Positron emission imaging of head and neck cancer, including thyroid carcinoma

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Abstract

Most positron emission tomography (PET) imaging studies in head and neck cancer are performed using the radiotracer 18-fluorodeoxyglucose (18FDG). PET with FDG has become a standard clinical imaging modality in patients with head and neck cancer. It contributes valuable information in localizing a primary tumor in patients with neck nodal metastases from an unknown primary, in the staging of primary head and neck cancer, and in the detection of recurrent disease. In addition, FDG-PET provides independent prognostic information in patients with newly diagnosed and recurrent head and neck cancer. PET/CT improves lesion localization and accuracy of FDG-PET and is strongly recommended in patients with head and neck cancer. After thyroidectomy, FDG-PET has proven useful in patients with clinical or serological evidence of recurrent or metastatic thyroid carcinoma but negative whole body iodine scan. PET shows metastatic disease in up to 90% of these patients, thereby providing a rational basis for further studies and therapy. In patients with medullary thyroid cancer with elevated calcitonin levels following thyroidectomy, FDG-PET has a sensitivity of 70–75% for localizing metastatic disease. Occasionally incidental intense FDG uptake is observed in the thyroid gland on whole body PET studies performed for other indications. Although diffuse FDG uptake usually indicates thyroiditis, focal uptake has been related to thyroid cancer in 25–50% of cases and should therefore be evaluated further if a proven malignancy would cause a change in patient management.

Section snippets

Neck metastases from an unknown primary tumor

This condition, also known as carcinoma of unknown primary, accounts for 3–15% of all cancer diagnoses and for approximately 1–2% of head and neck cancers. For practical purposes, this entity should be defined as the combination of: no history of previous malignancy, no clinical or laboratory evidence for primary neoplasm, neck mass that is histologically or cytologically proven to be carcinoma. Occurrence of nodal metastases in neck levels I-III increases the likelihood for a primary HNSCC.

PET for T-staging

At the time the patient is referred for staging (imaging) studies, the primary tumor has already been diagnosed, and a clinical head and neck examination has assessed the status of lymph nodes in the neck. The first goal of imaging studies is therefore to determine the extent of the primary tumor, in particular with regard to structures whose involvement may alter the surgical approach (eg, bone invasion, orbital invasion, skull base invasion, tumor “tracking” along nerves and blood vessels).

Treatment evaluation

Depending on the stage and location of the disease, treatment options in head and neck cancer include surgery, radiation therapy alone, or radiation therapy in combination with (concurrent) chemotherapy, possibly followed by surgery. If surgery is the primary treatment modality, this may be followed by radiation therapy, depending on the stage and aggressiveness of the primary tumor, status of surgical margins etc. With the latter approach, imaging studies during or immediately at the end of

Recurrent disease

The early detection of recurrent head and neck cancer is important in determining the ability to perform salvage surgery, which can improve the clinical outcome of these patients. For instance, patients with recurrent early stage HNSCC who undergo salvage surgery have a 70% 2-year relapse-free survival, whereas those with recurrent advanced stage disease HNSCC undergoing salvage surgery have a 22% 2-year RFS.52 It is therefore critically important to detect potential recurrences early in the

Prognostic value of FDG-PET imaging in patients with head and neck cancer

Several studies have shown that FDG-PET by itself can assess the aggressiveness and proliferation rate of HNSCC71, 72, 73 and therefore correlates with patient prognosis, regardless of the treatment modality used.43, 74, 75 In both untreated patients as well as patients with recurrent disease, the intensity of FDG uptake in the tumor appears to predict the ultimate outcome after therapy.68, 69, 74, 75 In patients with recurrent disease, clinical PET interpretation and SUV are independent

PET/CT in head and neck cancer

PET/CT is a new imaging modality providing an almost simultaneous acquisition of anatomic and metabolic imaging data. This modality has been used increasingly for clinical oncologic imaging since 2001. Emerging data demonstrate that the combination of PET and CT in one image set improves the anatomic localization of PET abnormalities, decreases the number of equivocal PET interpretations,76 and improves the diagnostic accuracy for staging of lung and colorectal cancer as compared with PET or CT

Thyroid carcinoma

Nuclear medicine has been one of the mainstays of management of thyroid disease. Since the early 1950s, the whole body scan (WBS) using tracer dose of iodine-131 (131I) has been widely used for detection of metastases of differentiated thyroid cancer (DTC). High dose 131I has also been widely used for the ablation of thyroid remnant in the neck as well as therapy of metastatic diseases detected on the diagnostic WBS. The efficacy of this approach has been confirmed repeatedly in multiple large

PET-FDG in differentiated thyroid cancer

Early studies found a low sensitivity of FDG-PET in the detection of metastases.89 However, others reported a sensitivity of 50% in 58 un-selected patients with DTC, thyroidectomy and prior radioactive iodine (RAI) ablation,90 and in a large multicenter study with un-selected patients, the sensitivity of FDG-PET for localizing metastatic disease in patients with DTC was 75%.91

As early as 1987, Joensuu and coworkers89 noticed metabolic heterogeneity between metastases of DTC in a study showing

Hürthle cell carcinoma

Hürthle cell cancer is a histologic subtype of DTC that is clinically more aggressive. The tumor frequently shows little or no iodine uptake, but can be identified with FDG-PET: a meta-analysis of two studies and 35 patients showed a sensitivity of 92% and specificity of 82%.107 A sensitivity of 92% (12/13 patients) was also reported in another recent study.108 More importantly, in 7 of these 13 patients PET showed disease not identified by other imaging methods.

Anaplastic thyroid cancer

This is an extremely aggressive tumor and imaging is not required for staging, since all patients are classified as stage IV at diagnosis. As a result FDG-PET scan in anaplastic thyroid cancer has not been studied systemically. However, in our own experience and in some case reports89 this malignancy usually shows intense FDG uptake, and in selected cases FDG-PET may be helpful in directing treatment and evaluating the efficacy of therapy (Fig 6).

Medullary thyroid cancer

Medullary thyroid cancer is a rare calcitonin secreting tumor originating from the parafollicular C cells. The primary treatment modality is surgical resection. A PET study may be requested in patients with high serum calcitonin level after surgery. The number of patients studied for this purpose is limited, but it appears that FDG-PET can identify metastatic disease more frequently than other imaging studies. A study of 20 patients reported a sensitivity of 76%,109 and in a large multicenter

Thyroid incidentaloma

The normal thyroid gland shows low grade FDG uptake or is usually not visualized on the whole-body FDG-PET scan. Occasionally, diffusely or focally increased FDG uptake is seen as an incidental finding in the thyroid gland. In a large series of patients it was proven that diffuse thyroid FDG uptake is usually an indicator of chronic thyroiditis.113 In contrast, focal FDG activity in the thyroid gland has been associated with malig-nancy.114, 115 In a retrospective review of over four thousand

PET imaging with 124Iodine

124Iodine has a half-life of 4.2 days and a relatively complex decay scheme, with 22% of the disintegrations producing positrons of relatively high energies (1532 keV and 2135 keV), as well as a number of high energy gamma and X-rays, with energy as high as 1691 keV. Despite the high abundance of high energy gamma photons images of satisfactory quality can be acquired and quantitation of tracer uptake can be performed with only minor degradation in image resolution and quantitation.118 Indeed,

Conclusion

PET imaging with FDG has become standard clinical practice in patients with head and neck cancer. It contributes valuable information in the primary staging of the disease, in particular in nodal staging of the neck and in the assessment for distant metastases or synchronous second primary malignancies. FDG-PET is very accurate in the detection of recurrent disease and should be used as the imaging modality of choice in patients at high risk for or with clinically suspected recurrence. PET can

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