Background: Thyroid cancer is the most common endocrine malignancy, with an increasing incidence globally and in Oman. The standard treatment for differentiated thyroid cancer (DTC) involves radioactive iodine (RAI). However, previous studies have suggested that RAI treatment may increase the risk of second primary malignancies (SPM). Despite the high incidence of thyroid cancer in Oman, to our knowledge, there are no published reports on the association between RAI treatment and the risk of SPM in Oman.

Objective: There is a lot of debate about the possibility of developing SPM in DTC patients after treatment with RAI. This research aimed to evaluate the incidence and estimate the risk of SPM in thyroid cancer patients treated with RAI.

Materials and Methods: A retrospective cohort study was conducted at Sultan Qaboos University Hospital (SQUH) for 500 DTC patients who received RAI treatment between January 2007 and December 2017. We collected patients’ information, including gender, age at diagnosis, thyroid cancer subtypes, site of SPM, cumulative RAI doses, and follow-up period. Descriptive statistics and Mann-Whitney test were used to analyze the data. SPM was defined as a new malignancy diagnosed at least one year after the first RAI dose.

Results: The mean follow-up period was 9.5 ± 3 years (range 5.1-15.8). During this period, four patients (0.8%) developed SPMs, all with the papillary subtype. The sites of the SPMs were the colon, bladder, breast, and liver. The Mann-Whitney test, comparing cumulative RAI doses between the high-dose and SPM groups, yielded a significant p-value (<0.001).

Conclusion: The incidence of SPMs in patients with thyroid cancer treated with RAI is low, and age at diagnosis was found to be the only significant predictor of SPM occurrence. Further studies with larger sample sizes and extended follow-up periods are recommended to confirm these findings.