Planning study for available dose of hypoxic tumor volume using fluorine-18-labeled fluoromisonidazole positron emission tomography for treatment of the head and neck cancer

Advances and residual knowledge gaps in the neck management of head and neck squamous cell carcinoma patients with advanced nodal disease undergoing definitive (chemo)radiotherapy for their primary

PURPOSE

Substantial changes have been made in the neck management of patients with head and neck squamous cell carcinomas (HNSCC) in the past century. These have been fostered by changes in cancer epidemiology and technological progress in imaging, surgery, or radiotherapy, as well as disruptive concepts in oncology. We aimed to review changes in nodal management, with a focus on HNSCC patients with nodal involvement (cN+) undergoing (chemo)radiotherapy.

METHODS

A narrative review was conducted to review current advances and address knowledge gaps in the multidisciplinary management of the cN+ neck in the context of (chemo)radiotherapy.

RESULTS

Metastatic neck nodes are associated with poorer prognosis and poorer response to radiotherapy, and have therefore been systematically treated by surgery. Radical neck dissection (ND) has gradually evolved toward more personalized and less morbid approaches, i.e., from functional to selective ND. Omission of ND has been made feasible by use of positron-emission tomography/computed tomography to monitor the radiation response in cN+ patients. Human papillomavirus-driven oropharyngeal cancers and their cystic nodes have shown dramatically better prognosis than tobacco-related cancers, justifying a specific prognostic classification (AJCC) creation. Finally, considering the role of lymph nodes in anti-tumor immunity, de-escalation of ND and prophylactic nodal irradiation in combination are intense areas of investigation. However, the management of bulky cN3 disease remains an issue, as aggressive multidisciplinary strategies or innovative combined treatments have not yet significantly improved their prognosis.

CONCLUSION

Personalized neck management is an increasingly important aspect of the overall therapeutic strategies in cN+ HNSCC.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

PET Imaging of Tumor Hypoxia in Head and Neck Cancer: A Primer for Neuroradiologists

Tumor hypoxia is a known independent prognostic factor for adverse patient outcomes in those with head and neck cancer. Areas of tumor hypoxia have been found to be more radiation resistant than areas of tumor with normal oxygenation levels. Hypoxia imaging may serve to help identify the best initial treatment option and to assess intratreatment monitoring of tumor response in case treatment changes can be made. PET imaging is the gold standard method for imaging tumor hypoxia, with 18F-fluoromisonidazole the most extensively studied hypoxic imaging tracer. Newer tracers also show promise.

Prediction of Response in Head and Neck Tumor: Focus on Main Hot Topics in Research

Radiation therapy is a cornerstone in the treatment of head and neck cancer patients; actually, their management is based on clinical and radiological staging with all patients at the same stage treated in the same way. Recently the increasing knowledge in molecular characterization of head and neck cancer opens the way for a more tailored treatment. Patient outcomes could be improved by a personalized radiotherapy beyond technological and anatomical precision. Several tumor markers are under evaluation to understand their possible prognostic or predictive value. In this paper we discuss those markers specific for evaluate response to radiation therapy in head and neck cancer for a shift toward a biological personalization of radiotherapy.

PET/CT in the Evaluation of Hypoxia for Radiotherapy Planning in Head and Neck Tumors: Systematic Literature Review

PET/CT combines imaging at the molecular level along with imaging at the anatomic level, which, with the administration of a hypoxia-sensitive radiopharmaceutical, allows evaluation of tissue oxygenation. Methods: This work consisted of a systematic literature review that included websites, books, and articles dated from July 1997 to December 2019. The aim was to identify the PET radiopharmaceuticals best suited to the detection of cell hypoxia and to recognize the benefits for planning intensity-modulated radiation therapy (IMRT) and volumetric arc therapy (VMAT). Results: Hypoxia affects the likelihood of cure for head and neck tumors, reducing the success rate. Radiopharmaceuticals such as ¹⁸F-fluoromisonidazole, ¹⁸F-fluoroerythronitromidazole, and ¹⁸F-HX4 (¹⁸F-3-fluoro-2-(4-((2-nitro-1H-imidazol-1-yl)methyl)-1H-1,2,3-triazol-1-yl)propan-1-ol) allow the delineation of hypoxic subvolumes within the target volume to optimize IMRT/VMAT. Conclusion: Identification of hypoxic areas with PET/CT imaging and use of subsequent IMRT/VMAT allows for possible escalation of radiation dose in radioresistant subvolumes, with a consequent decrease in relapses and an increased likelihood of disease-free survival.

Hypoxia Imaging and Adaptive Radiotherapy: A State-of-the-Art Approach in the Management of Glioma

Severe hypoxia [oxygen partial pressure (pO₂) below 5–10 mmHg] is more frequent in glioblastoma multiforme (GBM) compared to lower-grade gliomas. Seminal studies in the 1950s demonstrated that hypoxia was associated with increased resistance to low–linear energy transfer (LET) ionizing radiation. In experimental conditions, the total radiation dose has to be multiplied by a factor of 3 to achieve the same cell lethality in anoxic situations. The presence of hypoxia in human tumors is assumed to contribute to treatment failures after radiotherapy (RT) in cancer patients. Therefore, a logical way to overcome hypoxia-induced radioresistance would be to deliver substantially higher doses of RT in hypoxic volumes delineated on pre-treatment imaging as biological target volumes (BTVs). Such an approach faces various fundamental, technical, and clinical challenges. The present review addresses several technical points related to the delineation of hypoxic zones, which include: spatial accuracy, quantitative vs. relative threshold, variations of hypoxia levels during RT, and availability of hypoxia tracers. The feasibility of hypoxia imaging as an assessment tool for early tumor response to RT and for predicting long-term outcomes is discussed. Hypoxia imaging for RT dose painting is likewise examined. As for the radiation oncologist's point of view, hypoxia maps should be converted into dose-distribution objectives for RT planning. Taking into account the physics and the radiobiology of various irradiation beams, preliminary in silico studies are required to investigate the feasibility of dose escalation in terms of normal tissue tolerance before clinical trials are undertaken.

Integrating Small Animal Irradiators withFunctional Imaging for Advanced Preclinical Radiotherapy Research

Translational research aims to provide direct support for advancing novel treatment approaches in oncology towards improving patient outcomes. Preclinical studies have a central role in this process and the ability to accurately model biological and physical aspects of the clinical scenario in radiation oncology is critical to translational success. The use of small animal irradiators with disease relevant mouse models and advanced in vivo imaging approaches offers unique possibilities to interrogate the radiotherapy response of tumors and normal tissues with high potential to translate to improvements in clinical outcomes. The present review highlights the current technology and applications of small animal irradiators, and explores how these can be combined with molecular and functional imaging in advanced preclinical radiotherapy research.

Current State of Image Guidance in Radiation Oncology: Implications for PTV Margin Expansion and Adaptive Therapy

Image guidance technology has evolved and seen widespread application in the past several decades. Advancements in the diagnostic imaging field have found new applications in radiation oncology and promoted the development of therapeutic devices with advanced imaging capabilities. A recent example is the development of linear accelerators that offer magnetic resonance imaging for real-time imaging and online adaptive planning. Volumetric imaging, in particular, offers more precise localization of soft tissue targets and critical organs which reduces setup uncertainty and permit the use of smaller setup margins. We present a review of the status of current imaging modalities available for radiation oncology and its impact on target margins and use for adaptive therapy.

Clinical imaging of hypoxia: Current status and future directions

Tissue hypoxia is a key feature of many important causes of morbidity and mortality. In pathologies such as stroke, peripheral vascular disease and ischaemic heart disease, hypoxia is largely a consequence of low blood flow induced ischaemia, hence perfusion imaging is often used as a surrogate for hypoxia to guide clinical diagnosis and treatment. Importantly, ischaemia and hypoxia are not synonymous conditions as it is not universally true that well perfused tissues are normoxic or that poorly perfused tissues are hypoxic. In pathologies such as cancer, for instance, perfusion imaging and oxygen concentration are less well correlated, and oxygen concentration is independently correlated to radiotherapy response and overall treatment outcomes. In addition, the progression of many diseases is intricately related to maladaptive responses to the hypoxia itself. Thus there is potentially great clinical and scientific utility in direct measurements of tissue oxygenation. Despite this, imaging assessment of hypoxia in patients is rarely performed in clinical settings. This review summarises some of the current methods used to clinically evaluate hypoxia, the barriers to the routine use of these methods and the newer agents and techniques being explored for the assessment of hypoxia in pathological processes.

Impact of different biologically-adapted radiotherapy strategies on tumor control evaluated with a tumor response model

Motivated by the capabilities of modern radiotherapy techniques and by the recent developments of functional imaging techniques, dose painting by numbers (DPBN) was proposed to treat tumors with heterogeneous biological characteristics. This work studies different DPBN optimization techniques for virtual head and neck tumors assessing tumor response in terms of cell survival and tumor control probability with a previously published tumor response model (TRM). Uniform doses of 2 Gy are redistributed according to the microscopic oxygen distribution and the density distribution of tumor cells in four virtual tumors with different biological characteristics. In addition, two different optimization objective functions are investigated, which: i) minimize tumor cell survival (OF_surv) or; ii) maximize the homogeneity of the density of surviving tumor cells (OF_std). Several adaptive schemes, ranging from single to daily dose optimization, are studied and the treatment response is compared to that of the uniform dose. The results show that the benefit of DPBN treatments depends on the tumor reoxygenation capability, which strongly differed among the set of virtual tumors investigated. The difference between daily (fraction by fraction) and three weekly optimizations (at the beginning of weeks 1, 3 and 4) was found to be small, and higher benefit was observed for the treatments optimized using OF_surv. This in silico study corroborates the hypothesis that DPBN may be beneficial for treatments of tumors which show reoxygenation during treatment, and that a few optimizations may be sufficient to achieve this therapeutic benefit.

Integration of PET/MR Hybrid Imaging into Radiation Therapy Treatment

Hybrid PET/MR imaging is in early development for treatment planning. This article briefly reviews research and clinical applications of PET/MR imaging in radiation oncology. With improvements in workflow, more specific tracers, and fast and robust acquisition protocols, PET/MR imaging will play an increasingly important role in better target delineation for treatment planning and have clear advantages in the evaluation of tumor response and in a better understanding of tumor heterogeneity. With advances in treatment delivery and the potential of integrating PET/MR imaging with research on radiomics for radiation oncology, quantitative and physiologic information could lead to more precise and personalized RT.

18 F-Fluoromisonidazole positron emission tomography/CT-guided volumetric-modulated arc therapy-based dose escalation for hypoxic subvolume in nasopharyngeal carcinomas: A feasibility study

BACKGROUND

The purpose of this study is to investigate the feasibility of a simultaneously integrated boost to the hypoxic subvolume of nasopharyngeal carcinomas (NCPs) under the guidance of ¹⁸ F-fluoromisonidazole (FMISO) positron emission tomography (PET)/CT using volumetric-modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) techniques.

METHODS

Eight patients with NPC were treated with simultaneous integrated boost-IMRT (treatment plan named IMRT70) with dose prescriptions of 70 Gy, 66 Gy, 60 Gy, and 54 Gy to the gross tumor volume (GTV), positive neck nodes, the planning target volume (PTV), and the clinically negative neck, respectively. Based on the same datasets, experimental plans with the same dose prescription plus a dose boost of 14 Gy (an escalation of 20% of the prescription dose) to the hypoxic volume target contoured on the pretreatment ¹⁸ F-FMISO PET/CT imaging were generated using IMRT and VMAT techniques, respectively (represented by IMRT84 and VMAT84). Two or more arcs (approximately 2-2.5 arcs, totally rotating angle <1000 degrees) were used in VMAT plans and 9 equally separated fields in IMRT plans. Dosimetric parameters, total monitor units, and delivery time were calculated for comparative study of plan quality and delivery efficiency between IMRT84 and VMAT84.

RESULTS

In experimental plans, hypoxic target volumes successfully received the prescribed dose of 84 Gy in compliance with other dose constraints with either the IMRT technique or the VMAT technique. In terms of the target coverage, dose homogeneity, and organs at risk (OAR) sparing, there was no statistically significant difference between the actual treatment plan of IMRT70 and experimental plans. The total monitor unit of VMAT84 (525.7 ± 39.8) was significantly less than IMRT70 (1171.5 ± 167; P = .001) and IMRT84 (1388.3 ± 151.0; P = .001) per fraction, with 55.1% and 62.1% reduction. The average machine delivery time was 3.5 minutes for VMAT plans in comparison with approximately 8 minutes for IMRT plans, resulting in a reduction factor of 56.2%. For experimental plans, the 3D gamma index average was over 98.0% with no statistical significant difference when a 3%/3 mm gamma passing rate criteria was used.

CONCLUSION

With the guidance of ¹⁸ F-FMISO PET/CT imaging, dose escalation to hypoxic zones within NPC could be achieved and delivered efficiently with the VMAT technique in comparison with the IMRT technique.

Hypoxia imaging and radiotherapy: bridging the resolution gap

Oxygen distribution is a major determinant of treatment success in radiotherapy, with well-oxygenated tumour regions responding by up to a factor of three relative to anoxic volumes. Conversely, tumour hypoxia is associated with treatment resistance and negative prognosis. Tumour oxygenation is highly heterogeneous and difficult to measure directly. The recent advent of functional hypoxia imaging modalities such as fluorine-18 fluoromisonidazole positron emission tomography have shown promise in non-invasively determining regions of low oxygen tension. This raises the prospect of selectively increasing dose to hypoxic subvolumes, a concept known as dose painting. Yet while this is a promising approach, oxygen-mediated radioresistance is inherently a multiscale problem, and there are still a number of substantial challenges that must be overcome if hypoxia dose painting is to be successfully implemented. Current imaging modalities are limited by the physics of such systems to have resolutions in the millimetre regime, whereas oxygen distribution varies over a micron scale, and treatment delivery is typically modulated on a centimetre scale. In this review, we examine the mechanistic basis and implications of the radiobiological oxygen effect, the factors influencing microscopic heterogeneity in tumour oxygenation and the consequent challenges in the interpretation of clinical hypoxia imaging (in particular fluorine-18 fluoromisonidazole positron emission tomography). We also discuss dose-painting approaches and outline challenges that must be addressed to improve this treatment paradigm.

Radiation Oncology--New Approaches in Squamous Cell Cancer of the Head and Neck

The many advances in radiotherapy for squamous cell cancer of the head and neck described in this article will have significant effects on the ultimate outcomes of patients who receive this treatment. The technological and clinical advances should allow one to maintain or improve disease control, while moderating the toxicity associated with head and neck radiation therapy.

On the Reliability of Automatic Volume Delineation in Low-Contrast [(18)F]FMISO-PET Imaging

Hypoxia is a marker of poor prognosis in malignant tumors independent from the selected therapeutic method and the therapy should be intensified in such tumors. Hypoxia imaging with positron emission tomography (PET) is limited by low contrast to noise ratios with every available tracer. In radiation oncology appropriate delineation is required to allow therapy and intensification. While manual segmentation results are highly dependent from experience and observers condition (high inter- and intra observer variability), threshold- and gradient-based algorithms for automatic segmentation frequently fail in low contrast data sets. Likewise, calibration of these algorithms using phantoms is not useful. Complex computational models such as swarm intelligence-based algorithms are promising tools for optimized segmentation results and allow observer independent interpretation of multimodal and multidimensional imaging data.

Evaluation of tumour hypoxia during radiotherapy using [18F]HX4 PET imaging and blood biomarkers in patients with head and neck cancer

Background and purpose

Increased tumour hypoxia is associated with a worse overall survival in patients with head and neck squamous cell carcinoma (HNSCC). The aims of this study were to evaluate treatment-associated changes in [¹⁸F]HX4-PET, hypoxia-related blood biomarkers, and their interdependence.

Material and methods

[¹⁸F]HX4-PET/CT scans of 20 patients with HNSCC were acquired at baseline and after ±20Gy of radiotherapy. Within the gross-tumour-volumes (GTV; primary and lymph nodes), mean and maximum standardized uptake values, the hypoxic fraction (HF) and volume (HV) were calculated. Also, the changes in spatial uptake pattern were evaluated using [¹⁸F]HX4-PET/CT imaging. For all patients, the plasma concentration of CAIX, osteopontin and VEGF was assessed.

Results

At baseline, tumour hypoxia was detected in 69 % (22/32) of the GTVs. During therapy, we observed a significant decrease in all image parameters. The HF decreased from 21.7 ± 19.8 % (baseline) to 3.6 ± 10.0 % (during treatment; P < 0.001). Only two patients had a HV > 1 cm³ during treatment, which was located for >98 % within the baseline HV. During treatment, no significant changes in plasma CAIX or VEGF were observed, while osteopontin was increased.

Conclusions

[¹⁸F]HX4-PET/CT imaging allows monitoring changes in hypoxia during (chemo)radiotherapy whereas the blood biomarkers were not able to detect a treatment-associated decrease in hypoxia.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-016-3429-y) contains supplementary material, which is available to authorized users.

Newer positron emission tomography radiopharmaceuticals for radiotherapy planning: an overview

Positron emission tomography-computed tomography (PET-CT) has changed cancer imaging in the last decade, for better. It can be employed for radiation treatment planning of different cancers with improved accuracy and outcomes as compared to conventional imaging methods. (18)F-fluorodeoxyglucose remains the most widely used though relatively non-specific cancer imaging PET tracer. A wide array of newer PET radiopharmaceuticals has been developed for targeted imaging of different cancers. PET-CT with such new PET radiopharmaceuticals has also been used for radiotherapy planning with encouraging results. In the present review we have briefly outlined the role of PET-CT with newer radiopharmaceuticals for radiotherapy planning and briefly reviewed the available literature in this regard.

PET/CT imaging-guided dose painting in radiation therapy

Application of functional imaging to radiotherapy (RT) is a rapidly expanding field with the development of new modalities and techniques. Functional imaging of PET in conjunction with RT provides new avenues towards the clinical application of dose painting - a new RT strategy delivering optimized dose redistribution according to the functional imaging information to further improve tumour control. Two prototypical strategies of dose painting are reviewed: dose painting by contours (DPBC) and dose painting by numbers (DPBN). DPBN set a linear correlation of the boost dose and image intensity of this same voxel while homogeneous dose is given to the subvolume contoured by a threshold created in PET images in DPBC. Both comply with strict organs at risk (OAR) constraints and are alternatives for boosting subvolumes in clinical practice. This review focuses on the rationale, target validation, dose prescription verification and evaluation and recent clinical achievements in the field of integrating PET imaging into RT treatment planning. Further research is necessary in order to investigate unresolved problems in its routine clinical application thoroughly.

[Metabolic tailoring in radiotherapy for head and neck cancer]

Radiotherapy based on functional imaging consists to deliver a heterogeneity dose based on biological proprieties. This approach is termed biologically conformal radiotherapy or dose painting with biological target volume inside the gross tumor volume. Diffusion-weighted magnetic resonance imaging (MRI) and dynamic contrast-enhanced MRI can also be used to define a specific biological target volume. Three main tracers are used: ((18)F)-fluorodeoxyglucose to target the hypermetabolism, ((18)F)-fluoromizonidazole and ((18)F)- fluoroazomycin arabinoside to target areas of hypoxia. In this review, we give a practical approach to achieving a treatment-guided radiotherapy molecular and the main issues raised by this imaging technique. Despite the provision of all the technological tools to the radiotherapist, this new therapeutic approach is still evaluated in clinical studies to demonstrate a real clinical benefit compared to radiotherapy based on anatomic imaging.

Hypoxia in head and neck cancer in theory and practice: a PET-based imaging approach

Hypoxia plays an important role in tumour recurrence among head and neck cancer patients. The identification and quantification of hypoxic regions are therefore an essential aspect of disease management. Several predictive assays for tumour oxygenation status have been developed in the past with varying degrees of success. To date, functional imaging techniques employing positron emission tomography (PET) have been shown to be an important tool for both pretreatment assessment and tumour response evaluation during therapy. Hypoxia-specific PET markers have been implemented in several clinics to quantify hypoxic tumour subvolumes for dose painting and personalized treatment planning and delivery. Several new radiotracers are under investigation. PET-derived functional parameters and tracer pharmacokinetics serve as valuable input data for computational models aiming at simulating or interpreting PET acquired data, for the purposes of input into treatment planning or radio/chemotherapy response prediction programs. The present paper aims to cover the current status of hypoxia imaging in head and neck cancer together with the justification for the need and the role of computer models based on PET parameters in understanding patient-specific tumour behaviour.

Fusion of multi-tracer PET images for dose painting

PET imaging with FluoroDesoxyGlucose (FDG) tracer is clinically used for the definition of Biological Target Volumes (BTVs) for radiotherapy. Recently, new tracers, such as FLuoroThymidine (FLT) or FluoroMisonidazol (FMiso), have been proposed. They provide complementary information for the definition of BTVs. Our work is to fuse multi-tracer PET images to obtain a good BTV definition and to help the radiation oncologist in dose painting. Due to the noise and the partial volume effect leading, respectively, to the presence of uncertainty and imprecision in PET images, the segmentation and the fusion of PET images is difficult. In this paper, a framework based on Belief Function Theory (BFT) is proposed for the segmentation of BTV from multi-tracer PET images. The first step is based on an extension of the Evidential C-Means (ECM) algorithm, taking advantage of neighboring voxels for dealing with uncertainty and imprecision in each mono-tracer PET image. Then, imprecision and uncertainty are, respectively, reduced using prior knowledge related to defects in the acquisition system and neighborhood information. Finally, a multi-tracer PET image fusion is performed. The results are represented by a set of parametric maps that provide important information for dose painting. The performances are evaluated on PET phantoms and patient data with lung cancer. Quantitative results show good performance of our method compared with other methods.

Molecular imaging of tumor hypoxia with positron emission tomography

The problem of tumor hypoxia has been recognized and studied by the oncology community for over 60 years. From radiation and chemotherapy resistance to the increased risk of metastasis, low oxygen concentrations in tumors have caused patients with many types of tumors to respond poorly to conventional cancer therapies. It is clear that patients with high levels of tumor hypoxia have a poorer overall treatment response and that the magnitude of hypoxia is an important prognostic factor. As a result, the development of methods to measure tumor hypoxia using invasive and noninvasive techniques has become desirable to the clinical oncology community. A variety of imaging modalities have been established to visualize hypoxia in vivo. Positron emission tomography (PET) imaging, in particular, has played a key role for imaging tumor hypoxia because of the development of hypoxia-specific radiolabelled agents. Consequently, this technique is increasingly used in the clinic for a wide variety of cancer types. Following a broad overview of the complexity of tumor hypoxia and measurement techniques to date, this article will focus specifically on the accuracy and reproducibility of PET imaging to quantify tumor hypoxia. Despite numerous advances in the field of PET imaging for hypoxia, we continue to search for the ideal hypoxia tracer to both qualitatively and quantitatively define the tumor hypoxic volume in a clinical setting to optimize treatments and predict response in cancer patients.

A prospective clinical trial of tumor hypoxia imaging with 18F-fluoromisonidazole positron emission tomography and computed tomography (F-MISO PET/CT) before and during radiation therapy

To visualize intratumoral hypoxic areas and their reoxygenation before and during fractionated radiation therapy (RT), (18)F-fluoromisonidazole positron emission tomography and computed tomography (F-MISO PET/CT) were performed. A total of 10 patients, consisting of four with head and neck cancers, four with gastrointestinal cancers, one with lung cancer, and one with uterine cancer, were included. F-MISO PET/CT was performed twice, before RT and during fractionated RT of approximately 20 Gy/10 fractions, for eight of the 10 patients. F-MISO maximum standardized uptake values (SUVmax) of normal muscles and tumors were measured. The tumor-to-muscle (T/M) ratios of F-MISO SUVmax were also calculated. Mean SUVmax ± standard deviation (SD) of normal muscles was 1.25 ± 0.17, and SUVmax above the mean + 2 SD (≥1.60 SUV) was regarded as a hypoxic area. Nine of the 10 tumors had an F-MISO SUVmax of ≥1.60. All eight tumors examined twice showed a decrease in the SUVmax, T/M ratio, or percentage of hypoxic volume (F-MISO ≥1.60) at approximately 20 Gy, indicating reoxygenation. In conclusion, accumulation of F-MISO of ≥1.60 SUV was regarded as an intratumoral hypoxic area in our F-MISO PET/CT system. Most human tumors (90%) in this small series had hypoxic areas before RT, although hypoxic volume was minimal (0.0-0.3%) for four of the 10 tumors. In addition, reoxygenation was observed in most tumors at two weeks of fractionated RT.

Molecular PET imaging for biology-guided adaptive radiotherapy of head and neck cancer

Integration of molecular imaging PET techniques into therapy selection strategies and radiation treatment planning for head and neck squamous cell carcinoma (HNSCC) can serve several purposes. First, pre-treatment assessments can steer decisions about radiotherapy modifications or combinations with other modalities. Second, biology-based objective functions can be introduced to the radiation treatment planning process by co-registration of molecular imaging with planning computed tomography (CT) scans. Thus, customized heterogeneous dose distributions can be generated with escalated doses to tumor areas where radiotherapy resistance mechanisms are most prevalent. Third, monitoring of temporal and spatial variations in these radiotherapy resistance mechanisms early during the course of treatment can discriminate responders from non-responders. With such information available shortly after the start of treatment, modifications can be implemented or the radiation treatment plan can be adapted tailing the biological response pattern. Currently, these strategies are in various phases of clinical testing, mostly in single-center studies. Further validation in multicenter set-up is needed. Ultimately, this should result in availability for routine clinical practice requiring stable production and accessibility of tracers, reproducibility and standardization of imaging and analysis methods, as well as general availability of knowledge and expertise. Small studies employing adaptive radiotherapy based on functional dynamics and early response mechanisms demonstrate promising results. In this context, we focus this review on the widely used PET tracer (18)F-FDG and PET tracers depicting hypoxia and proliferation; two well-known radiation resistance mechanisms.

Interobserver agreement of qualitative analysis and tumor delineation of 18F-fluoromisonidazole and 3'-deoxy-3'-18F-fluorothymidine PET images in lung cancer

As the preparation phase of a multicenter clinical trial using (18)F-fluoro-2-deoxy-d-glucose ((18)F-FDG), (18)F-fluoromisonidazole ((18)F-FMISO), and 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) in non-small cell lung cancer (NSCLC) patients, we investigated whether 18 nuclear medicine centers would score tracer uptake intensity similarly and define hypoxic and proliferative volumes for 1 patient and we compared different segmentation methods.

METHODS

Ten (18)F-FDG, ten (18)F-FMISO, and ten (18)F-FLT PET/CT examinations were performed before and during curative-intent radiotherapy in 5 patients with NSCLC. The gold standards for uptake intensity and volume delineation were defined by experts. The between-center agreement (18 nuclear medicine departments connected with a dedicated network, SFMN-net [French Society of Nuclear Medicine]) in the scoring of uptake intensity (5-level scale, then divided into 2 levels: 0, normal; 1, abnormal) was quantified by κ-coefficients (κ). The volumes defined by different physicians were compared by overlap and κ. The uptake areas were delineated with 22 different methods of segmentation, based on fixed or adaptive thresholds of standardized uptake value (SUV).

RESULTS

For uptake intensity, the κ values between centers were, respectively, 0.59 for (18)F-FDG, 0.43 for (18)F-FMISO, and 0.44 for (18)F-FLT using the 5-level scale; the values were 0.81 for (18)F-FDG and 0.77 for both (18)F-FMISO and (18)F-FLT using the 2-level scale. The mean overlap and mean κ between observers were 0.13 and 0.19, respectively, for (18)F-FMISO and 0.2 and 0.3, respectively, for (18)F-FLT. The segmentation methods yielded significantly different volumes for (18)F-FMISO and (18)F-FLT (P < 0.001). In comparison with physicians, the best method found was 1.5 × maximum SUV (SUVmax) of the aorta for (18)F-FMISO and 1.3 × SUVmax of the muscle for (18)F-FLT. The methods using the SUV of 1.4 and the method using 1.5 × the SUVmax of the aorta could be used for (18)F-FMISO and (18)F-FLT. Moreover, for (18)F-FLT, 2 other methods (adaptive threshold based on 1.5 or 1.6 × muscle SUVmax) could be used.

CONCLUSION

The reproducibility of the visual analyses of (18)F-FMISO and (18)F-FLT PET/CT images was demonstrated using a 2-level scale across 18 centers, but the interobserver agreement was low for the (18)F-FMISO and (18)F-FLT volume measurements. Our data support the use of a fixed threshold (1.4) or an adaptive threshold using the aorta background to delineate the volume of increased (18)F-FMISO or (18)F-FLT uptake. With respect to the low tumor-on-background ratio of these tracers, we suggest the use of a fixed threshold (1.4).

High reproducibility of tumor hypoxia evaluated by 18F-fluoromisonidazole PET for head and neck cancer

Tumor hypoxia is well known to be radiation resistant. (18)F-fluoromisonidazole ((18)F-FMISO) PET has been used for noninvasive evaluation of hypoxia. Quantitative evaluation of (18)F-FMISO uptake is thus expected to play an important role in the planning of dose escalation radiotherapy. However, the reproducibility of (18)F-FMISO uptake has remained unclarified. We therefore investigated the reproducibility of tumor hypoxia by using quantitative analysis of (18)F-FMISO uptake.

METHODS

Eleven patients with untreated head and neck cancer underwent 2 (18)F-FMISO PET/CT scans ((18)F-FMISO(1) and (18)F-FMISO(2)) with a 48-h interval prospectively. All images were acquired at 4 h after (18)F-FMISO injection for 10 min. The maximum standardized uptake (SUVmax), tumor-to-blood ratio (TBR), and tumor-to-muscle ratio (TMR) of (18)F-FMISO uptake were statistically compared between the 2 (18)F-FMISO scans by use of intraclass correlation coefficients (ICCs). The hypoxic volume was calculated as the area with a TBR of greater than or equal to 1.5 or the area with a TMR of greater than or equal to 1.25 to assess differences in hypoxic volume between the 2 (18)F-FMISO scans. The distances from the maximum uptake locations of the (18)F-FMISO(1) images to those of the (18)F-FMISO(2) images were measured to evaluate the locations of (18)F-FMISO uptake.

RESULTS

The SUVmax (mean ± SD) for (18)F-FMISO(1) and (18)F-FMISO(2) was 3.16 ± 1.29 and 3.02 ± 1.12, respectively, with the difference between the 2 scans being 7.0% ± 4.6%. The TBRs for (18)F-FMISO(1) and (18)F-FMISO(2) were 2.98 ± 0.83 and 2.97 ± 0.64, respectively, with a difference of 9.9% ± 3.3%. The TMRs for (18)F-FMISO(1) and (18)F-FMISO(2) were 2.25 ± 0.71 and 2.19 ± 0.67, respectively, with a difference of 7.1% ± 5.3%. The ICCs for SUVmax, TBR, and TMR were 0.959, 0.913, and 0.965, respectively. The difference in hypoxic volume based on TBR was 1.8 ± 1.8 mL, and the difference in hypoxic volume based on TMR was 0.9 ± 1.3 mL, with ICCs of 0.986 and 0.996, respectively. The maximum uptake locations of the (18)F-FMISO(1) images were different from those of the (18)F-FMISO(2) images and were within the full width at half maximum of the PET/CT scanner, except in 1 case.

CONCLUSION

The values for (18)F-FMISO PET uptake and hypoxic volume in head and neck tumors between the 2 (18)F-FMISO scans were highly reproducible. Such high reproducibility of tumor hypoxia is promising for accurate radiation planning.

Hypoxia imaging with the nitroimidazole 18F-FAZA PET tracer: a comparison with OxyLite, EPR oximetry and 19F-MRI relaxometry

BACKGROUND AND PURPOSE

(18)F-FAZA is a nitroimidazole PET tracer that can provide images of tumor hypoxia. However, it cannot provide absolute pO(2) values. To qualify (18)F-FAZA PET, we compared PET images to pO(2) measured by OxyLite, EPR oximetry and (19)F-MRI.

MATERIALS AND METHODS

Male WAG/Rij rats grafted with rhabdomyosarcoma were used. Tumor oxygenation was modified by gas breathing (air or carbogen). The same day of PET acquisition, the pO(2) was measured in the same tumor either by OxyLite probes (measurement at 10 different sites), EPR oximetry using low frequency EPR or (19)F-relaxometry using 15C5 on an 11.7T MR system.

RESULTS

There was a good correlation between the results obtained by PET and EPR (R = 0.93). In the case of OxyLite, although a weaker correlation was observed (R = 0.55), the trend for two values to agree was still related to the inverse function theoretically predicted. For the comparison of (18)F-FAZA PET and (19)F-MRI, no change in T(1) was observed.

CONCLUSIONS

A clear correlation between (18)F-FAZA PET image intensities and tumor oxygenation was demonstrated, suggesting that (18)F-FAZA PET is a promising imaging technique to guide cancer therapy.

Small-molecule inhibitors of the HIF pathway and synthetic lethal interactions

INTRODUCTION

Activation of the hypoxia response pathway is a feature of many tumours and is one of the key mechanisms associated with tumour growth, chemoresistance and radioresistance. The major component of the hypoxia response pathway is the heterodimeric transcription factor, hypoxia-inducible factor (HIF), which is upregulated in many human cancers. Therefore, HIF is an attractive therapeutic target and several strategies have been developed to target it.

AREAS COVERED

Approaches used in targeting the hypoxia response pathway are discussed. Reviewed are agents that target upstream, directly and downstream of HIF, as well as some of the challenges in HIF-targeted therapy.

EXPERT OPINION

Many of the therapeutic agents that are in clinical use inhibit downstream HIF target genes, but ideally a molecule specific to HIF will have a more potent effect in inhibiting multiple HIF pathways. However, many anti-HIF molecules have multiple targets, which may increase non-specific cytotoxicity. In addition, many anti-HIF agents cannot discriminate between the different isoforms of HIF-α. So, it is important to assess whether targeting both HIF-1α and HIF-2α or each subunit selectively will provide better therapeutic effects.

Effect of smoking on oxygen delivery and outcome in patients treated with radiotherapy for head and neck squamous cell carcinoma--a prospective study

BACKGROUND

Head and neck cancer patients with high hemoglobin respond better to irradiation compared to patients with low hemoglobin possibly due to hypoxia induced radioresistance. The hemoglobin level is, however, a crude indicator of the amount of oxygen available to the tissue and may be influenced by a number of factors, smoking being of potential importance. The aim of the present study was to examine the effect of smoking on available oxygen to tumors and the effect on outcome in head and neck cancer patients treated with radiotherapy in a prospective study.

MATERIALS AND METHODS

A total of 232 consecutive patients with squamous cell carcinoma of the larynx, pharynx and oral cavity completed questionnaires on smoking habits prior to treatment. Venous blood samples were collected before and/or during treatment to determine the hemoglobin and carboxyhemoglobin level. Patients were treated with primary curative radiotherapy 62-68 Gy, 2 Gy/fx, 5 fx/week.

RESULTS

All but 12 patients had a history of smoking, 35 were long term quitters, 23 recent quitters, 54 moderate smokers and 108 heavy smokers (>1 pack/day). There was no relationship between total hemoglobin and carboxyhemoglobin, but effective hemoglobin and carboxyhemoglobin were linearly correlated. The amount of carboxyhemoglobin increased with increasing smoking status. Actuarial 5-year univariate analysis showed that heavy smokers had a significantly reduced probability of loco-regional control (44% vs. 65%, p = 0.001), disease-specific (56% vs. 77%, p = 0.003) and overall survival (39% vs. 66%, p = 0.0004) compared to non-smoking patients. Multivariate analyses showed that patients characterized as non-smokers, with low T and N classifications and high hemoglobin level had the best outcome measurements. A rise in carboxyhemoglobin significantly decreased the probability of loco-regional control and each additional pack year increased the risk of death. Smokers and former smokers develop secondary cancers.

CONCLUSION

The study showed a significant negative impact of smoking during radiotherapy for head and neck cancer and the risk of death was increased with each additional pack year of smoking. The effect on loco-regional control could be explained by a rise in carboxyhemoglobin level in smokers, e.g. a reduced oxygen supply to tumors. The data strongly advocate that smoking should be avoided in order to improve the therapeutic efficacy of radiotherapy and development of other smoking-related diseases and/or secondary cancers.

Accessing radiation response using hypoxia PET imaging and oxygen sensitive electrodes: a preclinical study

PURPOSE

Tumor hypoxia is a known cause of resistance to radiotherapy. The aim of this study was to investigate the prognostic value of hypoxia measured by (18)F-fluoroazomycin arabinoside ((18)F-FAZA) PET or the Eppendorf oxygen electrode in a pre-clinical tumor model.

MATERIAL/METHODS

Pretreatment (18)F-FAZA PET scans and blood sampling was conducted in 92 Female CDF1 mice with subcutaneous C3H mammary carcinomas grown in the right foot. Similarly, oxygenation status of 80 equivalent tumors was assessed using an invasive oxygen sensitive electrode. Tumors were then irradiated with a single dose of 55 Gy and local tumor control up to 90 days after the treatment was determined.

RESULTS

A significant difference in local tumor control between "more hypoxic" or "less hypoxic" groups separated either by a median (18)F-FAZA PET determined tumor-to-blood ratio (P=0.007; hazard ratio, HR=0.21 [95% CI: 0.06-0.74]), or the fraction of oxygen partial pressure (pO(2)) values ≤2.5 mmHg (P=0.018; HR=0.31 [95% CI: 0.11-0.87]), was found. Both assays showed that the more hypoxic tumors had significantly lower tumor control.

CONCLUSION

(18)F-FAZA PET analysis showed that pre treatment tumor hypoxia was prognostic of radiation response. Similar results were obtained when oxygenation status was assessed by the Eppendorf pO(2) Histograph. The results of this study support the role of (18)F-FAZA as a non-invasive prognostic marker for tumor hypoxia.

Hypoxic modification of radiotherapy in squamous cell carcinoma of the head and neck--a systematic review and meta-analysis

BACKGROUND

The importance of tumour hypoxia for the outcome of radiotherapy has been under investigation for decades. Numerous clinical trials modifying the hypoxic radioresistance in squamous cell carcinoma of the head and neck (HNSCC) have been conducted, but most have been inconclusive, partly due to a small number of patients in the individual trial. The present meta-analysis was, therefore, performed utilising the results from all clinical trials addressing the specific question of hypoxic modification in HNSCC undergoing curative intended primary radiotherapy alone.

METHODS

A systematic review of published and unpublished data identified 4805 patients with HNSCC treated in 32 randomized clinical trials, applying, normobaric oxygen or carbogen breathing (5 trials); hyperbaric oxygen (HBO) (9 trials); hypoxic radiosensitizers (17 trials) and HBO and radiosensitizer (1 trial). The trials were analysed with regard to the following endpoints: loco-regional control (32 trials), disease specific survival (30 trials), overall survival (29 trials), distant metastases (12 trials) and complications to radiotherapy (23 trials).

RESULTS

Overall hypoxic modification of radiotherapy in head and neck cancer did result in a significant improved therapeutic benefit. This was most dominantly observed when using the direct endpoint of loco-regional control with an odds ratio (OR) of 0.71, 95% cf.l. 0.63-0.80; p<0.001), but this was almost mirrored in the disease specific survival (OR: 0.73, 95% cf.l. 0.64-0.82; p<0.001), and to a lesser extent in the overall survival (OR: 0.87, 95% cf.l. 0.77-0.98; p=0.03). The risk of distant metastases was not significantly influenced although it appears to be less in the tumours treated with hypoxic modification (OR: 0.87, 95% cf.l. 0.69-1.09; p=0.22), whereas the radiation related late complications were not influenced by the overall use of hypoxic modifications (OR: 1.00, 95% cf.l. 0.82-1.23; p=0.96). The improvement in loco-regional control was found to be independent of the type of hypoxic modification. The trials have used different fractionation schedules, including large doses per fraction, which may result in relatively more hypoxia and greater benefit. However, analysis of HNSCC trials using conventional fractionation only, showed that the significant effect of hypoxic modification was maintained.

CONCLUSION

The meta-analysis thus demonstrates that there is level 1a evidence in favour of adding hypoxic modification to radiotherapy of squamous cell carcinomas of the head and neck.