Technetium-99m-MIBI scintigraphy of thyroid nodules in an endemic goiter area

Technetium-99m-methoxyisobutylisonitrile (MIBI) was introduced for myocardial imaging as an alternative to 201TI. According to biodistribution studies, MIBI also accumulates in the thyroid gland. The aim of this study was to find out which thyroid nodules retain MIBI and whether preoperative evaluation of malignancy is possible.

Single injection, dual-phase (30 min and 2 hr) thyroid scintigraphy with 99mTc-MIBI was performed on 62 patients who showed a cold nodule on previously performed 99mTc scintigraphy. MIBI scans were considered positive if there was a clear tracer retention in the late 120-min image compared to the early 30-min image. Sonographic examination and fine-needle aspiration biopsy, guided by ultrasonography, was also done on each patient. In the following days and weeks, all patients underwent surgery.

Histopathological diagnoses revealed a total of 12 thyroid carcinomas, five were MIBI positive and seven MIBI negative. Out of 27 patients with thyroid adenomas (nine microfollicular, ten follicular, eight oxyphilic), 18 were MIBI positive and nine MIBI negative. There was no MIBI retention on the scans of 22 patients with degenerative changes in the goiter and the one with Hashimoto's disease.

These results indicate that MIBI accumulation and retention is not specific for thyroid malignancy. Indeed, all evidence points to the fact that a positive MIBI scan is more likely to indicate thyroid adenoma than a malignant tumor.