Association Between Thyroid Function And Radiation Dose In Nasopharyngeal Carcinoma Patients

Background and ObjectiveNasopharyngeal carcinoma (NPC) is one of the most common malignant tumor in southern China.Radiotherapy is the main treatment for the patients,and more than 75% of whom combined with cervical lymph node metastasis when they go for a visit.In this case, radiation therapy will inevitably include part or the entire thyroid,and sometimes thyroid doses can up to 60Gy or more.Many scholars have observed that in the patients of long-term survival,there are some of hypothyroidism, thyroiditis, or thyroid nodules.Hypothyroidism is the most common complication. Typical symptoms and signs are cold, few words, easy fatigability, lack of facial expression,dry skin,memory loss,mental retardation,sinus bradycardia,anorexia,abdominal distension,constipation,loss of libido,etc,which greatly reduce the patient's life quality.Recent years,with the improvement of radiation therapy,patients survival rate has greatly increased,and how to improve the quality of life of patients has been paid increasing attention.But research about the association between thyroid function and radiation dose in nasopharyngeal carcinoma patients is relatively fewer.This study is mainly through monitoring FT3,FT4,TSH concentrations in serum of nasopharyngeal carcinoma patients before and after radiotherapy of 3,6,12,and 18 months,to understand the thyroid function changes,at the same time through monitoring patients thyroid volume change after radiation therapy of 6,12,and 18 months,to understand the thyroid morphological changes.In the end from the treatment planning system (TPS)we can read the irradiation dose of thyroid,so the relationship between thyroid function and radiation dose in nasopharyngeal carcinoma patients can be discuss.Methods and MaterialsPatients treated by radical chemo-radiotherapy between February and August 2008 received baseline thyroid hormones (fT3,fT4 and TSH) study and CT examination prior to radiotherapy.Subsequent hormone examinations were conducted at 3,6,12 and 18 months after completion of treatment,whereas follow up CT scans were performed at 6,12 and 18 months post-treatment.Patients with recurrent or residual diseases who required a second course of radiotherapy were excluded from the study.All these patients were confirmed by pathological examination,who had no history of head and neck surgery, endocrine or other thyroid disease.Results1. There was an overall decreasing trend in the thyroid gland volume post-RT,The mean volume was reduced from 17.2533±9.06402cm3 before radiotherapy to 14.1842±6.34759cm3 at 6 months post-RT,but the difference was not statistically significant(p=0.083),The mean volume at 12 months and 18 months post-RT was 12.8732±5.64541cm3,12.5698±5.53825cm3 respectively(p=0.014 and 0.009).The mean thyroid dose demonstrated a clear correlation with the percentage volume change at 6,12 and 18 months post-RT (P=0.034,0.002 and 0.001,r=-0.389,-0.544 and -0.594 respectively). Such correlation of the percentage volume change was not seen in the maximum dose in various time intervals(p=0.087,0.674,0.135,r=-0.318,-0.080 and -0.280 respectively).2. The mean values of fT3 hormone showed little change in the first 6 months and followed by a greater drop at 12 months post-RT.But a greater rise was observed at 18 months.The LSD test showed that differences between the 12 months post-RT value and those of the other 4 intervals (0,3,6 and 18 months post-RT) were all statistically significant (p<0.05).There were no correlations between the mean thyroid dose and changes of the fT3 levels.3. Similarly for the fT4 mean level,there were also mild changes in the first 6 months followed by a more obvious decrease at 12 months post-RT,A litter rise was observed at 18 months,The LSD test showed that differences between the 0 month value and those of the 12,18 months post-RT were both statistically significant (p<0.05).There were correlations between the mean thyroid dose and changes of the fT4 levels at 12 months post-RT(p=0.010,r=-0.461). 4. There was an overall increasing trend in the mean TSH level post-RT.The mean TSH level started to increase at 3 months post-RT(p=0.983).A greater rise was observed at 12 months(p=0.002),and at 18 months post-RT the mean TSH level rose to the highest(p=0.000).There were correlations between the mean thyroid dose and changes of the TSH levels at 12 months post-RT(p=0.038,r=0.381). ConclusionThe present study has demonstrated that radical external beam radiotherapy to NPC patients carry the risk of radiation induced hypothyroidism , which was manifested through the changes of physical gland size and peripheral thyroid hormone levels.The results also demonstrated the dose dependent phenomenon of the radiation induced hypothyroidism and any measures to reduce the thyroid dose in designing the treatment protocol should be considered.In addition,this study has also suggested a reference threshold mean thyroid dose of around 33 Gy during treatment planning stage so as to minimise the incidence of thyroid complications.