| Objective: To explore the anatomic and dose changes of parotid through the course of intensity-modulated radiotherapy(IMRT) for nasopharyngeal cancer patients. Analysis the rule of their variation to provide evidence for the necessary of replanning and selection of the best modification time.Methods: The study enrolled 18 nasopharyngeal cancer patients, each patient accepted once KV-CBCT scan every week before the 30 fractions. The outline of parotid glands were contoured on the images which were fused between planning CT images and kilovoltage cone beam CT images. And then the volume and location variation of parotid glands were measured and analyzed. A repeat CT scan was performed after 20 fractions. A hybrid plan1-2 was made by copy plan1 to the repeat CT scans. The magnitude of the parotid dosimetric changes between the two plans were observed. The xerostomia of patients was followed up one month after radiotherapy completed. The relevance between xerostomia and parotid glands variations were detected.Results: The volume of parotid gland narrowed significantly during the radiotherapy, the left and right parotid volume respectively reduced from 9.86~0.49 cm3 and 6.16~48.75 cm3 to 7.65~28.92 cm3 and 4.96~28.79 cm3 at the sixth week. The volume of left parotid narrowed most obviously in the fourth week, average shrinking 2.2%/d. The volume of right parotid narrowed most obviously in the third week, average shrinking 2.0%/d. The variations of parotid gland volume related to its initial volume and weight loss. Parotid volume narrowed mainly due to the lateral edge of gland shift. At the sixth week, the left and right lateral edge of parotid glands shifted to the midline with an average of 0.67cm、0.76cm(P<0.05) average shifting 0.11cm/d and 0.13cm/d. While the medial edge of parotid shifted insignificantly(P>0.05). The Dmean and D50 of left and right parotid increased by 17.1%、19.2%(P<0.05) and 18.8% and 18.7(P<0.05). The increase of parotid gland dose correlated with the degree of weight loss and its volume reduction. 1 months after radiotherapy, most of patients(61.1%) have moderate xerostomia, 16.7% and 22.2% patients express mild and severe xerostomia, respectively. The degree of xerostomia is associated with the Dmean of parotid gland, but has no correlation with parotid volume changes.Conclusion: During the radiotherapy of nasopharyngeal carcinoma(NPC), the volume of parotid gland and displacement had changed significantly. Parotid gland gradually moved to the high dose area, make its actual illuminated dose beyond plan set dose. In order to protect the parotid gland function. A new plan indicated in the middle of the radiotherapy(3~4 weeks) can further reduce the illuminated dose of parotid gland. |
