18F-PET/CT imaging of metastasis to the thyroid gland: Imaging findings and effect on patient management

Abstract

Purpose: While metastasis to the thyroid from a primary cancer remote to the thyroid is uncommon, current imaging techniques have improved detection of these intrathyroid metastases. The purpose of this study was to evaluate the ¹⁸F-PET/CT appearance of intrathyroid metastases and assess the impact of detection on patient management.

Methods: The ¹⁸F-PET/CT appearance of intrathyroid metastasis, including standardized uptake value (SUV), disease extent, and the effect on patient management following diagnosis were retrospectively reviewed. Inclusion criteria included ¹⁸F-PET/CT imaging and diagnosis of the intrathyroid metastasis matching the remote primary tumor.

Results: Intrathyroid metastasis were detected in 24 patients. The intrathyroid metastases presented on ¹⁸F-PET/CT as focal nodular uptake (n = 21), multiple nodular uptake (n = 2), or diffuse uptake/infiltration of the thyroid gland (n = 1). The SUV ranged between 3.9 and 42 (median 12.5 ± 7.5); in 2 patients, the FDG-avidity was minimal. On ¹⁸F-PET/CT, distant metastases were present outside the neck (n = 18), or limited to the neck (n = 6). In 2 of these 6 patients, the thyroid was the only site of metastatic disease. Due to the metastatic disease, the therapy was changed in 23 of 24 patients; 1 patient was lost to follow-up.

Conclusion: In any patient with a previous or current history of an extrathyroid malignancy, an ¹⁸FDG-avid thyroid mass or diffuse infiltration of the thyroid on ¹⁸F-PET/CT should be considered a potential intrathyoid metastasis until proven otherwise. Knowledge of an intrathyroid metastasis may impact patient management, especially if the thyroid or neck are the only sites of metastatic disease.

Keywords: Metastasis; PET/CT; Standardized uptake value (SUV); Thyroid.

Figure 1

A 54-year-old female with breast cancer (patient #1). ¹⁸F-PET/CT, axial plane, shows a solitary nodular focus of uptake in the right thyroid gland (SUV = 15.4) (arrow).

Figure 2

A 67-year-old male with synovial sarcoma of the C1 ring (patient #14). A) ¹⁸F-PET/CT, axial plane, shows multiple nodular foci of uptake (SUV = 21.8) in the thyroid and isthmus (arrows). B) ¹⁸F-PET/CT, coronal plane, multiple nodular foci of uptake in the right thyroid (large arrows) and right mid neck node (small arrow). Note lack of FDG activity in the left lobe.

Figure 3

A 51-year-old male with history of lung cancer (patient #23). A) ¹⁸F-PET/CT, axial plane, show diffuse uptake throughout the thyroid gland (SUV = 9.4) (arrows). B) ¹⁸F-PET/CT, 3D image, diffuse uptake in the thyroid gland (large arrows) and a left lung metastasis (small arrow).

Figure 4

A 44-year-old female with lung cancer (patient #12). A) ¹⁸F-PET/CT, axial plane, demonstrates minimal FDG avidity in the left lobe of the thyroid gland (arrow). B) ¹⁸F-PET/CT, axial plane, shows bilateral lower neck FDG avid nodes (arrows).