Risk factors associated with repeat Radioactive Iodine Therapy for differentiated Thyroid Cancer in a tertiary care center in the Philippines: A retrospective cohort study

ABSTRACT

Introduction@#Thyroid carcinoma remains the most common endocrine malignancy and incidence has increased due to improved diagnosis. Most Differentiated Thyroid Cancers (DTC) are indolent and easily cured with surgery, radioactive iodine (RAI) therapy and TSH suppression. However, persistent and recurrent disease is not uncommon among Filipinos. RAI therapy is being used more frequently even for low risk patients due to this observed aggressiveness of DTC in our population. This study sought to identify factors that are associated with failure of initial RAI for DTC, leading to repeat doses.@*Methods@#This is a single-center, retrospective cohort study conducted in the Nuclear Medicine Department of a tertiary care center in the Philippines, involving 325 patients who underwent RAI from 2006-2016@*Results@#Out of 570 patients who underwent RAI therapy for DTC, only 325 were included. Majority (n=280, 86%) had PTC and the rest had FTC (n=45, 14%). Twenty four percent (n=67) of the PTC group and 31% (n=14) of the FTC had subsequent RAI therapy after initial therapy due to either persistent or recurrent disease, with a mean interval of 21-22 months. Distant metastasis at presentation (M1), uptake in distant tissues on the initial post-therapy whole body scan (WBS) and TNM stage 4 were predictive for repeat RAI for FTC. A negative post-therapy WBS was found to be associated with no need for repeat RAI. On the other hand, the initial RAI dose of 150 mCi or higher was noted to be associated with repetition of RAI for PTC. Other risk factors noted were the presence of lymph nodes and distant metastasis at presentation and loco-regional uptake on the posttherapy WBS. Conversely, a negative post-treatment scan appeared to be protective against repeat RAI, as in FTC. However, multivariate analysis of risk factors showed that only metastasis at presentation (LN or distant) was associated with repeat RAI therapy. @*Conclusion@#The only risk factor associated with failure of initial RAI for patients with PTC and FTC in this study was distant metastasis at presentation. Nodal involvement at presentation was noted to be a significant factor for among those with PTC.