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Unusual uptake on whole-body scan after gastric bypass surgery
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A 38-year-old woman was referred for evaluation of metastatic thyroid cancer. During a routine physical exam 10 years earlier, her primary care physician noted an enlarged thyroid. Her thyroid function was normal with a thyroid-stimulating hormone of 2.08 uIU/mL. She had no family history of thyroid disease or thyroid cancer; no prior history of thyroid disease or head and neck radiation; and no symptoms of obstruction, including dysphagia, change in voice or cough.
Because of the large size of the goiter at >80 g, she was referred for thyroidectomy. The pathology revealed two micropapillary thyroid carcinomas with classical histology measuring 0.6 cm and 1 cm. The smaller lesion had extensive capsular invasion, but neither showed extrathyroidal extension. No nodes were removed at the time of surgery.
Stephanie L. Lee
The patient was treated with 100 mCi of radioactive iodine (I-131) while hypothyroid. Her post-therapy scan showed uptake only in thyroid remnant in the thyroid bed. She was considered a low-risk patient with an American Joint Committee on Cancer stage I and American Thyroid Association low risk. She moved to another state and did not receive further care for the thyroid cancer.
Focal uptake on follow-up
At age 33 years, the patient underwent Roux-en-Y gastric bypass for morbid obesity (Figure 1), leaving a small gastric pouch (Figure 2). She lost ˃100 lb after the surgery. Her PCP urged her to obtain follow-up care for her thyroid cancer. While on levothyroxine therapy, her TSH was 1.5 uIU/mL and her thyroglobulin antibodies were elevated at 47 IU/mL (normal, <20 IU/mL) with a thyroglobulin of <0.1 ng/mL. the endocrinologist correctly understood that the thyroglobulin measurement was not reliable because of the presence of thyroglobulin antibodies and ordered a recombinant human TSH stimulated I-123 whole-body scan. After the recombinant human TSH injections, the patient’s TSH was elevated to 100 uIU/mL with a stimulated thyroglobulin of <0.1 IU/mL and an elevated thyroglobulin antibody of 42 IU/mL (normal, <20 IU/mL). The report of the scan suggested a focal uptake consistent with recurrent thyroid cancer to the left of the midline in the abdomen (Figure 3).
Figure 1. Roux-en-Y gastric bypass diagram. Anatomy and flow of food before and after gastric bypass. After surgery, a small pouch is made from the upper stomach. Food bypasses most of the stomach and duodenum and passes into the jejunum.
Reprinted with permission from: Stephanie L. Lee, MD, PhD, ECNU.
Figure 2. Barium swallow. Liquid barium is swallowed, enters into the stomach pouch and then passes into the jejunum. The size of the pouch is small, with a volume of 15 mL to 30 mL.
Figure 3. Radioactive iodine (I-131) whole-body scan. Anterior (A) and posterior (B) I-131 whole-body scan demonstrated a focal area of uptake to the left of midline (red arrow). Anterior images of the whole-body scans of two other patients (C, D) showing normal stomach trapping of I-131 (red arrow). The source of I-131 in the stomach is iodine secreted in saliva from the salivary glands and in gastric acid secreted by the stomach. Nonspecific excretion of I-131 can be seen in the nasopharynx (green arrow), urine in the bladder (yellow arrow) and urine contamination of the perineum (blue arrow). I-131 uptake in thyroid remnants is shown in panel D (white arrow).
A CT scan showed no masses in the upper abdomen, liver or lung bases. The patient was referred for a second opinion. Review of her I-131 whole-body scan revealed a focal uptake just left of midline in the upper abdomen (Figures 3A and 3B). When compared with I-131 whole-body scans of two patients with normal stomachs, this focal area of uptake overlapped with the I-131 trapping in the stomach. The area of the stomach for this patient is much smaller because of the reduced size of the remaining pouch after the gastric bypass surgery (Figure 1). Because her CT scan showed no masses in this area, the focal uptake was normal trapping of I-131 in the stomach pouch. The small size of the pouch made this trapping appear to be focal. To confirm the absence of thyroid cancer, a thyroglobulin by radioimmunoassay was performed and found to be <0.5 ng/mL.
Gastric bypass, false-positive scan
False-positive I-131 whole-body scans can have many causes. This case is an example of the anatomical changes of gastric bypass resulting in a false-positive scan because of radioactive saliva and gastric secretions in the small pouch creating a focal iodine “uptake.” Presence of thyroglobulin antibodies produces a false-negative thyroglobulin level in the typical immunometric assay, and so this value should be measured by alternative methods, including radioimmunoassay and high-performance liquid chromatography (HPLC)/mass spectroscopy assay of proteolytic fragments of thyroglobulin (Quest Diagnostics and ARUP Laboratories). The fundus of the excluded stomach is filled with air, fluid and contrast material, which on CT scan can mimic postoperative loculated fluid collection.
In patients who are at high risk for tumor recurrence, other imaging techniques should be used to detect recurrent tumor, including neck ultrasound scan, CT scan with contrast of the neck and chest or fluorine-18 fluorodeoxyglucose (18F-FDG)-PET scan.
For more information:
- Cooper DS. Thyroid. 2009;19:1167-1214.
- Yu J. Radiology. 2004;231:753-760.
Disclosures:
- Stephanie L. Lee, MD, PhD, ECNU, is an Associate Professor of Medicine and Associate Chief, in the Section of Endocrinology, Diabetes and Nutrition at Boston Medical Center. Lee can be reached at Boston Medical Center, 88 E. Newton St., Endocrinology Evans 201, Boston, MA 02118; email: stephanie.lee@bmc.org. She reports no relevant financial disclosures.