Thyroid Cancer

Fast Facts:

• 1-1.5% of all cancers.
• FDG-avid thyroid nodules incidentally seen in up to 4% of scans.
• 1/3 of FDG-avid nodules are malignant.
• 1/3 of benign thyroid nodules are FDG-avid.

Types:

• Papillary: ≈ 80%
• Follicular: ≈ 10%
• Medullary: 5-10%
• Hurthle Cell & Anaplastic (rare)

Indications for PET/CT Imaging:

Papillary & Follicular Carcinoma:

• Evaluating potential recurrence in patients with elevated thyroglubilin levels and a normal I-131 exam.
• Patients with high clinical suspicion of recurrence despite normal chemistries and negative I-131 imaging.
• Some advocate TSH stimulation prior to PET/CT imaging for patients with thyroglobulin levels below 10 ng/mL, by either withdrawing thyroid hormone or administering recombinant TSH.

Medullary Carcinoma: 

• May be useful in the setting of elevated chemical markers and negative (or equivocal) conventional imaging.

Hurthle Cell & Anaplastic:

• Generally do not concentrate Iodine, but are usually FDG-avid.
• May be useful in assessing loco-regional and distant metastatic disease.

What We Report:  

We report the size, metabolic activity & location of the primary lesion and representative metastatic lesions. 

• Primary Lesion: Size, SUV & location.
• Loco-Regional Disease: Usually hypermetabolic cervical lymph nodes.
• Distant Disease: Most commonly to distant lymph nodes, lung, bones, liver and brain.

Incidental Focal Thyroid Uptake:

• An incidental focus of thyroid uptake is seen in up to 4% of scans.
• Up to 33% of FDG-avid thyroid nodules will represent a primary thyroid malignancy (the nodule itself may be difficult to visualize on the non-contrast co-registered CT images).
• Therefore, any focus of uptake above background metabolic activity warrants further clinical assessment, typically ultrasound and biopsy.

Diffuse Thyroid Uptake:

• Mild diffuse thyroid uptake is considered normal.
• Moderate to intense diffuse thyroid uptake can be normal, or can reflect thyroiditis. In such cases, we typically report, “Diffuse intense thyroid uptake is noted, which can reflect thyroiditis.”

False Negatives:

• Lesions < 8.0 mm: Such lesions are considered “beneath the resolution of PET”, and will often be undetected.  Any focal uptake in a lesion less than 8.0 mm should be considered suspicious.
• Hot Nodule Masked By Diffuse Thyroid Uptake:  Do not reflexively dismiss diffuse thyroid uptake without carefully searching for an underlying hot nodule being masked by the diffuse uptake (best accomplished by manually decreasing the intensity of the gland). 

False Positives:

• 33% of benign thyroid nodules are FDG-avid.
• Parathyroid adenomas can be hypermetabolic (we do not include this in our differential unless there is a high clinical suspicion for a parathyroid adenoma).