Lymph node staging with dual-modality PET/CT: Enhancing the diagnostic accuracy in oncology

https://doi.org/10.1016/j.ejrad.2005.12.042Get rights and content

Abstract

Lymph node staging according to the TNM criteria is an essential part of tumor evaluation. Several morphological and functional imaging procedures are used complementarily in this setting. Dual-modality PET/CT scanners are able to provide anatomical and functional data sets in a single session with accurate image co-registration. Comparative studies between morphological imaging procedures, such as MRI and CT, with co-registered PET/CT demonstrated significantly better lymph node staging with PET/CT than with anatomical procedures alone, regardless of the staged body compartment (head and neck, thorax or abdominal area). Based on more accurate staging results, PET/CT was able to alter the patients’ therapy in a significant number of studies. Functional imaging with FDG–PET ([18F]-2-fluoro-2-desoxy-d-glucose–positron emission tomography) demonstrated outstanding results in lymph node staging of different tumor diseases. By adding anatomical information to PET, PET/CT outperforms PET alone when assessing the TNM-stage of different malignant diseases.

This paper provides an overview concerning the performance of PET/CT in staging lymph nodes for malignant spread and points out benefits and limitations of this new imaging modality.

Introduction

The TNM classification is the worldwide accepted and used system for categorizing and staging of all solid tumors. The T-stage indicates the tumor size, whereas the N-stage shows the extent of metastatic spread to lymph nodes. The M-stage accounts for solid metastases in distant organs.

Radiological imaging procedures are an essential step in cancer diagnosis as they provide the referring physician with information about the patients’ tumor status. Imaging procedures for tumor staging are based on morphological and functional data. However, morphological imaging like MRI, CT, and ultrasound are based on strictly anatomical information such as lymph node size, shape, and the pattern of contrast enhancement, and are known to be of only of limited sensitivity and specificity when staging lymph nodes for metastatic spread. Positron emission tomography (PET) using [18F]-2-fluoro-2-desoxy-d-glucose (FDG) as a radioactive tracer has proven superior to morphologic imaging procedures when assessing the N-stage based on functional data evaluating tumor metabolism [1]. Tumor cells metabolise more glucose compared to normal adjacent tissue, leading to increased uptake of the glucose analogue FDG in the majority of solid tumors. However, the major downside of functional imaging alone is the limited morphological information which can render the exact anatomical localisation of an area of increased glucose metabolism difficult. Therefore, the combination of morphological and functional imaging represents the optimal approach for lymph node staging and general staging. Several attempts have been made to fuse both imaging modalities, but apart from accurate fusion of brain data sets, image co-registration in other parts of the body has remained unsatisfactorily so far for clinical routine, based on the time required as well as limitations associated with respiratory motion and organ movement. Dual-modality PET/CT scanners have demonstrated to overcome these limitations. In this hybrid imaging tool, the CT and the PET are acquired in direct succession with the patient scanned in the same position during CT and PET for accurate image fusion. Thus, limitations of image fusion associated with different patient positions are minimized. Several studies evaluating patients with different oncological diseases have reported outstanding results concerning the TNM staging when using PET/CT instead of separate morphological and functional data sets [2], [3], [4], [5]. Therefore, dual-modality FDG–PET/CT is expected to play an important role in lymph node diagnostics as it will enhance the tumor staging accuracy which is essential for tailored oncological therapy. This article summarized advantages and limitations of PET/CT imaging in lymph node staging.

Section snippets

Technical considerations

The underlying principle of currently available PET/CT systems is the acquisition of two dedicated imaging procedures, CT and PET, in a single step followed by image co-registration. Both modalities are physically aligned in one setting which is served by a single examination table (Fig. 1). The CT is acquired first followed by the PET. Since the patient is scanned with CT and PET while in the same position on the examination table, both data sets can be accurately co-registered. Depending on

PET/CT and morphological imaging procedures

The appearance of non suspicious lymph nodes with morphological imaging procedures is classified by their shape, size, density and, if applied, contrast enhancement. Benign nodes usually tend to have a fatty hilum, an oval shape and frequently do not measure more than 1 cm in the short axis diameter. Size-based measurements are the main criteria for differentiating benign from malignant lymph nodes. Physiologic lymph node size may vary based on their anatomical localisation [10], [11].

Thus,

PET/CT and positron emission tomography

PET imaging using FDG as a radioactive tracer has been reported a very sensitive and specific tool for the detection of lymph node metastases. In comparison with morphological imaging procedures higher sensitivities and specificities are based on the functional data provided by PET which offer visualization of tumor metabolism.

In the head and neck, PET was found to be 87% sensitive and 94% specific for staging of lymph nodes [14]. These results are well comparable with the published literature

Limitations of PET/CT

Most malignant tumors are associated with an increase in glucose metabolism leading to increased FDG-uptake on FDG–PET. However, there are tumors which are found to be frequently FDG–PET negative based on normal or even decreased glucose metabolism (Table 1). For these tumor entities implementation of alternative radioactive tracers into the clinical routine may improve the staging results with PET and PET/CT. For neuroendocrine tumors DOTATOC, a somatostatine analog, labelled with a

Conclusion

Accurate tumor staging is essential for patients’ therapy planning. Compared with morphological imaging procedures PET/CT provides additional functional data which increase the diagnostic accuracy over morphology alone when staging lymph nodes for malignant spread. Furthermore, morphological data in accurate fusion with functional images from PET have further improved the accuracy when detecting malignant lymph nodes with PET/CT compared to PET alone. Compared to both, morphology and function

Acknowledgements

The authors would like to thank Sandra Pabst, RT, Janina Marchese, RT and Bearbel Terschüren, RT as well as Slavco Maric and Stergios Gradas, RT for the acquisition and preparation of the PET/CT images.

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