Background: To identify thyroid dose-volume thresholds for radiotherapy (RT)-related hypothyroidism (HT) in patients with nasopharyngeal carcinoma (NPC) treated with intensity-modulated RT (IMRT). In this way, we desired to guide the design of treatment plans and, finally, lower HT prevalence.
Methods: In total, 345 NPC patients treated with IMRT were evaluated retrospectively during a median follow-up of 45.2 (range, 11.3-64.9) months. Serum-based assessments of thyroid function before and after IMRT were monitored periodically. Thyroid dose-volume parameters were analyzed for their association with HT risk.
Results: In total, 44.1% of patients (152/345) developed primary HT. Analyses of thyroid dose-volume parameters identified a stringent dose-volume histogram (DVH) threshold defined by V25Gy (the percentage thyroid volume that receives >25 Gy, not the absolute volume) ≤60%, V35Gy ≤ 55%, and V45Gy ≤ 45%. Patients whose thyroid DVHs satisfied these constraints had a lower prevalence of 2-year HT compared with the overall prevalence (13.2% vs 25.8%, P < .001). Another DVH was defined by V25Gy > 95%, V35Gy > 90%, and V45Gy > 75%, and patients whose thyroid DVHs satisfied with these constraints had a higher prevalence of 2-year HT than the overall incidence (36.0% vs 25.8%, P < .001).
Conclusion: We recommend V25Gy ≤ 60%, V35Gy ≤ 55%, and V45Gy ≤ 45% as the "stringent" DVH line, and V25Gy > 95%, V35Gy > 90%, and V45Gy > 75% as the "inhibition" DVH line, under the precondition of not compromising the target coverage. These findings could help in the design of individual treatment plans and, eventually, to lowering of HT prevalence.
Keywords: hypothyroidism; intensity-modulated radiotherapy; nasopharyngeal carcinoma; prevalence; risk factors.
© 2019 The Authors. Cancer Medicine published by John Wiley & Sons Ltd.