Adjuvant External Beam Radiotherapy Reduces Local Recurrence in Poorly Differentiated Thyroid Cancer : A Multicenter Retrospective Cohort Study Describing Outcomes in the Treatment of Resectable Poorly Differentiated Thyroid Cancer.

BACKGROUND

Poorly differentiated thyroid carcinoma (PDTC) accounts for 5% of all thyroid cancers and is responsible for a large proportion of thyroid cancer-related deaths. The optimal treatment approach is not clear. This study aimed to evaluate the effect of postoperative intensity-modulated radiotherapy (IMRT) on the treatment of resectable PDTC. Additionally, treatment-related morbidity, characteristics of I-refractory disease, and factors affecting survival were assessed.

METHODS

The study included consecutive PDTC cases from 1997 to 2018, defined according to Turin criteria and treated in two tertiary referral centers. Surgery, IMRT, I, and systemic therapies were administered based on multidisciplinary team recommendations. The primary study outcome was 5-year local control after IMRT in cases with positive resection margins (micro- and macroscopic). The secondary outcomes were treatment-related morbidity within 30-days after completion of treatment (Clavien-Dindo and Common Terminology Criteria for Adverse Events [CTC-AE] 5.0), I-refractory disease characteristics using standardized definitions, and factors influencing survival.

RESULTS

Among 51 PDTC cases, 53% presented with metastatic disease. Adjuvant IMRT improved 5-year local control (100% vs. 17.5%; p = 0.02), with a higher number of grades 1 to 3 complications (p = 0.005) versus cases without IMRT. Within 13 months, I-refractory disease occurred in 62.7% of the patients and was more common in non-survivors (86.6% vs. 52.8%; p = 0.01). Positive resection margins and extrathyroidal extension were associated with poor survival in the univariate analysis, but were not significant in the multiple regression analysis.

CONCLUSION

Adjuvant IMRT may reduce thyroid bed recurrence in resectable PDTC with positive resection margins, but is associated with increased treatment-related complications. I-refractory disease occurs frequently, with non-survivors progressing earlier to I resistance.