Clonal analysis of early-stage bilateral papillary thyroid cancer identifies field cancerization

Introduction Bilaterality is a newly identified indicator for aggressive tumor behavior and poor outcome in papillary thyroid cancer. However, the clonal origin of these bilateral tumors remains unclear. Methods Here we analyzed 28 pairs of early-stage papillary thyroid cancers (stage I–II without extra-thyroidal extension, lymph node metastasis or distant metastasis) that underwent surgery at First Affiliated Hospital of Zhejiang University School of Medicine (Hangzhou, China). Genomic DNA was extracted from paraffin-embedded tissues after microdissection and analyzed for BRAF mutation and X-chromosome inactivation. Results A total of 16 patients (16/28, 57.1%) harbored different BRAF status in bilateral tumors. Fourteen patients were available for X-chromosome inactivation assay and 10 of them achieved informative results. Bilateral tumors from four cases had distinct patterns of X-chromosome inactivation. Combining the results of X-chromosome inactivation and BRAF analysis, we demonstrated that at least 64.3% (18/28) cases harbored discordant X-chromosome inactivation or BRAF status, indicating their independent clonal origin in bilateral tumors. Conclusions The present study confirms “field cancerization” in early-stage bilateral thyroid cancers, suggesting that these subtype papillary thyroid cancers should be treated as independent and localized tumors.