Uptake of 131I by a papillary meningioma

Radioiodine whole body scan pitfalls in differentiated thyroid cancer

PURPOSE

whole body scan (WBS) performed following diagnostic or therapeutic administration of I-131 is useful in patients with differentiated thyroid carcinoma. However, it can be falsely positive in various circumstances. We aimed to report a series of pitfalls in a clinical perspective.

METHODS

A search in the database PubMed utilizing the following terms: "false radioiodine uptake" and "false positive iodine 131 scan" has been made in January 2023. Among the 346 studies screened, 230 were included in this review, with a total of 370 cases collected. Physiological uptakes were excluded. For each patient, sex, age, dose of I-131 administered, region and specific organ of uptake and cause of false uptake were evaluated.

RESULTS

370 cases of false radioiodine uptake were reported, 19.1% in the head-neck region, 34.2% in the chest, 14.8% in the abdomen, 20.8% in the pelvis, and 11.1% in the soft tissues and skeletal system. The origin of false radioiodine uptake was referred to non-tumoral diseases in 205/370 cases (55.1%), benign tumors in 108/370 cases (29.5%), malignant tumors in 25/370 cases (6.7%), and other causes in 32/370 cases (8.7%).

CONCLUSIONS

WBS is useful in the follow-up of patients with differentiated thyroid carcinoma, however it can be falsely positive in various circumstances. For this reason, it is critically important to correlate the scintigraphic result with patient's medical history, serum thyroglobulin levels, additional imaging studies and cytologic and/or histologic result.

The interpretation of 131I scans in the evaluation of thyroid cancer, with an emphasis on false positive findings

Radioiodine has aided the management of differentiated thyroid cancer for several decades. Most thyroid cancers retain the ability to trap iodine, and radionuclides of iodine can be used both diagnostically and therapeutically. The availability of sensitive diagnostic tests, coupled with the ability to deliver targeted therapy, gives physicians the ability to manage thyroid cancer better than with any other type of cancer. The correct interpretation of radioiodine scans is critical in the appropriate management of patients with thyroid cancer. False positive findings do occur. A radioiodine scan showing abnormal uptake outside the thyroid bed must be studied carefully and alternative reasons for the finding must be considered. The scan should be analysed systematically. Is there residual thyroid? If so, what is the 48 or 72 h neck uptake? Radioiodine uptake in the salivary glands, stomach, gastrointestinal and urinary tracts should be acknowledged as physiological. Diffuse uptake is seen in the liver in most patients with functioning thyroid at the time of their post-therapy scan. When there is uptake of the radioiodine outside these regions, contamination must be considered. A variety of cases illustrating true positive, true negative, and false positive findings is presented in this review, and the causes and consequences of misinterpretation of radioiodine scans are discussed.

Serum thyroglobulin measurement. Utility in clinical practice

Serum thyroglobulin measurement has greatly facilitated the clinical management of patients with differentiated thyroid cancer and a variety of other thyroid disorders. Thyroglobulin autoantibodies remain a significant obstacle to the clinical use of thyroglobulin measurement. The interpretation of any given thyroglobulin value requires the careful synthesis of all pertinent clinical and laboratory data available to the clinician. The diagnostic use of rhTSH-stimulated thyroglobulin levels has greatly facilitated the follow-up of low-risk patients with thyroid cancer. Although the measurement of thyroglobulin mRNA from peripheral blood is likely to affect the future management of these patients, it is expected that serum thyroglobulin measurement will continue to have a principal role in the care of patients with differentiated thyroid cancer.

Artifacts, anatomical and physiological variants, and unrelated diseases that might cause false-positive whole-body 131-I scans in patients with thyroid cancer

The whole body 131-I scan remains an important component in the postoperative treatment of patients with well-differentiated thyroid cancer. Because normal thyroid tissue remnants and residual or metastatic foci of well-differentiated thyroid cancer have the unique ability to concentrate, organify, and store 131-I, the whole body scan provides a depiction of those tissues that can be ablated with therapeutic doses of 131-I. Over time, it has become obvious that the whole body scan may also reveal foci of 131-i uptake owing to a wide variety of other causes. We provide a detailed pathophysiological classification of the artifacts, anatomic and physiological variants, and nonthyroidal diseases that may give rise to false-positive whole body scans in postoperative patients with thyroid cancer. These include ectopic foci of normal thyroid tissue; nonthyroidal physiological sites (eg, choroid plexus, salivary glands, gastric mucosa, urinary tract); contamination by physiological sections; ectopic gastric mucosa; other gastrointestinal abnormalities; urinary tract abnormalities; mammary abnormalities; serous cavities and cysts; inflammation and infection; nonthyroidal neoplasms; and currently unexplained causes. This article also provides a detailed review of the widely scattered English language literature in which these phenomena were originally described.

Spurious thyroid cancer bone metastases on radioiodine scan due to external contamination

In patients with differentiated thyroid cancer, bone uptake on radioiodine whole body scan indicates functional metastasis and is usually followed by treatment with a high dose of 131I. However, apparent radioiodine uptake in the bone may be due to other causes. We describe 19 radioiodine artifacts that mimicked bone metastases and were proven to be due to external contamination as they disappeared on reimaging after skin cleaning and garment change (15 artifacts) or on a repeat study (four artifacts). All artifacts were noted only on anterior views; 12 in the skull, five in the shoulder girdle and two in the upper extremity. Thirteen artifacts were focal and six were smeared. The prevalence of scans showing such artifacts was 0.5% of the total number of radioiodine scans performed over a 3-year period. Distinguishing contamination artifacts from real bone metastases based on scintigraphic appearance is rather difficult. Repeat post-cleaning images should be obtained in all newly identified cases of apparent bone metastasis.

False positive 131I total body scan due to an ectasia of the common carotidis

A cervical accumulation of radioiodine at Total Body Scan (TBS) in a patient who had been thyroidectomized for a follicular thyroid carcinoma led to unnecessary radioiodine treatment. Thyroglobulin measurements indicated constantly low levels. Following ultrasound scanning of the neck, echo-doppler examination of the cervical vessels and angiography with 99Tc, this area of increased activity at TBS imaging was shown to be due to an ectasia of the right common carotidis causing blood stasis and reverse flow.

Misinterpretation of iodine uptake in papillary thyroid carcinoma and primary lung adenocarcinoma

This is the first case report of an iodine-storing metastasising carcinoma of extrathyroidal origin and the simultaneous presence of a papillary thyroid carcinoma. In the course of the disease, an increase in pathological iodine uptake was observed after repeated iodine-131 ablation, so that the original tentative diagnosis of a metastasising, differentiated thyroid carcinoma appeared to be confirmed; moreover, the histological work-up of a femur metastasis--without adequate immunohistochemical assessment--was misinterpreted. What is also remarkable about this case report is a pathological iodine-131 uptake both in the extrathyroidal primary tumor and the metastases.