This study aimed to reduce radiation-induced thyroid injury without compromising control of the cervical region by optimizing the delineation of the cervical lymph node drainage area.

Since the neck target area of nasopharyngeal carcinoma is close to the thyroid gland, the average dose to the thyroid gland is usually around 54Gy, which is difficult to meet the protection requirements for thyroid protection. In the current guidelines, the inner edge of the common carotid artery is used as the inner boundary of the lymph nodes in levels III and IVa, and the lymphatic drainage area needs to include the inner edge of the common carotid artery. Several studies have found that the centers of positive lymph nodes in cervical regions III and IVa are located outside the inner border of the common carotid artery. In order to better protect the thyroid gland, the investigators have improved the delineation of the neck target area. For patients with negative lymph nodes in regions II and IVa (accounting for about 60% of the total patients), a modified neck target volume delineation method was used to define the inner boundary of regions III and IVa as the outer edge of the common carotid artery. For patients with positive lymph nodes in regions III and IVa (accounting for about 40% of the total patients), the inner boundary of regions III and IVa is defined as the inner edge of the common carotid artery or the outer edge of the thyroid. This study aimed to reduce radiation-induced thyroid injury without compromising control of the cervical region by optimizing the delineation of the cervical lymph node drainage area.

Inclusion Criteria:

1. The pathological diagnosis was newly diagnosed non-keratinizing squamous cell carcinoma nasopharyngeal carcinoma.
2. The clinical stage is T1-4N0-3M0 (stage I~IVa, UICC/AJCC 8th edition);
3. There is no history of hypothyroidism or hyperthyroidism before treatment;
4. Using intensity-modulated radiotherapy technology,
5. Complete radical dose radiotherapy;
6. KPS≥70 points;
7. Age 18-70 years old;
8. Male or female who is not pregnant or lactating;
9. Hematological examination: white blood cells > 4×109/L, neutrophils > 2×109/L, hemoglobin > 90g/L, platelets > 100×109/L;
10. Liver function: ALT, AST < 1.5×ULN, ALP < 2.5×ULN, bilirubin < ULN.
11. Renal function: creatinine > 60 ml/min;
12. The patient signs the informed consent form;

Exclusion Criteria:

1. WHO pathological classification is keratinizing squamous cell carcinoma or basaloid carcinoma;
2. palliative treatment;
3. Have a history of other malignant tumors;
4. Pregnant or lactating women (women of childbearing age should consider taking a pregnancy test, and patients are required to use effective contraception during treatment);
5. Have a history of neck surgery;
6. Have a history of hyperthyroidism or hypothyroidism.
7. Previous history of head and neck radiation therapy;