The Impact of Dynamic Parotid Volume Changes and Body Weight Loss on Dose Distributions for Patients with Nasopharyngeal Cancer Receiving Intensity-Modulated Radiation Therapy

Abstract

Background: Intensity-modulated radiation therapy (IMRT) has become the new promising treatment of nasopharyngeal cancer (NPC) in terms of providing excellent tumor coverage with significant sparing of the parotid glands. In this study, we intend to quantify the importance of the volume shrinkage of parotid gland and its influence on the dose distribution of parotid glands among patients with or without significant body weight loss. Material and Methods: From Feb. 2003 to Oct. 2005, 30 patients with NPC were enrolled into this study. 13 patients suffered from significant body weight loss (＞6% of their origin body weight at the end of the 4th week of the entire RT course). A 2nd plan CT scan images were acquired after every patient had received 41.4 to 46.8 Gy of prescribed dose. A second IMRT plan was generated for the cone-down treatment. However, we also created a 2nd plan (Hybrid 2nd plan) for the 1st CT images to assume if no 2nd plan CT images were available. Results: The reductions of body contours are highly correlated with the body weight loss of the patients. The parotid glands decrease in size no matter the patients had significant body weight loss or not. When comparing the dosimetric effects of Hybrid and New 2nd plan, there were no significant dosimetric differences of target coverage. However, increases in the doses of normal tissue such as brainstem, spinal cord, left and right parotid glands were noted. In the subgroup analysis, there were significant increase of spinal cord, brainstem, R't parotid gland doses and hot spots in PTV(subscript CTV1) for patients with significant body weight loss. For patients without significant body weight loss, there were increases of spinal cord and brainstem doses but the target dose coverage remained satisfactory. Conclusions: We conclude that the volume of parotid glands decreased significantly during the course of IMRT even in patients without significant body weight loss. A 2nd plan CT and a new 2nd IMRT plan based on the new CT scan are needed for the IMRT treatment of NPC.