CASE 1320: RAI Uptake and Serum Prolactin in Thyroid Cancer

Radioactive iodine (RAI) is a radioisotope used to ablate thyroid gland remnant after thyroidectomy in patients diagnosed with differentiated thyroid carcinoma (DTC). A whole body scan (WBS) is performed to not only evaluate for iodine uptake by the native thyroid tissue but also to observe for uptake in other areas of the body, which could be physiological or indicative of iodide-avid metastases. Research has shown a correlation between breast cancer and thyroid cancer. Patients with DTC have been found to have elevated levels of serum prolactin, which could lead to mammary gland dysfunction. As such, this study will evaluate for breast uptake on the RAI WBS. Research has shown that patients with DTC may have uptake on RAI WBS.

There is a concern in the molecular imaging community and oncology community that increased I-131 retention in breast tissue may increase the risk of future breast malignancy. The notion that increased serum prolactin concentration increases iodine uptake in breast tissue comes mostly from case series. A correlation between high prolactin as a result of withdrawal from thyroid hormone treatment vs stimulation by recombinant human thyrotropin and uptake by thyroid tissue is the main research question. This research will not be able to answer the question of whether such breast uptake is associated with the future risk breast malignancy. The later research question should be undertaken by other researchers in a larger study with longer follow up. Metastases to the breast from thyroid cancer are rare. Uptake in a discrete lesion in the breast will need to be investigated. It is unclear whether breast adenocarcinoma will concentrate iodine.

In a previous retrospective study of 194 patients with DTC and no breast cancer, it was demonstrated that patients prepared by withdrawal from thyroid hormone had 5-fold higher peak I-131 uptake in breast tissue on post-therapy scan compared to patients prepared by recombinant human thyrotropin (rh-TSH). In other research, DTC patients with hyperprolactinemia have been found to have increased uptake by the mammary gland. The hypothesis is that patients prepared by withdrawal would have increased serum prolactin compared to patients prepared by rh-TSH who are not made hypothyroid. This study plans to prospectively evaluate serum prolactin and breast uptake on the RAI WBS and assess the impact of pre-scan preparation on those variables. This would be the first prospective study of its kind in this set of patients to look at whether the method of pre-RAI scan preparation might impact RAI uptake with correlation to prolactin levels.

Inclusion criteria

• Female patients with diagnosis of differentiated thyroid carcinoma who have had thyroidectomy performed previously
• Patients able to understand and sign informed consent for imaging
• Patients already scheduled to undergo radioactive iodine whole body scanning (RAI WBS)

Exclusion criteria

• Males
• Patients with a history of breast cancer
• Patients with a known history of hyperprolactinemia
• Patients receiving medications known to raise serum prolactin