BackgroundTo 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.MethodsIn 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.ResultsIn 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 V 25Gy (the percentage thyroid volume that receives >25 Gy, not the absolute volume) ≤60%, V 35Gy ≤ 55%, and V 45Gy ≤ 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 V 25Gy > 95%, V 35Gy > 90%, and V 45Gy > 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).ConclusionWe recommend V 25Gy ≤ 60%, V 35Gy ≤ 55%, and V 45Gy ≤ 45% as the “stringent” DVH line, and V 25Gy > 95%, V 35Gy > 90%, and V 45Gy > 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.

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