Thyroid tissue uptake represents confounding variable in detection of regional metastasis
A 70-year-old woman with type 2 diabetes underwent an uncomplicated total thyroidectomy in March for removal of a biopsy-proven papillary thyroid carcinoma in the left thyroid lobe.
There was no lymph node metastasis. The patient presented a week after surgery with episodes of nausea and vomiting.
A review of systems was otherwise negative. On physical exam, she was not found to have an enlarged neck mass. Post-thyroidectomy laboratory evaluation in late March revealed a markedly elevated level of thyroid-stimulating hormone (TSH) of 9.13 mIU/L, as well as decreased free thyroxine of 0.8 mcg/dL. Thyroglobulin levels were elevated at 121 ng/dL. An adrenocorticotropic hormone stimulation test was within normal limits.
Source: Images courtesy of M. Ghesani, MD, reprinted with permission.
Ultrasound of the neck performed in April revealed no abnormal soft tissue in the thyroidectomy bed (Figure 1). However, there was a 2.7 cm x 2.7 cm x 2.6 cm non-hypervascular mass just above the right clavicle at the midline, adjacent to the subclavian vein, suspicious for local metastasis or papillary thyroid cancer recurrence.
Midline mass identified
In May, the patient was administered 0.9 mg of recombinant TSH for 2 days before oral administration of 850 mcCi I-123 sodium iodide. Whole-body scintigraphic images acquired approximately 24 hours later revealed an abnormal focus of intense tracer uptake in the midline-lower neck (Figure 2B, 2C), which corresponded to the lobulated heterogeneous mass seen in the prior ultrasound. The findings were most likely due to regional nodal metastasis of papillary thyroid carcinoma. Two foci of moderate uptake in the region of thyroid bed are consistent with remnant thyroid tissue.
Interestingly, the midline mass seen on postoperative I-123 whole-body scan in May was not seen on the preoperative I-123 thyroid scan in January (Figure 2A), which had demonstrated a diffuse photopenic region of uptake in the right thyroid lobe proven to be an adenomatoid nodule, and a photopenic region in the left upper thyroid lobe proven to be papillary carcinoma.
These findings raise the possibility that the midline metastatic lesion rapidly grew in size between January and May, or that the previously present thyroid tissue had taken up a majority of the I-123 tracer and depleted the blood pool such that the midline metastatic lesion appeared relatively photopenic.
Papillary thyroid carcinomas are treated by means of surgical resection and radioiodine therapy.
Although indolent, papillary thyroid carcinomas have a high likelihood of local recurrence in the thyroid resection bed and cervical nodal chains. Focused neck ultrasonography combined with thyroglobulin testing has been accepted as the initial screening tool to detect recurrence.
Kim and colleagues conducted a retrospective study of 542 patients with papillary thyroid carcinoma who underwent thyroidectomy and simultaneous lymph node dissection. They reported the prevalence of central lymph node metastasis was as high as 39.6%.
For papillary thyroid carcinomas measuring greater than 1 cm on initial diagnosis, central lymph node metastasis is associated with a significantly worse DFS than those without metastasis.
Given the indolent nature of papillary thyroid carcinoma, it is rare to observe a marked increase in radiotracer uptake along cervical nodal chains within a 4-month period.
We speculate that these findings represent either a rare occurrence of aggressive nodal metastasis of papillary thyroid carcinoma, or that the previously present thyroid tissue had depleted the blood pool of I-123 tracer in a normal “thyroid steal phenomenon” such that lymph node metastasis appear more photopenic.
Normal thyroid tissue uptake, therefore, likely represents a potential confounding variable in the detection of regional metastasis.
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Munir Ghesani, MD, is an attending radiologist at St. Luke’s-Roosevelt Hospital Center and Beth Israel Medical Center, an associate clinical professor of radiology at Columbia University College of Physicians and Surgeons, and a HemOnc Today section editor. He can be reached at Department of Radiology, Beth Israel Medical Center, First Avenue at 16th Street, New York, NY 10003; email: email@example.com.
Yi C. Zhang, MD, is a radiology resident at St. Luke’s-Roosevelt Hospital Center.
Rick A. Wray, MD, is a nuclear medicine resident at St. Luke’s-Roosevelt Hospital Center.