Risk of Thyroid Cancer after Therapeutic Irradiation in Adult Patients: An Age-Based SEER Analysis

Abstract

One risk of radiation therapy exposure (XRTe) is second primary thyroid cancer (SPTC). Previous reports examined this in children, but no population-based studies have explored XRTe and SPTC across all ages or stratified by histological subtypes. To examine trends in the histological subtypes of SPTC after therapeutic XRTe based on patient age and tumor characteristics. We report patient characteristics of a SEER dataset of SPTC (n=4,669) using Chi square and t-test. Odds ratio (OR) for SPTC was determined based on age, histology, and XRTe compared to expected values in the SEER database. Kaplan-Meier survival and Cox-proportional hazard ratios were reported to determine factors influencing latent survival (LS, time from initial diagnosis to SPTC) and overall survival (OS) in univariate and multivariate models. Extrathyroid extension and node status based on XRTe were similar (p=0.684 and p=0.776 respectively). XRTe patients were more likely to have smaller tumors (17.6 vs 19.3mm, p=0.007). XRTe patients were diagnosed with SPTC at younger ages (59.8 vs 62.7, p<0.001) without a difference in LS (7.45 vs 7.50 years, p=0.426). Patients age 1-14 and 15-29 at diagnosis of first cancer are at higher risk of SPTC after XRTe (OR=1.89, p=0.005 and OR=2.35, p=0.001, respectively), unlike patients age 30-44 and 45+ (OR=1.03, p=0.823 and OR=0.97, p=0.624, respectively). This difference is not present for follicular and medullary SPTC. Patients 30+ receiving XRT do not have an increased risk of SPTC. Deliberation is necessary in recommending, planning, and delivering XRT to minimize risk of subsequent malignancy in younger patients.