Can FDG-PET assist in radiotherapy target volume definition of metastatic lymph nodes in head-and-neck cancer?

[PET technology: Latest advances and potential impact on radiotherapy]

Multimodal imaging has become a standard for planning radiation therapy via magnetic resonance imaging (MRI) or positron emission tomography (PET) in many cancers. However, its use is now old, and its impact has not been much discussed in light of technological improvements in imaging and advances in radiotherapy. However, in 20 years, the exclusive functional imaging has been replaced by hybrid imaging (functional and anatomical) with successive improvements (flight time, detector modifications, digitisation, etc.) have enabled us to go from centimetric resolution to the current 3 to 4mm resolution. This article will specifically review PET technology, its latest advances and the potential impact on radiotherapy, particularly head and neck cancers.

PET segmentation of bulky tumors: Strategies and workflows to improve inter-observer variability

Background

PET-based tumor delineation is an error prone and labor intensive part of image analysis. Especially for patients with advanced disease showing bulky tumor FDG load, segmentations are challenging. Reducing the amount of user-interaction in the segmentation might help to facilitate segmentation tasks especially when labeling bulky and complex tumors. Therefore, this study reports on segmentation workflows/strategies that may reduce the inter-observer variability for large tumors with complex shapes with different levels of user-interaction.

Methods

Twenty PET images of bulky tumors were delineated independently by six observers using four strategies: (I) manual, (II) interactive threshold-based, (III) interactive threshold-based segmentation with the additional presentation of the PET-gradient image and (IV) the selection of the most reasonable result out of four established semi-automatic segmentation algorithms (Select-the-best approach). The segmentations were compared using Jaccard coefficients (JC) and percentage volume differences. To obtain a reference standard, a majority vote (MV) segmentation was calculated including all segmentations of experienced observers. Performed and MV segmentations were compared regarding positive predictive value (PPV), sensitivity (SE), and percentage volume differences.

Results

The results show that with decreasing user-interaction the inter-observer variability decreases. JC values and percentage volume differences of Select-the-best and a workflow including gradient information were significantly better than the measurements of the other segmentation strategies (p-value<0.01). Interactive threshold-based and manual segmentations also result in significant lower and more variable PPV/SE values when compared with the MV segmentation.

Conclusions

FDG PET segmentations of bulky tumors using strategies with lower user-interaction showed less inter-observer variability. None of the methods led to good results in all cases, but use of either the gradient or the Select-the-best workflow did outperform the other strategies tested and may be a good candidate for fast and reliable labeling of bulky and heterogeneous tumors.

FDG PET/CT in cancer therapy monitoring

Objectives: We developed and tested a software tool for computer-assisted analysis of FDG-PET/CT in cancer therapy monitoring. The tool provides automatic semi-quantitative analysis of a baseline scan together with up to two follow-up scans (standardized uptake values, glycolytic volume). The tool also supports visual analysis by local spatial registration which allows display of tumor lesions with the same orientation in all scans. The tool’s stability and accuracy was tested at typical everyday image quality. Patients, methods: Ten unselected cancer patients in whom three FDG PET/CT scans had been performed were included. A total of 18 lesions were analyzed. Results: Automatic lesion tracking worked properly in all lesions but one. In this lesion local coregistration had to be adjusted manually tuwhich, however, is easily performed with the tool. Semi-automatic lesion segmentation and fully automatic semi-quantitative analysis worked properly in all cases. Computer-assisted analysis was significantly less time consuming than manual analysis. Conclusions: The novel software tool appears useful for analysis of FDGPET/ CT in cancer therapy monitoring in clinical routine patient care.

Uniform FDG-PET guided GRAdient Dose prEscription to reduce late Radiation Toxicity (UPGRADE-RT): study protocol for a randomized clinical trial with dose reduction to the elective neck in head and neck squamous cell carcinoma

BACKGROUND

In definitive radiation therapy for head and neck cancer, clinically uninvolved cervical lymph nodes are irradiated with a so-called 'elective dose' in order to achieve control of clinically occult metastases. As a consequence of high-resolution diagnostic imaging, occult tumor volume has significantly decreased in the last decades. Since the elective dose is dependent on occult tumor volume, the currently used elective dose may be higher than necessary. Because bilateral irradiation of the neck contributes to dysphagia, xerostomia and hypothyroidism in a dose dependent way, dose de-escalation to these regions can open a window of opportunity to reduce toxicity and improve quality of life after treatment.

METHODS

UPGRADE-RT is a multicenter, phase III, single-blinded, randomized controlled trial. Patients to be treated with definitive radiation therapy for a newly diagnosed stage T2-4 N0-2 M0 squamous cell carcinoma of the oropharynx, hypopharynx or larynx are eligible. Exclusion criteria are recurrent disease, oncologic surgery to the head and neck area, concomitant chemotherapy or epidermal growth factor receptor inhibitors. In total, 300 patients will be randomized in a 2:1 ratio to a treatment arm with or without de-escalation of the elective radiation dose and introduction of an intermediate dose-level for selected lymph nodes. Radiation therapy planning FDG-PET/CT-scans will be acquired to guide risk assessment of borderline-sized cervical nodes that can be treated with the intermediate dose level. Treatment will be given with intensity-modulated radiation therapy or volumetric arc therapy with simultaneous-integrated boost using an accelerated fractionation schedule, 33 fractions in 5 weeks. The primary endpoint is 'normalcy of diet' at 1 year after treatment (toxicity). The secondary endpoint is the actuarial rate of recurrence in electively irradiated lymph nodes at 2 years after treatment (safety).

DISCUSSION

The objective of the UPGRADE-RT trial is to investigate whether de-escalation of elective radiation dose and the introduction of an intermediate dose-level for borderline sized lymph nodes in the treatment of head and neck cancer will result in less radiation sequelae and improved quality of life after treatment without compromising the recurrence rate in the electively treated neck.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02442375 .

Importance of PET/CT scan use in planning radiation therapy for lymphoma

BACKGROUND

Radiation therapy is a key part of the combined modality treatment for Hodgkin's lymphoma (HL) and non-Hodgkin's lymphoma (NHL), which can achieve locoregional control of disease. The 3D-conformal radiation oncology can be extended-field (EFRT), involved-field (IFRT) and involved node (INRT). New techniques have resulted in a smaller radiation field and lower dose for critical organs such as lung heart and breast.

MATERIALS AND METHODS

In our research, we made a virtual simulation for one patient who was treated in four different radiotherapeutic techniques: mantle field (MFRT), EFRT, IFRT and INRT. After delineatiion we compared dose-volume histograms for each technique. The fusion of CT for planning radiotherapy with the initial PET/CT was made using Softver Xio 4.6 in the Focal program. The dose for all four techniques was 36Gy.

RESULTS

Our results support the use of PET/CT in radiation therapy planning. With IFRT and INRT, the burden on the organs at risk is less than with MFRT and EFRT. On the other hand, the dose distribution in the target volume is much better with the latter.

CONCLUSIONS

The aim of modern radiotherapy of HL and NHL is to reduce the intensity of treatment and therefore PET/CT should be used to reduce and not increase the amount of tissue receiving radiation.

Application of positron emission tomography/computed tomography in radiation treatment planning for head and neck cancers

18-fluorodeoxygluocose positron emission tomography/computed tomography (¹⁸FDG-PET/CT) provides significant information in multiple settings in the management of head and neck cancers (HNC). This article seeks to define the additional benefit of PET/CT as related to radiation treatment planning for squamous cell carcinomas (SCCs) of the head and neck through a review of relevant literature. By helping further define both primary and nodal volumes, radiation treatment planning can be improved using PET/CT. Special attention is paid to the independent benefit of PET/CT in targeting mucosal primaries as well as in detecting nodal metastases. The utility of PET/CT is also explored for treatment planning in the setting of SCC of unknown primary as PET/CT may help define a mucosal target volume by guiding biopsies for examination under anesthesia thus changing the treatment paradigm and limiting the extent of therapy. Implications of the use of PET/CT for proper target delineation in patients with artifact from dental procedures are discussed and the impact of dental artifact on CT-based PET attenuation correction is assessed. Finally, comment is made upon the role of PET/CT in the high-risk post-operative setting, particularly in the context of radiation dose escalation. Real case examples are used in these settings to elucidate the practical benefits of PET/CT as related to radiation treatment planning in HNCs.

PET-based radiation therapy planning

In this review, we review the literature on the use of PET in radiation treatment planning, with an emphasis on describing our institutional methodology (where applicable). This discussion is intended to provide other radiation oncologists with methodological details on the use of PET imaging for treatment planning in radiation oncology, or other oncologists with an introduction to the use of PET in planning radiation therapy.

Functional imaging in radiation therapy planning for head and neck cancer

Functional imaging and its application to radiotherapy (RT) is a rapidly expanding field with new modalities and techniques constantly developing and evolving. As technologies improve, it will be important to pay attention to their implementation. This review describes the main achievements in the field of head and neck cancer (HNC) with particular remarks on the unsolved problems.

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.

Pathology-based validation of FDG PET segmentation tools for volume assessment of lymph node metastases from head and neck cancer

PURPOSE

FDG PET is increasingly incorporated into radiation treatment planning of head and neck cancer. However, there are only limited data on the accuracy of radiotherapy target volume delineation by FDG PET. The purpose of this study was to validate FDG PET segmentation tools for volume assessment of lymph node metastases from head and neck cancer against the pathological method as the standard.

METHODS

Twelve patients with head and neck cancer and 28 metastatic lymph nodes eligible for therapeutic neck dissection underwent preoperative FDG PET/CT. The metastatic lymph nodes were delineated on CT (NodeCT) and ten PET segmentation tools were used to assess FDG PET-based nodal volumes: interpreting FDG PET visually (PETVIS), applying an isocontour at a standardized uptake value (SUV) of 2.5 (PETSUV), two segmentation tools with a fixed threshold of 40% and 50%, and two adaptive threshold based methods. The latter four tools were applied with the primary tumour as reference and also with the lymph node itself as reference. Nodal volumes were compared with the true volume as determined by pathological examination.

RESULTS

Both NodeCT and PETVIS showed good correlations with the pathological volume. PET segmentation tools using the metastatic node as reference all performed well but not better than PETVIS. The tools using the primary tumour as reference correlated poorly with pathology. PETSUV was unsatisfactory in 35% of the patients due to merging of the contours of adjacent nodes.

CONCLUSION

FDG PET accurately estimates metastatic lymph node volume, but beyond the detection of lymph node metastases (staging), it has no added value over CT alone for the delineation of routine radiotherapy target volumes. If FDG PET is used in radiotherapy planning, treatment adaptation or response assessment, we recommend an automated segmentation method for purposes of reproducibility and interinstitutional comparison.

The reliability of quantitative thresholding methods for PET aided delineation of GTVs in Head and Neck tumours

Introduction: PET–CT scans are commonly used for the purpose of gross tumour volume (GTV) delineation in head and neck cancers. Qualitative visual methods (QVM) are currently employed in most radiotherapy departments but these are subject to inter- and intra-observer variability. Quantitative thresholding methods which appear in the published literature are evaluated with respect to their reliability for delineation of GTVs in head and neck cancers.

Discussion: Image segmentation involves the application of a distinct value to all pixels or voxels in an image dataset. This is a complex process affected by numerous variables. Some of the following segmentation thresholds may be applied to automatically delineate specified regions. Standardised uptake value (SUV) is commonly used to apply a threshold for GTV delineation, however this leads to inappropriately large GTVs. A further common quantitative threshold is based on the maximum signal on the PET image relative to the background uptake, known as signal to background ratio (SBR). This method generates GTVs that correlate well with surgically removed tumour volumes. Applying a fixed threshold of a percentage of the maximal intensity uptake is also documented in the literature but was found to be unsuitable for the purpose of head and neck GTV contouring. Systems based on the physical features of the PET-CT images are also discussed and are found to produce very promising results.

Conclusion: A number of quantitative techniques are evaluated and currently the most suitable is found to be SBR, however even this method was not found to be entirely reliable. More promising techniques need further evaluation before they could be implemented clinically and a Radiation Oncologist or Nuclear Medicine Radiologist must still validate all GTVs produced by quantitative methods.

Defining PET standardized uptake value threshold for tumor delineation with metastatic lymph nodes in head and neck cancer

OBJECTIVE

Hot spots of F-18 fluorodeoxyglucose positron emission tomograms are variable in size according to window settings of standardized uptake values. The purpose of this study was to determine the standardized uptake value threshold that represents the target volume.

METHODS

Sixty-three patients who underwent fluorodeoxyglucose positron emission tomographic computed tomography and were diagnosed as having head and neck cancer with cervical lymphadenopathy were studied. The horizontal and vertical diameters of metastatic lymph nodes (LN-CT) were measured at the center of computed tomographic images. Of the corresponding nodes, the maximal standardized uptake value (SUVmax) and standardized uptake value profiles along the central horizontal and vertical axes were calculated on positron emission tomographic images (LN-PET). On the standardized uptake value profiles, the standardized uptake value levels (SUVeq) where the size of LN-PET was equivalent to the diameters of LN-CT were obtained. The regression formula between SUVeq and SUVmax was obtained. The regression formula of SUVeq was validated in subsequent 30 positron emission tomographic computed tomography studies.

RESULTS

The mean horizontal and vertical diameters of LN-CT were 14.9 and 16.4 mm, respectively. SUVmax ranged from 1.88 to 9.07, and SUVeq was between 1.16 and 6.42. The regression formula between SUVeq and SUVmax was as follows: SUVeq = 1.21 + 0.34 × SUVmax (coefficient of correlation: R = 0.69). The validation study resulted in a good correlation between the volume of lymph nodes on computed tomography and positron emission tomographic computed tomography (R(2) = 0.93).

CONCLUSIONS

The formula with a relatively high coefficient of correlation is considered to indicate that SUVeq is not constant, but is a complex of an absolute standardized uptake value and is proportional to SUVmax.

The role of nuclear medicine in modern therapy of cancer

Nuclear medicine is a multidisciplinary field that develops and uses instrumentation and tracers (radiopharmaceuticals) to study physiological processes and noninvasively diagnose, stage, and treat diseases. Particularly, it offers a unique means to study cancer biology in vivo and to optimize cancer therapy for individual patients. A tracer is either a radionuclide alone, such as iodine-131 or a radiolabel in a carrier molecule such as (18)F in fluorodeoxyglucose ((18)F-FDG), or other feasible radionuclide attached to a drug, a protein, or a peptide, which when introduced into the body, would accumulate in the tissue of interest. Nuclear medicine imaging, including single-photon emission computer tomography and positron emission tomography, can provide important quantitative and functional information about normal tissues or disease conditions, in contrast to conventional, anatomical imaging techniques such as ultrasound, computed tomography, or magnetic resonance imaging. For treatment, tumor-targeting agents, conjugated with therapeutic radionuclides, may be used to deposit lethal radiation at tumor sites. This review outlines the role of nuclear medicine in modern cancer therapy.

The Impact of (18)F-FDG PET CT Prior to Chemoradiotherapy for Stage III/IV Head and Neck Squamous Cell Carcinoma

Introduction. To determine the value of a FDG-PET-CT scan in patients with locally advanced head and neck squamous cell carcinoma (HNSCC) prior to chemoradiotherapy. Materials and Methods. Consecutive patients with stage III or IV HNSCC who had undergone a staging FDG-PET-CT scan prior to chemoradiotherapy between August 2008 and April 2011 were included. Clinical details and conventional imaging (CT and/or MRI) were, retrospectively, reviewed, a TNM stage was assigned, and levels of cervical lymph node involvement were documented. This process was repeated with the addition of FDG-PET-CT. Radiotherapy plans were reviewed for patients with an alteration identified on TNM staging and/or nodal level identification with FDG-PET-CT and potential alterations in radiotherapy planning were documented. Results. 55 patients were included in the analysis. FDG-PET-CT altered the TNM stage in 17/55 (31%) of patients, upstaging disease in 11 (20%) and downstaging in 6 (11%); distant metastases were identified by FDG-PET-CT in 1 (2%) patient. FDG-PET-CT altered the lymph node levels identified in 22 patients (40%), upclassifying disease in 16 (29%) and downclassifying in 6 (11%). Radiotherapy plans were judged retrospectively to have been altered by FDG-PET-CT in 10 patients (18%). Conclusions. The use of FDG-PET-CT potentially impacts upon both treatment decisions and radiotherapy planning.

Evaluation of a metal artifact reduction technique in tonsillar cancer delineation

PURPOSE

Metal artifacts can degrade computed tomographic (CT) simulation imaging and impair accurate delineation of tumors for radiation treatment planning purposes. We investigated a Digital Imaging and Communications in Medicine-based metal artifact reduction technique in tonsillar cancer delineation.

METHODS AND MATERIALS

Eight patients with significant artifact and tonsil cancer were evaluated. Each patient had a positron emission tomography (PET)-CT and a contrast-enhanced CT obtained at the same setting during radiotherapy simulation. The CTs were corrected for artifact using the metal deletion technique (MDT). Two radiation oncologists independently delineated primary gross tumor volumes (GTVs) for each patient on native (CTnonMDT), metal corrected (CTMDT), and reference standard (CTPET/nonMDT) imaging, 1 week apart. Mixed effects models were used to determine if differences among GTVs were statistically significant. Two diagnostic radiologists and 2 radiation oncologists independently qualitatively evaluated CTs for each patient. Ratings were on an ordinal scale from -3 to +3, denoting that CTMDT was markedly, moderately, or slightly worse or better than CTnonMDT. Scores were compared with a Wilcoxon signed-rank test.

RESULTS

The GTVPET/nonMDT were significantly smaller than GTVnonMDT (P = .004) and trended to be smaller than GTVMDT (P = .084). The GTVnonMDT and GTVMDT were not significantly different (P = .93). There was no significant difference in the extent to which GTVnonMDT or GTVMDT encompassed GTVPET/nonMDT (P = .33). In the subjective assessment of image quality, CTMDT did not significantly outperform CTnonMDT. In the majority of cases, the observer rated the CTMDT equivalent to (53%) or slightly superior (41%) to the corresponding CTnonMDT.

CONCLUSIONS

The MTD modified images did not produce GTVMDT that more closely reproduced GTVPET/nonMDT than did GTVnonMDT. Moreover, the MTD modified images were not judged to be significantly superior when compared to the uncorrected images in terms of subjective ability to visualize the tonsilar tumors. This study failed to demonstrate value of the adjunctive use of a CT corrected for artifacts in the tumor delineation process. Artifacts do make tumor delineation challenging, and further investigation of other body sites is warranted.

Correlation between metabolic tumor volume and pathologic tumor volume in squamous cell carcinoma of the oral cavity

PURPOSE

To explore the relationship between pathologic tumor volume and volume estimated from different tumor segmentation techniques on (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in oral cavity cancer.

MATERIALS AND METHODS

Twenty-three patients with squamous cell carcinoma of the oral tongue had PET-CT scans before definitive surgery. Pathologic tumor volume was estimated from surgical specimens. Metabolic tumor volume (MTV) was defined from PET-CT scans as the volume of tumor above a given SUV threshold. Multiple SUV thresholds were explored including absolute SUV thresholds, relative SUV thresholds, and gradient-based techniques.

RESULTS

Multiple MTV's were associated with pathologic tumor volume; however the correlation was poor (R(2) range 0.29-0.58). The ideal SUV threshold, defined as the SUV that generates an MTV equal to pathologic tumor volume, was independently associated with maximum SUV (p=0.0005) and tumor grade (p=0.024). MTV defined as a function of maximum SUV and tumor grade improved the prediction of pathologic tumor volume (R(2)=0.63).

CONCLUSIONS

Common SUV thresholds fail to predict pathologic tumor volume in head and neck cancer. The optimal technique that allows for integration of PET-CT with radiation treatment planning remains to be defined. Future investigation should incorporate biomarkers such as tumor grade into definitions of MTV.

Can FDG PET predict radiation treatment outcome in head and neck cancer? Results of a prospective study

Purpose

In head and neck cancer (HNC) various treatment strategies have been developed to improve outcome, but selecting patients for these intensified treatments remains difficult. Therefore, identification of novel pretreatment assays to predict outcome is of interest. In HNC there are indications that pretreatment tumour ¹⁸F-fluorodeoxyglucose (FDG) uptake may be an independent prognostic factor. The aim of this study was to assess the prognostic value of FDG uptake and CT-based and FDG PET-based primary tumour volume measurements in patients with HNC treated with (chemo)radiotherapy.

Methods

A total of 77 patients with stage II–IV HNC who were eligible for definitive (chemo)radiotherapy underwent coregistered pretreatment CT and FDG PET. The gross tumour volume of the primary tumour was determined on the CT (GTV_CT) and FDG PET scans. Five PET segmentation methods were applied: interpreting FDG PET visually (PET_VIS), applying an isocontour at a standardized uptake value (SUV) of 2.5 (PET_2.5), using fixed thresholds of 40% and 50% (PET_40%, PET_50%) of the maximum intratumoral FDG activity (SUV_MAX) and applying an adaptive threshold based on the signal-to-background (PET_SBR). Mean FDG uptake for each PET-based volume was recorded (SUV_mean). Subsequently, to determine the metabolic volume, the integrated SUV was calculated as the product of PET-based volume and SUV_mean. All these variables were analysed as potential predictors of local control (LC), regional recurrence-free survival (RRFS), distant metastasis-free survival (DMFS), disease-free survival (DFS) and overall survival (OS).

Results

In oral cavity/oropharynx tumours PET_VIS was the only volume-based method able to predict LC. Both PET_VIS and GTV_CT were able to predict DMFS, DFS and OS in these subsites. Integrated SUVs were associated with LC, DMFS, DFS and OS, while SUV_mean and SUV_MAX were not. In hypopharyngeal/laryngeal tumours none of the variables was associated with outcome.

Conclusion

There is no role yet for pretreatment FDG PET as a predictor of (chemo)radiotherapy outcome in HNC in daily routine. However, this potential application needs further exploration, focusing both on FDG PET-based primary tumour volume, integrated SUV and SUV_MAX of the primary tumour.

Positron emission tomography imaging approaches for external beam radiation therapies: current status and future developments

In an era in which it is possible to deliver radiation with high precision, there is a heightened need for enhanced imaging capabilities to improve tumour localisation for diagnostic, planning and delivery purposes. This is necessary to increase the accuracy and overall efficacy of all types of external beam radiotherapy (RT), including particle therapies. Positron emission tomography (PET) has the potential to fulfil this need by imaging fundamental aspects of tumour biology. The key areas in which PET may support the RT process include improving disease diagnosis and staging; assisting tumour volume delineation; defining tumour phenotype or biological tumour volume; assessment of treatment response; and in-beam monitoring of radiation dosimetry. The role of PET and its current developmental status in these key areas are overviewed in this review, highlighting the advantages and drawbacks.

An imageable retinoid acid derivative to detect human cancer xenografts and study therapeutic dosing to reduce its toxicity

Developing agents with 'seek, treat and see' capability is critical for personalized molecular medicine. Those agents will specifically target the disease markers for diagnosis and apply the biologically effective dose for treatment. Retinoids regulate a multitude of biological processes. In addition, retinoic acid can reverse premalignancy, significantly decrease second primary tumors and provide a treatment benefit in head and neck, lung, esophagus, colon and bladder cancer. These data suggest that cancer cells can take up retinoids. Therefore, retinoids are potential tumor-imaging agents. We developed near-infrared (NIR)-labeled retinoid agents to detect human cancers, visualize drug redistribution within the body, determine the optimal biological dose and reduce systemic toxicity. Our data demonstrate that the retinoid agent, but not the free dye, binds to the human tumor cells and is internalized, where it permits the imaging of human cancer xenografts. The high dose of retinoid agent is significantly associated with systemic toxicity. In summary, synthetic NIR-labeled retinoid agents can be used to detect multiple human cancer xenografts as the agent is internalized by cancer cells. The binding of the agent to the tumor xenografts is dependent on the redistribution of the agent. Therapeutic agents labeled with reporters will interrogate tumor-drug interactions and permit analysis of biodistribution, pharmacokinetics and pharmacodynamics in real time. At the same time, we can apply the biologically effective dose for therapy, instead of the traditional maximum tolerated dose, to reduce systemic toxicity.

Assessing hypoxia in animal tumor models based on pharmocokinetic analysis of dynamic FAZA PET

Positron emission tomography (PET) allows non-invasive detection and mapping of tumor hypoxia. However, slow tracer kinetics and low resolution, results in limited tumor-to-normal tissue contrast and the risk of missing areas where hypoxic cells are intermixed with necrosis. The shape of tumor time activity curves (TACs), as deduced from dynamic scans, may allow further separation of tumors/tumor sub-volumes that are inseparable based on static scans. This study was designed to define the added value of dynamic scans.

MATERIAL AND METHODS

Three squamous cell carcinoma tumor models were grown in mice. Mice were injected with the (18)F-labeled PET hypoxia-tracer fluoroazomycin arabinoside (FAZA) and the immunologically-detectable hypoxia-marker pimonidazole, and PET scanned dynamically for three to six hours. Subsequently, microregional tracer retention (autoradiography) and the distribution of pimonidazole-retaining cells (immunohistology) and necrosis were analyzed in tumor tissue sections. Dynamic PET data were analysed based on a two-compartment model with irreversible tracer binding generating estimates of the putative hypoxia surrogate markers k(3) (tracer trapping rate constant) and K(i) (influx rate constant from plasma into irreversible bound tracer).

RESULTS/DISCUSSION

High tumor-to-reference tissue ratios and a strong linear correlation (R∼0.7 to 0.95) between density of hypoxic cells and FAZA concentration was observed three hours after tracer administration, suggesting that late time PET images provides an accurate measure of hypoxia against which kinetic model estimates can be validated. Tumor TACs varied widely (ranging from distinctly wash-out to accumulative type) among tumor types although pimonidazole-stainings revealed extensive hypoxia in all models. Kinetic analysis of tumor sub-volumes showed that k(3) correlated poorly with late time FAZA retention regionally in two of the three tumor models. The influx rate constant K(i) displayed far less variability and correlated strongly with late time FAZA retention (hypoxia) in two of three tumor models, whereas a non-consistent relationship was observed in the last tumor model. Our study demonstrates the potential usefulness of dynamic PET, but also that a simple two-compartment model may be inappropriate in some tumor models.

The use of FDG-PET to target tumors by radiotherapy

Fluorodeoxyglucose positron emission tomography (FDG-PET) plays an increasingly important role in radiotherapy, beyond staging and selection of patients. Especially for non-small cell lung cancer, FDG-PET has, in the majority of the patients, led to the safe decrease of radiotherapy volumes, enabling radiation dose escalation and, experimentally, redistribution of radiation doses within the tumor. In limited-disease small cell lung cancer, the role of FDG-PET is emerging. For primary brain tumors, PET based on amino acid tracers is currently the best choice, including high-grade glioma. This is especially true for low-grade gliomas, where most data are available for the use of (11)C-MET (methionine) in radiation treatment planning. For esophageal cancer, the main advantage of FDG-PET is the detection of otherwise unrecognized lymph node metastases. In Hodgkin's disease, FDG-PET is essential for involved-node irradiation and leads to decreased irradiation volumes while also decreasing geographic miss. FDG-PET's major role in the treatment of cervical cancer with radiation lies in the detection of para-aortic nodes that can be encompassed in radiation fields. Besides for staging purposes, FDG-PET is not recommended for routine radiotherapy delineation purposes. It should be emphasized that using PET is only safe when adhering to strictly standardized protocols.

Segmentation of positron emission tomography images: some recommendations for target delineation in radiation oncology

Positron emission tomography can be used in radiation oncology for the delineation of target volumes in the treatment planning stage. Numerous publications deal with this topic and the scientific community has investigated many methodologies, ranging from simple uptake thresholding to very elaborate probabilistic models. Nevertheless, no consensus seems to emerge. This paper reviews delineation techniques that are popular in the literature. Special attention is paid to threshold-based techniques and the caveats of this methodology are pointed out by formal analysis. Next, a simple model of positron emission tomography is suggested in order to shed some light on the difficulties of target delineation and how they might be eventually overcome. Validation aspects are considered as well. Finally, a few recommendations are gathered in the conclusion.

Role of positron emission tomography in the treatment of occult disease in head-and-neck cancer: a modeling approach

PURPOSE

To determine under what conditions positron emission tomography (PET) imaging will be useful in decisions regarding the use of radiotherapy for the treatment of clinically occult lymph node metastases in head-and-neck cancer.

METHODS AND MATERIALS

A decision model of PET imaging and its downstream effects on radiotherapy outcomes was constructed using an influence diagram. This model included the sensitivity and specificity of PET, as well as the type and stage of the primary tumor. These parameters were varied to determine the optimal strategy for imaging and therapy for different clinical situations. Maximum expected utility was the metric by which different actions were ranked.

RESULTS

For primary tumors with a low probability of lymph node metastases, the sensitivity of PET should be maximized, and 50 Gy should be delivered if PET is positive and 0 Gy if negative. As the probability for lymph node metastases increases, PET imaging becomes unnecessary in some situations, and the optimal dose to the lymph nodes increases. The model needed to include the causes of certain health states to predict current clinical practice.

CONCLUSION

The model demonstrated the ability to reproduce expected outcomes for a range of tumors and provided recommendations for different clinical situations. The differences between the optimal policies and current clinical practice are likely due to a disparity between stated clinical decision processes and actual decision making by clinicians.

PET-guided delineation of radiation therapy treatment volumes: a survey of image segmentation techniques

Historically, anatomical CT and MR images were used to delineate the gross tumour volumes (GTVs) for radiotherapy treatment planning. The capabilities offered by modern radiation therapy units and the widespread availability of combined PET/CT scanners stimulated the development of biological PET imaging-guided radiation therapy treatment planning with the aim to produce highly conformal radiation dose distribution to the tumour. One of the most difficult issues facing PET-based treatment planning is the accurate delineation of target regions from typical blurred and noisy functional images. The major problems encountered are image segmentation and imperfect system response function. Image segmentation is defined as the process of classifying the voxels of an image into a set of distinct classes. The difficulty in PET image segmentation is compounded by the low spatial resolution and high noise characteristics of PET images. Despite the difficulties and known limitations, several image segmentation approaches have been proposed and used in the clinical setting including thresholding, edge detection, region growing, clustering, stochastic models, deformable models, classifiers and several other approaches. A detailed description of the various approaches proposed in the literature is reviewed. Moreover, we also briefly discuss some important considerations and limitations of the widely used techniques to guide practitioners in the field of radiation oncology. The strategies followed for validation and comparative assessment of various PET segmentation approaches are described. Future opportunities and the current challenges facing the adoption of PET-guided delineation of target volumes and its role in basic and clinical research are also addressed.