Post-therapy whole-body scan with abnormal radioactive iodine localized to the head of two women

Issue: January 2011

ByStephanie L. Lee, MD, PhD, ECNU

A 25-year-old woman was referred for radioactive iodine remnant ablation for multifocal micropapillary thyroid carcinoma.

A thyroidectomy performed after a fine-needle aspiration, suspicious for papillary thyroid carcinoma, was found and revealed three papillary microcarcinoma measuring 0.2 cm, 0.6 cm and 0.6 cm in the right lobe and a 0.2-cm papillary microcarcinoma in the left lobe. One perithyroidal lymph node was removed with no tumor identified. The patient was referred by her outside endocrinologist for radioactive iodine (RAI) ablation. Despite a long discussion stressing that she was at very low risk for death and recurrence indicated by her young age, American Joint Committee on Cancer (AJCC) stage I and her low risk for recurrence per the 2009 American Thyroid Association thyroid nodule and cancer guidelines, the patient was anxious and worried, and requested RAI ablation.

Stephanie L. Lee, MD, PhD
Stephanie L. Lee

She had remnant ablation with 50 mCi I-131 with a thyroid-stimulating hormone of 73.07 mU/L and a low thyroglobulin 1.4 ng/mL with a negative thyroglobulin antibody level less than 20 IU/mL. Seven days later, a post-therapy whole-body scan was obtained that showed a focus of avid uptake in the thyroid bed from a postsurgical thyroid remnant. There was also diffuse tracer localized along the periphery of her head in the location of the skull (Figures 1A, B). The patient had washed her hair between the dose and the 7-day post-therapy scan, but she complained of extreme anxiety and sweating during the post-therapy scan.

Another patient

A second patient, a 54-year-old woman, was referred for RAI ablation. She had a total thyroidectomy for a multifocal micropapillary thyroid carcinoma with feature of a tall cell variant with lymphovascular and capsule invasion with positive margins. Four metastatic lymph nodes were removed from the lateral neck (levels 3 and 4). Her past medical history was positive for oligomenorrhea, but since her surgery 2 months before, she was amenorrheic with severe daytime and night sweats. This patient was at moderate risk for death and recurrence based on the histological variant of her tumor, lymphovascular and capsule invasion and lateral neck nodes (AJCC stage IV A).

She had RAI therapy with 149 mCi I-131 with a TSH of 49.02 mU/L and a low thyroglobulin 3.6 ng/mL with a negative thyroglobulin antibody level less than 20 IU/mL. Seven days later, a post-therapy whole-body scan was obtained that showed a focus of avoid uptake in the thyroid bed from the postsurgical thyroid remnant and tracer located along the periphery of her head in the location of the skull (Figures 1C, D). The patient noted that she was very anxious and had a drenching sweating episode during her whole-body scan.

Figure 1. One week post-therapy RAI whole-body scan. A. Patient 1, anterior view. B. Patient 1, posterior view. C. Patient 2, anterior view. D. Patient 2, posterior view. Red arrows: RAI contamination of scalp by sweat (helmet sign) seen best on the posterior views. Green arrows: RAI contamination by sweaty hands. Yellow arrows: Thyroid remnant. Blue arrows: RAI contamination of perineum by urine. Pink arrows: Liver uptake of iodinated proteins released by normal and malignant thyroid tissue. Red asterisk: Physiological excretion of RAI in nasopharyngeal secretions.

Photo courtesy of: Stephanie L. Lee, MD, PhD

Pattern of uptake

One year after the initial RAI therapy, both patients had a recombinant human TSH-stimulated RAI whole-body scan that demonstrated no abnormal uptake, and the stimulated thyroglobulin was unmeasurable, suggesting the absence of persistent disease.

The specificity of I-131 whole-body scans for the initial staging and detecting postoperative local and distant metastatic disease is excellent. Radioiodine whole-body imaging is the most accurate method in the diagnosis of metastases from differentiated thyroid cancer. However, false-positive images occur at sites of body fluid stasis such as in a renal cyst, sinus mucocele and hematoma or cutaneous contamination by radioiodine in either saliva or urine.

It was felt that the diffuse uptake along the peripherally of the skull in both cases was a contamination artifact likely due to physiological scalp sweat due to anxiety and postmenopausal diaphoresis. It is very unlikely, with the low level of thyroglobulin levels in the presence of a significantly elevated TSH that this peripheral skull trapping was due to widely metastatic disease to the calvarium. This pattern of uptake on a RAI whole-body scan has been previously described and named the â€œhelmet sign.â€�

Stephanie L. Lee, MD, PhD, is associate chief in the section of endocrinology, diabetes and nutrition and associate professor of medicine at Boston Medical Center. She is also an Endocrine Today Editorial Board member.

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