# Clinical factors affecting colonic iodine-131 distribution after radioactive iodine therapy for thyroid cancer

## TL;DR

This study identifies clinical factors like thyroid hormone withdrawal and laxative type that influence colonic radiation exposure after radioactive iodine therapy for thyroid cancer.

## Contribution

The study identifies specific clinical risk factors and laxative types that affect colonic iodine-131 distribution after RAIT.

## Key findings

- Thyroid hormone withdrawal, higher RAIT doses, and fewer defecations increase colonic I-131 distribution.
- Stimulant laxatives reduce colonic I-131 distribution compared to osmotic laxatives in THW patients.
- Multivariate analysis confirms significant associations between clinical factors and colonic radiation exposure.

## Abstract

Radioactive iodine therapy (RAIT) is used to treat patients with thyroid cancer at high risk of recurrence or those with distant metastases. Small amounts of iodine-131 (I-131) are excreted in the stool after RAIT. Thyroid hormone withdrawal (THW) before RAIT can cause constipation, increasing radiation exposure to the colon. Although measuring colonic radiation using I-131 dosimetry would be challenging, colonic radiation dose can be estimated using I-131 whole-body scintigraphy post-RAIT. Therefore, we aimed to determine the clinical risk factors, including THW, associated with colonic distribution on I-131 scintigraphy post-RAIT.

We analyzed 251 patients who received 530 RAITs, categorized into two groups based on the preparation method. We compared the relationship between clinical risk factors (including preparation method) and colonic I-131 distribution 3 d post-RAIT. In addition, we compared the frequency and degree of colonic I-131 distribution between patients who received RAITs with stimulant laxatives and those who received RAITs with osmotic laxatives. Four subgroup analyses were conducted based on the preparation method and defecation frequency.

We performed 253 RAITs (47.7%) using recombinant human thyrotropin, while the remaining 277 RAITs (52.3%) were performed using THW. In the multivariate analysis, THW, higher RAIT dose (≥ 3.7 GBq), and fewer defecation frequencies (≤ 2 times) were significantly associated with a higher frequency of colonic I-131 distribution (p = 0.0206, 0.0020, and 0.0006, respectively). Of the patients treated using THW RAITs, which relieved constipation, those treated with RAITs with stimulant laxatives had significantly lower colonic I-131 distribution than did those treated with RAITs with osmotic laxatives (p = 0.0378).

THW, high-dose RAIT, and defecation frequency were significantly associated with colonic I-131 distribution. Pre-treatment strategies, such as the use of stimulant laxatives should be considered to reduce colonic radiation exposure.