumor heterogeneity in blood perfusion in orthotopic human melanoma xenografts assessed by dynamic contrast-enhanced magnetic resonance imaging

PURPOSE

To determine the intratumor heterogeneity in blood perfusion of orthotopic human melanoma xenografts by use of gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

MATERIALS AND METHODS

Orthotopic xenografts of an amelanotic human melanoma cell line (A-07) were scanned sagittally, coronally, and axially in three subsequent DCE-MRI sessions, using spoiled gradient recalled sequences, a voxel size of 0.31x0.62x2.0 mm3, and an interleaving acquisition method to avoid slice gaps. Tumor images of E . F (E is initial extraction fraction and F is perfusion) were produced by subjecting the DCE-MRI data to Kety analysis. E . F was used as a parameter for tumor blood perfusion, since E for Gd-DTPA is close to unity in A-07 tumors.

RESULTS

All A-07 tumors subjected to investigation showed anisotropic radial heterogeneity in blood perfusion. The blood perfusion was low in the center of the tumors and increased toward the tumor periphery in the cranial, dorsal, caudal, and ventral directions, but not in the lateral and medial directions. In addition, 9 of 10 tumors showed blood perfusion hot spots in central or nonperipheral regions. The hot spots differed significantly between tumors in size, shape, location, and intensity, and appeared to be governed by stochastic processes. This heterogeneity superimposed the radial heterogeneity, but did not overshadow it in any tumor.

CONCLUSION

Orthotopic human melanoma xenografts show significant intratumor heterogeneity in blood perfusion. This heterogeneity is made up of two distinctly different components, one stochastic and one nonstochastic radial component. The radial component is anisotropic and dominant and is superimposed by the stochastic component.