Clinical analysis for weekly changes of parotid glands during intensity-modulated radiotherapy for nasopharyngeal carcinoma patients

Background and purpose: It is reported that the volume and dosimetry of parotid glands would change,however,the weekly changes of them and when to attain the most distinguished moment are controversial and the results can not effectively guide the clinical practice.The study aimed to analysis weekly volume,displacement,dimension of C2 and dosimetric changes of parotid glands during intensity-modulated radiotherapy for nasopharyngeal carcinoma patients,and the relationship between dimension of C2 and xerostomia in order to guide the best time for altering the radiation planning.Methods: Ten nasopharyngeal patients receiving intensity-modulated radiotherapy had repeat CT scanning at weeks 1,2,3,4 and 5 during radiation.All repeat CTs were fused with the planned CT and contoured the parotid glands.Weekly volume,displacement,dimension of C2 and dosimetric changes of parotid glands were calculated and the xerostomia of patients was followed after 3 months of radiotherapy.Results: In the fifth week of radiotherapy,the volume of left and right parotid glands decreased by 29%-44.9% and 28%-57.1%,the lateral displacement of centers of both parotid glands were ?4.1 to 9.2 mm and ?2.8 to 9.8 mm,and the dosimetry increased to 148.3%-259.4% and 198.1%-274.7%,respectively.The dimension of C2 decreased gradually.We can detected the most distinguished changes of volume,displacement and dosimetry for the left parotid gland in the third week,however their most distinguished changes for the right parotid gland were in the third week,second week and fourth week respectively.The dimensional change of C2 was correlated with post radiotherapy xerostomia significantly.Conclusion: The most distinguished changes of dosimetric changes for both parotid glands occur in the third week,and it is the best time to modify the radiotherapy plan.