Subsequent Thyroid Carcinomas in Children and Adolescents Registered in the German MET Consortium (1997–2023)

<b><i>Introduction:</i></b> Among childhood cancer survivors, the cumulative incidence rate of differentiated thyroid carcinomas (DTCs) is estimated to be 8-11%. Although the association of DTC with prior radiotherapy is well-studied, the association with chemotherapy remains less understood. Most studies focused on young adults, leaving a knowledge gap on subsequent DTC occurring in childhood and adolescence. <b><i>Methods:</i></b> In this retrospective cohort study, we analyzed DTCs in children and adolescents under 18 years of age who were registered with the national multicenter Malignant Endocrine Tumor studies in Germany (1997-2023). We compared patients with first primary DTC to those with subsequent DTC that developed after a history of childhood cancer or hematopoietic stem cell transplantation (HSCT). In the subsequent DTC subgroup, we compared DTCs following chemotherapy only to those following chemo- and radiotherapy. <b><i>Results:</i></b> Of 505 patients with DTCs, 66 (13.1%) (38 male, 28 female) were subsequent DTCs. The median age at subsequent DTC diagnosis was 12.7 years (range, 5.1-17.9), with a median latency of 7.3 years (range, 2.2-15.6) from the first malignancy or HSCT. The 5-year overall survival (OS) and thyroid-related adverse event-free survival (EFS) estimates from the diagnoses of a subsequent DTC were 100.0% and 82.5%, respectively. Prior treatment included chemotherapy in 64 patients, with 18 receiving chemotherapy alone. In all, 46 subsequent DTC patients had a history of external radiotherapy, including 2 treated with radiotherapy only and 14 with total body irradiation. Two patients received ¹³¹I-metaiodobenzylguanidine treatment. Subsequent DTCs versus first DTCs were smaller in size but more frequently multifocal. Subsequent DTCs following chemotherapy only, compared with chemo- and radiotherapy, developed after a shorter latency (median 6.2 vs. 7.8 years), and were larger (median 1.86 vs. 1.18 cm). Patients with subsequent DTCs following chemotherapy only were younger at diagnosis (median 11.5 vs. 13.7 years). No differences were observed for OS and EFS. <b><i>Conclusions:</i></b> Presenting features of subsequent DTCs differ from primary counterparts, although the prognosis is not significantly different. Subsequent DTCs following chemotherapy only versus chemo- and radiotherapy DTCs were larger and diagnosed in younger patients after a shorter latency. More research is needed to identify risk factors and mechanisms potentially contributing to thyroid tumorigenesis post-chemotherapy.