Imaging vascular physiology to monitor cancer treatment

Vascular damage in tumors: a key player in stereotactic radiation therapy?

The use of stereotactic radiation therapy (SRT) for cancer treatment has grown in recent years, showing excellent results for some tumors. The greatly increased doses per fraction in SRT compared to conventional radiotherapy suggest a 'new biology' that determines treatment outcome. Proposed mechanisms include significant damage to tumor blood vessels and enhanced antitumor immune responses, which are also vasculature-dependent. These ideas are mostly based on the results of radiation studies in animal models because direct observations in humans are limited. However, even preclinical findings are somewhat incomplete and result in ambiguous conclusions. Current evidence of vasculature-related mechanisms of SRT is reviewed. Understanding them could result in better optimization of SRT alone or in combination with immune or other cancer therapies.

The Anti-VEGF(R) Drug Discovery Legacy: Improving Attrition Rates by Breaking the Vicious Cycle of Angiogenesis in Cancer

Resistance to anti-vascular endothelial growth factor (VEGF) molecules causes lack of response and disease recurrence. Acquired resistance develops as a result of genetic/epigenetic changes conferring to the cancer cells a drug resistant phenotype. In addition to tumor cells, tumor endothelial cells also undergo epigenetic modifications involved in resistance to anti-angiogenic therapies. The association of multiple anti-angiogenic molecules or a combination of anti-angiogenic drugs with other treatment regimens have been indicated as alternative therapeutic strategies to overcome resistance to anti-angiogenic therapies. Alternative mechanisms of tumor vasculature, including intussusceptive microvascular growth (IMG), vasculogenic mimicry, and vascular co-option, are involved in resistance to anti-angiogenic therapies. The crosstalk between angiogenesis and immune cells explains the efficacy of combining anti-angiogenic drugs with immune check-point inhibitors. Collectively, in order to increase clinical benefits and overcome resistance to anti-angiogenesis therapies, pan-omics profiling is key.

Imaging methods to evaluate tumor microenvironment factors affecting nanoparticle drug delivery and antitumor response

Standard small molecule and nanoparticulate chemotherapies are used for cancer treatment; however, their effectiveness remains highly variable. One reason for this variable response is hypothesized to be due to nonspecific drug distribution and heterogeneity of the tumor microenvironment, which affect tumor delivery of the agents. Nanoparticle drugs have many theoretical advantages, but due to variability in tumor microenvironment (TME) factors, the overall drug delivery to tumors and associated antitumor response are low. The nanotechnology field would greatly benefit from a thorough analysis of the TME factors that create these physiological barriers to tumor delivery and treatment in preclinical models and in patients. Thus, there is a need to develop methods that can be used to reveal the content of the TME, determine how these TME factors affect drug delivery, and modulate TME factors to increase the tumor delivery and efficacy of nanoparticles. In this review, we will discuss TME factors involved in drug delivery, and how biomedical imaging tools can be used to evaluate tumor barriers and predict drug delivery to tumors and antitumor response.

Perfusion CT - A novel quantitative and qualitative imaging biomarker in gastric cancer

OBJECTIVES

The aim of this research was to examine whether Perfusion Computed Tomography (P-CT) can qualitatively and quantitatively help detect gastric cancer neoangiogenesis in vivo as well as treatment response evaluation. We attempted to explore which P-CT parameters are best used in neoangiogenesis and neoadjuvant therapy for most effective evaluation. We also tried to recognize a positive prediction value of P-CT in early responders and non-responders patients identification.

MATERIALS AND METHODS

Twenty-four patients with positive biopsy results and/or clinically proven gastric cancer were enrolled in the P-CT exam. Patients were qualified for systemic treatment (16 patients received chemotherapy and 8 patients received radiochemotherapy). The baseline Perfusion-CT exam and after neoadjuvant treatment Perfusion-CT exam were conducted using a 64-row GE tomograph based on a deconvolution model in first-pass protocol perfusion. The P-CT examined the following parameters: Blood Flow (BF), Blood Volume (BV), Mean Transit Time (MTT) and Permeability Surface (PS). Positive clinical response to neoadjuvant treatment (CHT and RCT) was defined as tumor size reduction 25% or more.

RESULTS

Tumor dimension reduction after neoadjuvant therapy was significantly correlated with the BF and the PS. Neoadjuvant therapy was more effective for patients with higher output BF and PS values. We did not register a significant relationship between BV and MTT parameters and tumor dimension reduction. Patients with a positive treatment response showed a decrease in BF, BV and PS perfusion parameters with an increase in MTT.

CONCLUSIONS

P-CT examination allows a noninvasive neoangiogenesis assessment in vivo, leading to early identification of responding and non-responding patients. As a standard procedure, a full evaluation of treatment response should include a P-CT exam assessing neoangiogenesis.

High and low frequency subharmonic imaging of angiogenesis in a murine breast cancer model

This project compared quantifiable measures of tumor vascularity obtained from contrast-enhanced high frequency (HF) and low frequency (LF) subharmonic ultrasound imaging (SHI) to 3 immunohistochemical markers of angiogenesis in a murine breast cancer model (since angiogenesis is an important marker of malignancy and the target of many novel cancer treatments). Nineteen athymic, nude, female rats were implanted with 5×10(6) breast cancer cells (MDA-MB-231) in the mammary fat pad. The contrast agent Definity (Lantheus Medical Imaging, N Billerica, MA) was injected in a tail vein (dose: 180μl/kg) and LF pulse-inversion SHI was performed with a modified Sonix RP scanner (Analogic Ultrasound, Richmond, BC, Canada) using a L9-4 linear array (transmitting/receiving at 8/4MHz in SHI mode) followed by HF imaging with a Vevo 2100 scanner (Visualsonics, Toronto, ON, Canada) using a MS250 linear array transmitting and receiving at 24MHz. The radiofrequency data was filtered using a 4th order IIR Butterworth bandpass filter (11-13MHz) to isolate the subharmonic signal. After the experiments, specimens were stained for endothelial cells (CD31), vascular endothelial growth factor (VEGF) and cyclooxygenase-2 (COX-2). Fractional tumor vascularity was calculated as contrast-enhanced pixels over all tumor pixels for SHI, while the relative area stained over total tumor area was calculated from specimens. Results were compared using linear regression analysis. Out of 19 rats, 16 showed tumor growth (84%) and 11 of them were successfully imaged. HF SHI demonstrated better resolution, but weaker signals than LF SHI (0.06±0.017 vs. 0.39±0.059; p<0.001). The strongest overall correlation in this breast cancer model was between HF SHI and VEGF (r=-0.38; p=0.03). In conclusion, quantifiable measures of tumor neovascularity derived from contrast-enhanced HF SHI appear to be a better method than LF SHI for monitoring angiogenesis in a murine xenograft model of breast cancer (corresponding in particular to the expression of VEGF); albeit based on a limited sample size.

Dependence of DCE-MRI biomarker values on analysis algorithm

Background

Dynamic contrast-enhanced MRI (DCE-MRI) biomarkers have proven utility in tumors in evaluating microvascular perfusion and permeability, but it is unclear whether measurements made in different centers are comparable due to methodological differences.

Purpose

To evaluate how commonly utilized analytical methods for DCE-MRI biomarkers affect both the absolute parameter values and repeatability.

Materials and Methods

DCE-MRI was performed on three consecutive days in twelve rats bearing C6 xenografts. Endothelial transfer constant (K^trans), extracellular extravascular space volume fraction (v_e), and contrast agent reflux rate constant (k_ep) measures were computed using: 2-parameter (“Tofts” or “standard Kety”) vs. 3-parameter (“General Kinetic” or “extended Kety”) compartmental models (including blood plasma volume fraction (v_p) with 3-parameter models); individual- vs. population-based vascular input functions (VIFs); and pixel-by-pixel vs. whole tumor-ROI. Variability was evaluated by within-subject coefficient of variation (wCV) and variance components analyses.

Results

DCE-MRI absolute parameter values and wCVs varied widely by analytical method. Absolute parameter values ranged, as follows, median K^trans, 0.09–0.18 min^-1; k_ep, 0.51–0.92 min^-1; v_e, 0.17–0.23; and v_p, 0.02–0.04. wCVs also varied widely by analytical method, as follows: mean K^trans, 32.9–61.9%; k_ep, 11.6–41.9%; v_e, 16.1–54.9%; and v_p, 53.9–77.2%. K^trans and k_ep values were lower with 3- than 2-parameter modeling (p<0.0001); k_ep and v_p were lower with pixel- than whole-ROI analyses (p<0.0006). wCVs were significantly smaller for v_e, and larger for k_ep, with individual- than population-based VIFs.

Conclusions

DCE-MRI parameter values and repeatability can vary widely by analytical methodology. Absolute values of DCE-MRI biomarkers are unlikely to be comparable between different studies unless analyses are carefully standardized.

Reproducibility of magnetic resonance perfusion imaging

Dynamic MR biomarkers (T2*-weighted or susceptibility-based and T1-weighted or relaxivity-enhanced) have been applied to assess tumor perfusion and its response to therapies. A significant challenge in the development of reliable biomarkers is a rigorous assessment and optimization of reproducibility. The purpose of this study was to determine the measurement reproducibility of T1-weighted dynamic contrast-enhanced (DCE)-MRI and T2*-weighted dynamic susceptibility contrast (DSC)-MRI with two contrast agents (CA) of different molecular weight (MW): gadopentetate (Gd-DTPA, 0.5 kDa) and Gadomelitol (P792, 6.5 kDa). Each contrast agent was tested with eight mice that had subcutaneous MDA-MB-231 breast xenograft tumors. Each mouse was imaged with a combined DSC-DCE protocol three times within one week to achieve measures of reproducibility. DSC-MRI results were evaluated with a contrast to noise ratio (CNR) efficiency threshold. There was a clear signal drop (>95% probability threshold) in the DSC of normal tissue, while signal changes were minimal or non-existent (<95% probability threshold) in tumors. Mean within-subject coefficient of variation (wCV) of relative blood volume (rBV) in normal tissue was 11.78% for Gd-DTPA and 6.64% for P792. The intra-class correlation coefficient (ICC) of rBV in normal tissue was 0.940 for Gd-DTPA and 0.978 for P792. The inter-subject correlation coefficient was 0.092. Calculated K^trans from DCE-MRI showed comparable reproducibility (mean wCV, 5.13% for Gd-DTPA, 8.06% for P792). ICC of K^trans showed high intra-subject reproducibility (ICC = 0.999/0.995) and inter-subject heterogeneity (ICC = 0.774). Histograms of K^trans distributions for three measurements had high degrees of overlap (sum of difference of the normalized histograms <0.01). These results represent homogeneous intra-subject measurement and heterogeneous inter-subject character of biological population, suggesting that perfusion MRI could be an imaging biomarker to monitor or predict response of disease.

MRI of blood-brain barrier permeability in cerebral ischemia

Quantitative measurement of blood-brain barrier (BBB) permeability using MRI and its application to cerebral ischemia are reviewed. Measurement of BBB permeability using MRI has been employed to evaluate ischemic damage during acute and subacute phases of stroke and to predict hemorrhagic transformation. There is also an emerging interest on the development and use of MRI to monitor vascular structural changes and angiogenesis during stroke recovery. In this review, we describe MRI BBB permeability and susceptibility-weighted MRI measurements and its applications to evaluate ischemic damage during the acute and subacute phases of stroke and vascular remodeling during stroke recovery.

Real time dynamic imaging and current targeted therapies in the war on cancer: a new paradigm

In biology, as every science student is made to learn, ontology recapitulates phylogeny. In medicine, however, oncology recapitulates polemology, the science of warfare: The medical establishment is transitioning from highly toxic poisons that kill rapidly dividing normal and malignant cells with little specificity to tailored therapies that target the tumors with the lethality of the therapeutic warhead. From the advent of the information age with the incorporation of high-tech intelligence, reconnaissance, and surveillance has resulted in "data fusion" where a wide range of information collected in near real-time can be used to redesign most of the treatment strategies currently used in the clinic. The medical community has begun to transition from the 'black box' of tumor therapy based solely on the clinical response to the 'glass box' of dynamic imaging designed to bring transparency to the clinical battlefield during treatment, thereby informing the therapeutic decision to 'retreat or repeat'. The tumor microenvironment is dynamic, constantly changing in response to therapeutic intervention, and therefore the therapeutic assessment must map to this variable and ever-changing landscape with dynamic and non-static imaging capabilities. The path to personalized medicine will require incorporation and integration of dynamic imaging at the bedside into clinical practice for real-time, interactive assessment of response to targeted therapies. The application of advanced real time imaging techniques along with current molecularly targeted anticancer therapies which alter cellular homeostasis and microenvironment can enhance therapeutic interventions in cancer patients and further improve the current status in clinical management of patients with advanced cancers.

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

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

Novel agents in renal carcinoma: a reality check

The discovery of the molecular mechanisms underlying development of renal cell carcinoma have allowed for the development of novel targeted therapy for treatment of this disease. Recently, multiple agents have become approved by regulatory authorities for the treatment of advanced renal cell carcinoma, including sunitinib, sorafenib, bevacizumab (with interferon alpha), pazopanib, temsirolimus and everolimus. While these therapies have generated excitement and have clearly altered the treatment paradigm, multiple limitations have been elucidated over time. These include but are not limited to the fact that treatment is not associated with complete responses, a significant number of patients are primarily refractory to treatment, and clinical trials mostly include clear cell histology. Furthermore, the role of these therapies in the treatment of brain metastases remains unclear and therapies can have considerable toxicities. RECIST criteria (Response Evaluation Criteria In Solid Tumors) can be inadequate for the assessment of these modalities' treatment efficacy, and biomarkers predictive of individual patient benefit have been elusive. This review summarizes the major clinical data and discusses these limitations.

CT and MRI findings correlate with the time-course of unresectable cavernous haemangioma of the liver after fractionated radiotherapy

We present the case of a 79-year-old female with symptomatic cavernous haemangioma of the liver. The patient had experienced progressive right lateral abdominal pain for years despite increased painkiller use. Surgical resection or transarterial embolisation was not recommended because of the patient's age, cardiovascular comorbidities and large tumour size. Therefore, the patient was treated with 3-dimensional conformal radiotherapy (RT) with a total dose of 30 Gy in 15 fractions. Following RT, the painkillers were tapered from the third month, and complete symptomatic remission was achieved after the ninth month. The measured tumour volume from serial images pre-RT and 3, 9 and 15 months post-RT was 400 ml, 372 ml, 185 ml and 140 ml, respectively. The most dramatic volumetric reduction was found between 3 and 9 months post-RT, whereas the change before or after this period was minimal. The time course of the radiological volumetric changes correlated with that of the clinical symptoms. In addition, the observed vascular changes on serial imaging studies were consistent with the assumed radiobiological effects after fractionated RT.

Tracer kinetic modelling in MRI: estimating perfusion and capillary permeability

The tracer-kinetic models developed in the early 1990s for dynamic contrast-enhanced MRI (DCE-MRI) have since become a standard in numerous applications. At the same time, the development of MRI hardware has led to increases in image quality and temporal resolution that reveal the limitations of the early models. This in turn has stimulated an interest in the development and application of a second generation of modelling approaches. They are designed to overcome these limitations and produce additional and more accurate information on tissue status. In particular, models of the second generation enable separate estimates of perfusion and capillary permeability rather than a single parameter K(trans) that represents a combination of the two. A variety of such models has been proposed in the literature, and development in the field has been constrained by a lack of transparency regarding terminology, notations and physiological assumptions. In this review, we provide an overview of these models in a manner that is both physically intuitive and mathematically rigourous. All are derived from common first principles, using concepts and notations from general tracer-kinetic theory. Explicit links to their historical origins are included to allow for a transfer of experience obtained in other fields (PET, SPECT, CT). A classification is presented that reveals the links between all models, and with the models of the first generation. Detailed formulae for all solutions are provided to facilitate implementation. Our aim is to encourage the application of these tools to DCE-MRI by offering researchers a clearer understanding of their assumptions and requirements.

MRI & MRS assessment of the role of the tumour microenvironment in response to therapy

MRI and MRS techniques are being applied to the characterisation of various aspects of the tumour microenvironment and to the assessment of tumour response to therapy. For example, kinetic parameters describing tumour blood vessel flow and permeability can be derived from dynamic contrast-enhanced MRI data and have been correlated with a positive tumour response to antivascular therapies. The ongoing development and validation of noninvasive, high-resolution anatomical/molecular MR techniques will equip us with the means to detect specific tumour biomarkers early on, and then to monitor the efficacy of cancer treatments efficiently and reliably, all within a clinically relevant time frame. Reliable tumour microenvironment imaging biomarkers will provide obvious advantages by enabling tumour-specific treatment tailoring and potentially improving patient outcome. However, for routine clinical application across many disease types, such imaging biomarkers must be quantitative, robust, reproducible, sufficiently sensitive and cost-effective. These characteristics are all difficult to achieve in practice, but image biomarker development and validation have been greatly facilitated by an increasing number of pertinent preclinical in vivo cancer models. Emphasis must now be placed on discovering whether the preclinical results translate into an improvement in patient care and, therefore, overall survival.

Imaging tumor perfusion and oxidative metabolism in patients with head-and-neck cancer using 1- [11C]-acetate PET during radiotherapy: preliminary results

BACKGROUND

A growing body of in vitro evidence links alterations of the intermediary metabolism in cancer to treatment outcome. This study aimed to characterize tumor oxidative metabolism and perfusion in vivo using dynamic positron emission tomography (PET) with 1- [(11)C]-acetate (ACE) during radiotherapy.

METHODS AND MATERIALS

Nine patients with head-and-neck cancer were studied. Oxidative metabolic rate (k(mono)) and perfusion (rF) of the primary tumors were assessed by dynamic ACE-PET at baseline and after 15, 30, and 55 Gy was delivered. Tumor glucose uptake (Tglu) was evaluated with [(18)F]-fluorodeoxyglucose PET at baseline. Patients were grouped into complete (CR, n = 6) and partial responders (PR, n = 3) to radiotherapy.

RESULTS

The 3 PR patients died within a median follow-up period of 33 months. Baseline k(mono) was almost twice as high in CR as in PR (p = 0.02) and Tglu was lower in CR than in PR (p = 0.04). k(mono) increased during radiotherapy in PR (p = 0.004) but remained unchanged in CR. There were no differences in rF between CR and PR at any dosage. k(mono) and rF were coupled in CR (p = 0.001), but not in PR.

CONCLUSIONS

This study shows that radiosensitive tumors might rely predominantly on oxidative metabolism for their bioenergetic needs. The impairment of oxidative metabolism in radioresistant tumors is potentially reversible, suggesting that therapies targeting the intermediary metabolism might improve treatment outcome.

Radionuclide imaging of perfusion and hypoxia

PURPOSE

We present a review of radionuclide imaging of tumour vascular physiology as it relates to angiogenesis. We focus on clinical trials in human subjects using PET and SPECT to evaluate tumour physiology, in particular blood flow and hypoxia.

METHODS

A systematic review of literature based on MEDLINE searches updated in February 2010 was performed.

RESULTS

Twenty-nine studies were identified for review: 14 dealt with (15)O-water PET perfusion imaging, while 8 dealt with (18)F-fluoromisonidazole PET hypoxia imaging. Five used SPECT methods. The studies varied widely in technical quality and reporting of methods.

CONCLUSIONS

A subset of radionuclide methods offers accurate quantitative scientific observations on tumour vascular physiology of relevance to angiogenesis and its treatment. The relationship between cellular processes of angiogenesis and changing physiological function remains poorly defined. The promise of quantitative functional imaging at high specificity and low administered dose sustains interest in radionuclide methods.

Morphological and functional MDCT: problem-solving tool and surrogate biomarker for hepatic disease clinical care and drug discovery in the era of personalized medicine

This article explains the significant role of morphological and functional multidetector computer tomography (MDCT) in combination with imaging postprocessing algorithms served as a problem-solving tool and noninvasive surrogate biomarker to effectively improve hepatic diseases characterization, detection, tumor staging and prognosis, therapy response assessment, and novel drug discovery programs, partial liver resection and transplantation, and MDCT-guided interventions in the era of personalized medicine. State-of-the-art MDCT depicts and quantifies hepatic disease over conventional CT for not only depicting lesion location, size, and extent but also detecting changes in tumor biologic behavior caused by therapy or tumor progression before morphologic changes. Color-encoded parameter display provides important functional information on blood flow, permeability, leakage space, and blood volume. Together with other relevant biomarkers and genomics, the imaging modality is being developed and validated as a biomarker to early response to novel, targeted anti-VEGF(R)/PDGFR or antivascular/angiogenesis agents as its parameters correlate with immunohistochemical surrogates of tumor angiogenesis and molecular features of malignancies. MDCT holds incremental value to World Health Organization response criteria and Response Evaluation Criteria in Solid Tumors in liver disease management. MDCT volumetric measurement of future remnant liver is the most important factor influencing the outcome of patients who underwent partial liver resection and transplantation. MDCT-guided interventional methods deliver personalized therapies locally in the human body. MDCT will hold more scientific impact when it is fused with other imaging probes to yield comprehensive information regarding changes in liver disease at different levels (anatomic, metabolic, molecular, histologic, and other levels).

Imaging as a surveillance tool in rectal cancer

Despite advances in diagnosis and treatment, half of patients with treated rectal cancer will die owing to recurrent disease. There is no evidence of benefit on survival from an intensive surveillance program, even if presymptomatic recurrent disease is detected. The aim of this article is to review the results described for the different imaging techniques in diagnosing rectal cancer recurrence in different sites and to discuss their relative clinical impact. The sensitivity of imaging techniques is related to the performance of the machines and the site being examined. Computed tomography is the most used technique owing to its availability, speed, panoramic images and ease of use, while MRI of the pelvis and the liver produces the highest resolution, sensitivity and specificity in these anatomical areas. Owing to its high cost, [(18)F] fluorodeoxyglucose-PET should be used as a third-level examination, a 'problem-solving' method when the site of recurrence is unknown or to rule out other possible sites of recurrence before a second surgery, and, finally, because it offers the possibility to investigate the whole body. The follow-up must be designed for individual patients, taking into account a number of factors. In the near future, whole-body imaging, probably by MRI, that is free from radiation will become the method of choice for screening for recurrent disease.

Functional imaging of colorectal cancer: positron emission tomography, magnetic resonance imaging, and computed tomography

In the past 10 years, overall survival and disease-free survival of patients with colorectal cancer (CRC) has improved substantially because of a combination of factors: (1) more accurate staging as a result of advances in imaging technology; (2) refinements in surgical technique; (3) 'curative' metastasectomy for patients with limited metastatic disease; (4) improvements in radiation therapy planning and greater precision of radiation therapy delivery; and (5) increasing chemotherapeutic options, including antiangiogenic and vascular targeting drugs. In this era of 'personalized medicine,' the increasingly individualized treatment of patients with CRC has highlighted the need for functional imaging techniques in addition to conventional anatomic-based imaging. This review discusses the contribution of positron emission tomography to the clinical management of CRC. In addition, evolving techniques such as dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), DCE computed tomography (perfusion CT), diffusion-weighted MRI, and blood oxygenation level-dependent MRI that might have a future role will be covered.

Tumor vasculature and microenvironment normalization: a possible mechanism of antiangiogenesis therapy

Tumor antiangiogenesis therapy has been in application for more than 30 years; however, its mechanism remains obscure. An intriguing hypothesis, which has recently gained acceptance, explores the possibility that antiangiogenesis therapy may transiently normalize tumor vasculature and its microenvironment, thus enhancing chemoradiotherapy efficacy. As the equilibrium between proangiogenesis and antiangiogenesis factors is perturbed in the tumor and tips to the former, tumor vasculature tends to exhibit abnormal structure and function. Abnormal vasculature is tightly associated with an uncharacteristic microenvironment, including uneven perfusion, hypoxia, and increased interstitial fluid pressure: This malignant microenvironment hinders the delivery of chemotherapeutics to tumor cells and desensitizes the malignant cells to radiation. Antiangiogenesis therapy can reverse the imbalance and transiently normalize this microenvironment and gives a new perspective for combining antiangiogenesis therapy and traditional chemoradiotherapy.

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.

PET Imaging of Angiogenesis

Angiogenesis is a highly-controlled process that is dependent on the intricate balance of both promoting and inhibiting factors, involved in various physiological and pathological processes. A comprehensive understanding of the molecular mechanisms that regulate angiogenesis has resulted in the design of new and more effective therapeutic strategies. Due to insufficient sensitivity to detect therapeutic effects by using standard clinical endpoints or by looking for physiological improvement, a multitude of imaging techniques have been developed to assess tissue vasculature on the structural, functional and molecular level. Imaging is expected to provide a novel approach to noninvasively monitor angiogenesis, to optimize the dose of new antiangiogenic agents and to assess the efficacy of therapies directed at modulation of the angiogenic process. All these methods have been successfully used preclinically and will hopefully aid in antiangiogenic drug development in animal studies. In this review article, the application of PET in angiogenesis imaging at both functional and molecular level will be discussed. For PET imaging of angiogenesis related molecular markers, we emphasize integrin alpha(v)beta(3), VEGF/VEGFR, and MMPs.

Imaging angiogenesis

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

What can be expected from nuclear medicine tomorrow?

Imaging can take advantage of developments in "omics" approaches and go from routine individual biomarkers to multiple-scale biomarker profiles. Imaging structural, functional, metabolic, cellular, and molecular changes will be made possible by multimodality hybrid techniques, such as positron emission tomography-magnetic resonance imaging. Imaging should predict treatment response, look at stratification for specific treatment modalities, and look at the "omic" characterization of an individual patient or a specific tumor. This should lead to the development of "personalized" medicine. In cancer radiotherapy, patient responses should be accurately predicted. In specific cases, proton and hadrontherapy will be further enhanced by the irradiation dose delivered to the tumors. For disseminated or metastatic disease, targeted radionuclide therapy is an effective addition to the arsenal against cancer. The clinical efficacy of radiolabeled antibodies has been clearly demonstrated in lymphoma as well as that of radiolabeled peptides derived from somatostatin in the treatment of neuroendocrine tumors. Preliminary studies now show interesting results in solid tumors, too. Even if the number of objective clinical responses based on tumor shrinkage is small, targeted radionuclide therapy increases progression-free survival or overall survival in some specific cases where tumor burden is small. Avenues for further improvement are multiple and include combination with other therapeutic modalities, development of new approaches (e.g., small molecules, pretargeting, and antibody alternatives). Using alpha-emitting radionuclides is another possibility for specific diseases, such as leukemias, multiple myeloma, or brain tumor remnants.

Imaging techniques to evaluate the response to treatment in oncology: current standards and perspectives

Response evaluation in solid tumours currently uses radiological imaging techniques to measure changes under treatment. Imaging requires a well-defined anatomical lesion to be viewed and relies on the measurement of a reduction in tumour size during treatment as the basis for presumed clinical benefit. However, with the development of anti-angiogenesis agents, anatomical imaging has became inappropriate as certain tumours would not reduce in size. Functional studies are therefore necessary and dynamic contrast enhanced magnetic resonance imaging (DCE-MRI), DCE-computed tomography (CT) and DCE-ultrasonography (US) are currently being evaluated for monitoring treatments. Diffusion-weighted MR imaging (DW-MRI) and magnetic resonance spectroscopy (MRS) are also capable of detecting changes in cell density and metabolite content within tumours. In this article, we review anatomical and functional criteria currently used for monitoring therapy. We review the published data on DCE-MRI, DCE-CT, DCE-US, DW-MRI and MRS. This literature review covers the following area: basic principles of the technique, clinical studies, reproducibility and repeatability, limits and perspectives in monitoring therapy. Anatomical criteria such as response evaluation criteria in solid tumours (RECIST) will require adaptation to employ not only new tools but also different complementary techniques such as functional imaging in order to monitor therapeutic effects of conventional and new anti-cancer agents.

Anti-VEGF therapy: the search for clinical biomarkers

Despite the large diffusion and rapid development of anti-VEGF therapy in clinical practice and in contrast to the consolidated evidence with imatinib and trastuzumab that demonstrated a direct correlation between pre-treatment target expression and drug activity, it is very difficult, at present, to identify validated and useful biomarkers to monitor the efficacy of these compounds and to appropriately select patients most likely to benefit from such treatments. However, emerging data suggest that this is not presently feasible for antiangiogenic drugs. Although tumoral and/or circulating VEGF levels have been associated with tumor progression and/or poor prognosis, to date, there is no validated evidence suggesting their role as potential predictive biomarkers of response to anti-VEGF therapy. Recently, many studies have documented promising results with the evaluation of circulating endothelial cells and/or progenitors, and the use of several imaging techniques, such as dynamic contrast-enhanced MRI, PET, dynamic CT scan and functional ultrasound. These preliminary data need a validation in larger prospective trials.

The expanding role of PET technology in the management of patients with colorectal cancer

The therapeutic options and subsequent survival of colorectal cancer (CRC) patients has increased substantially over recent years. While surgical excision of the primary cancer results in cure of approximately 50% of patients, recurrence and metastatic disease still remains a significant cause of death. Although resection of liver or lung metastases can result in cure, relapse rates remain high, indicating that patient selection needs improvement. Positron emission tomography (PET) technology has a great deal to offer with respect to CRC management, particularly in the setting of patient selection for metastasectomy and in the evaluation of possible recurrent disease, however it has not yet become a routine part of the management of all CRC patients. This review article aims to discuss the current and future implications of PET technology in the optimal management of CRC patients throughout their care pathway.

Correlation between tumor blood flow assessed by perfusion CT and effect of neoadjuvant therapy in advanced esophageal cancers

BACKGROUND

Redundant blood supply by angiogenesis is a great advantage for the continuous growth of solid tumors. However, with chemotherapy (ChT), it can be disadvantageous due to increased drug delivery. An objective, reproducible, and non-invasive method for evaluating tumor blood supply is offered by perfusion CT.

METHODS

Fifty-five advanced esophageal cancers (95% squamous cell carcinomas) were evaluated for hemodynamic parameters, including blood flow (BF), blood volume (BV), and mean transit time (MTT) of primary tumor by perfusion CT scan. Forty-six underwent neoadjuvant therapy (36 ChT and 10 chemoradiotherapy (CRT)) as primary treatment of the cancer and 36 underwent esophagectomy.

RESULTS

Clinicopathological parameters were not significantly associated with hemodynamic parameters, except for hematogenic metastasis, which was not frequent (seven patients), but significantly associated with high BF (P = 0.0165). High BF and low MTT correlated significantly with a good response by neoadjuvant therapies (P = 0.0004 and P = 0.0124), while BV did not. The patients with high BF more frequently underwent esophagectomy and displayed better prognosis than those with lower values, while neither BV nor MTT was associated with patient survival.

CONCLUSIONS

Tumor BF by perfusion CT can partly predict the effect of ChT and CRT and survival. Further large cohort studies in homogeneous patient groups will reveal its clinical usefulness.

Design of clinical trials of radiation combined with antiangiogenic therapy

Clinical trials showing longer survival when chemotherapy is combined with antiangiogenic agents (AAs) have led to growing interest in designing combined modality protocols that exploit abnormalities in tumor vasculature. Approved agents include bevacizumab, a recombinant monoclonal antibody that binds to vascular endothelial growth factor, and two small molecule multitargeted tyrosine kinase inhibitors of angiogenesis (SU11248 and BAY-43-9006) that have been approved for therapy of renal cancer. Targeting tumor vasculature has a strong biological rationale in radiation therapy, and preclinical studies consistently show an increase in radiosensitization with combined treatment. Preclinical studies indicate that excessive damage to tumor vasculature can result in radioresistance in some situations, and early clinical data suggest that treatment sequencing may be important when combining AAs with radiation. Radiation itself appears to antagonize any hypoxia that can be induced by long-term administration of AAs. The optimal biological doses of AAs with radiotherapy are unknown, and surrogate markers of efficacy remain to be validated. Early clinical trials should therefore include studies designed to identify mechanisms of interaction and increases in tumor hypoxia. This review highlights preclinical and early clinical data that are relevant for clinical trial design. Optimal radiation planning and delivery is required to minimize the volume of irradiated normal organs and to establish safe dose-volume parameters for phase II-III clinical trials.

Precision in measurements of perfusion and microvascular permeability withT1-weighted dynamic contrast-enhanced MRI

Dynamic contrast-enhanced MRI is used to estimate microvascular parameters by tracer kinetics analysis. The time for the contrast agent to travel from the artery to the tissue of interest (bolus arrival time (BAT)) is an important parameter that must be measured in such studies because inaccurate estimates or neglect of BAT contribute to inaccuracy in model fitting. Furthermore, although the precision with which these parameters are estimated is very important, it is rarely reported. To address these issues, two investigations were undertaken. First, simulated data were used to validate an independent method for estimation of BAT. Second, the adiabatic approximation to the tissue homogeneity model was fitted to experimental data acquired in prostate and muscle tissue of 22 patients with prostate cancer. A bootstrap error analysis was performed to estimate the precision of parameter estimates. The independent method of estimating BAT was found to be more accurate and precise than a model-fitting approach. Estimated precisions for parameters measured in the prostate gland were 14% for extraction fraction (median coefficient of variation), 19% for blood flow, 28% for permeability-surface area product, 35% for volume of the extravascular-extracellular space, and 36% for blood volume. Techniques to further reduce uncertainty are discussed.

Nonmetastatic renal-cell carcinoma: is it really possible to define rational guidelines for post-treatment follow-up?

Defining rational follow-up guidelines in patients treated for cancer is important, from both a medical and an economical perspective. Renal-cell carcinoma is reputed to be unpredictable in its course and only a few, and often contradictory, follow-up guidelines exist for patients treated for nonmetastatic renal-cell carcinoma. Recent advances in tumor biology have contributed to a better understanding of this cancer and have indicated that personalized follow-up regimens, based on tumor and host molecular characteristics, might be possible in the near future.