Positron emission tomography (PET) is a nuclear medicine functional imaging modality with direct applications to oncology, cardiology and neurology. At our facility in SGH, we currently perform many oncologic PET/CT imaging which combines PET with CT within a single scanner. PET imaging exploits the glucose metabolic pathway, through the use of the most commonly used PET radio-tracer [F-18] fluoro-deoxyglucose (FDG). Various molecular derangements in malignant cells, including increased glycolytic rates and upregulated glucose transporters, result in increased cellular uptake of FDG. A FDG-PET scan can detect and localize such abnormal concentrations of FDG. This short account can only provide a very brief overview of some current indications of PET imaging in head and neck cancers (excluding CNS, NPC and thyroid cancers).

Primary tumour

Apart from nasopharyngeal carcinoma (NPC), squamous cell carcinomas (SCC) account for about 90% of head and neck tumours. Staging is critical in the management of such tumours and provides prognostic information. Cellular cell-types such as squamous cell carcinomas are highly FDG-avid. Several studies have indicated that FDG-PET has good sensitivity in excess of 90% for detecting primary tumour, with failures in very small lesions. However, it should be noted that the clinical information it provides is complementary to that of structural imaging such as MRI. The standardized uptake value (SUV, a semiquantitative index of glucose uptake) obtained may also have prognostic significance.

Evaluation of nodes and distant metastases

PET compares with CT or MRI in terms of sensitivity for detecting nodal involvement, although the reported sensitivities can be as low as 50% and as high as 94%. The use of PET in staging lymph nodes of primary head and neck tumours shows that it can detect disease in a segment of patients with N0 neck although its exact role is still evolving. [1]. In cases of advanced primary tumours, PET imaging allows the concomitant detection of distant metastases, which may alter the plan of management. Patients with head and neck cancer also have an increased risk of developing secondary cancers in the head and neck, esophagus or lung. One report suggested that PET is able to detect distant metastases or a synchronous secondary cancer in about 10% of patients at initial staging [2].

Post chemo-radiation neck

PET is useful for assessing residual disease or tumour recurrence in the post-chemo-radiation neck, which may be difficult to assess by structural imaging. A post-therapy PET scan should be performed at 8-12 weeks. A negative scan has high negative predictive value. Nonetheless, it is important to recognise that post-treatment changes such as inflammation can mimic tumour uptake.

Detection of unknown primary

Patients with neck metastases of squamous cell cancer are very likely to have the primary lesion within the head and neck region. PET is often useful in detecting some of these tumours when no obvious tumour has been found by other imaging modalities or panendoscopy.

Some limitations for PET imaging in the head and neck include the relative low uptake by some well-differentiated thyroid carcinomas. Certain benign lesions such as parotid Warthin’s tumours may give rise to rather high uptake. PET is not optimal for evaluating intra-cranial abnormalities as the physiological uptake of glucose by the normal gray matter precludes a proper delineation of tumours that occur at the base of skull or in the cavernous sinus.


Man with cervical adenopathy. PET/CT shows abnormal area of hypermetabolic uptake at the right fossa of Rosenmuller, which proved to be nasopharyngeal carcinoma.

Conclusion

In conjunction with structural imaging, PET and PET/CT provide clinical information on status of the primary tumour, nodal and distant metastases and the presence of unsuspected synchronous second cancer. It is useful in the evaluation of post-treatment neck and the detection of occult primary tumours.

References

[1] Heiko Schoder et al. J Nucl Med 2006; 47:755–762.

[2] Goerres G W. PET and PET/CT of head and neck tumours. In Clinical Molecular Anatomic Imaging. (ed) G K von Schulthess.

David Chee-Eng Ng, Senior Consultant

Ong Seng Chuan, Consultant

Anthony S W Goh, Head and Senior Consultant

Department of Nuclear Medicine and PET
Singapore General Hospital.