Abstract: Background and purpose: When the patients with nasopharyngeal carcinoma (NPC) receive radiotherapy, their thyroids are inevitably involved. As a result, thyroid damage occurs. This study aimed to explore the effects of intensity modulated radiation therapy (IMRT) on dynamics of thyroid blood flow in patients with NPC. Methods: A total number of 68 patients with NPC were enrolled in the study who received primary treatment of radical radiation and chemotherapy from Jul. 2012 to Oct. 2013. And the TMN stage was from Ⅱ to Ⅳc according to UICC 2010. The treatment method consisted of 2 cycles of TPF induction treatment, concurrent radiation therapy (IMRT) with 2 cycles of DDP and 2 cycles of adjuvant therapy sequentially. Before radiotherapy, at the end of radiotherapy, 3 and 6 months after radiotherapy, serum free triiodothyronine (FT3), free thyroxin (FT4) and thyroid-stimulating hormone (TSH) concentrations of all cases were detected by electrochemiluminescence. The highest systolic velocity, mean velocity, minimum diastolic velocity, resistance index, and the value of all thyroid diameter lines were measured by type-B ultrasound. Results: All the patients were followed up for 6 months. Hypothyroidism: the incidence of immediate clinical hypothyroidism after radiotherapy was 5.9%; 3 months later, the incidence was 13.2%; and 6 months later, the incidence was 26.5%. The difference in volume change between before radiotherapy and at the end of radiotherapy had no statistical significance (P>0.05). The difference in volume change between 3 and 6 months after radiotherapy had statistical significance (P<0.05). The difference in FT3, FT4 and FSH between the end of radiotherapy and before radiotherapy had no statistical significance, while there was statistically significant difference between at the end of radiotherapy and 3 months after radiotherapy. The thyroid volume correlated with the average dose at the end of radiotherapy, 3 and 6 months after radiotherapy as shown by the single factor correlation analysis (P<0.05). The results of sinlge factor correlation analysis also showed that the occurrence of hypothyroidism correlated with thyroid dose-volume parameter V40 at the end of radiotherapy (P<0.05). The correlation between hypothyroidism and the average dose on thyroid 6 months after radiotherapy was demonstrated by independent t test (P<0.05). Hypothyroidism had no correlation with thyroid artery systolic maximum velocity and resistance index at the end of radiotherapy, 3 and 6 months after radiotherapy (P>0.05). Conclusion: The incidence of hypothyroidism may increase with time after radiotherapy. The volume may decrease with the increased dose of radiotherapy and the follow-up time. The patients with NPC after radiotherapy should be tested for thyroid lesions routinely. The thyroid dose-volume parameter V40 may be a predictor for acute radioactive thyroid lesions. The study did not reveal temporarily that hypothyroidism was associated with thyroid ultrasound blood flow velocity.

Key words: Radio therapy, Nasopharyngeal carcinoma, Thyroid hormone, Hypothyroidism, B-ultrasound