SU‐E‐T‐605: A Modified Adaptive Radiotherapy Based On CT‐CT Image Fusion with a Repeat CT in the VMAT Treatment of Nasopharyngeal Cancer Patients

Abstract

Purpose: To investigate the volumetric and dosimetric changes of a CT‐CT image fusion based adaptive radiotherapy by using a repeat CT as contour references. Methods: Nine nasopharyngeal cancer (NPC) patients underwent a modified adaptive volumetric modulated arc radiotherapy (VMAT) based on CT‐CT image fusion were enrolled in this study. The volumetric and dosimetric changes of gross tumor volume (GTV) and organs at risk (OARs) were investigated with a repeat CT at 23rd fraction. A commercially deformable registration software package, MIMVista, was applied to verify the contour and dosimetric changes accuracy of the CT‐CT image fusion based modified adaptive radiotherapy. Results: The average volume of GTV decreased from78.83±38.42 cm3 in the initial planning CT to 71.44±37.46 cm3 in the repeat CT, but with no significant difference (p=0.66). The average volume of the left parotid and right parotid decreased from 19.91±4.89 cm3 and 21.58±6.16 cm3 in initial planning CT to 11.80±2.79 cm3 and 13.29±4.17 cm3 in the repeat CT (both p<0.01), respectively. The volume of other OARs did not shrink very much. Dice similarity coefficient (DSC) and mass center distance (MCD) verified the accuracy of CT‐CT image fusion. No significant difference on GTV and CTV coverage was observed. Significant differences in volume of left parotid (p=0.03) and right parotid (p<0.01) receiving 5 Gy were observed between the initial plan and the re‐optimized plan. The mean dose of left parotid (p=0.08) and right parotid (p<0.01) were also significantly different. There was no significant difference on the dose delivered to other OARs. Conclusion: Patients with NPC undergoing VMAT have significant anatomic and dosimetric changes to parotids. Repeat CT as an anatomic changes reference and re‐optimization with initial CT based on CT‐CT image fusion was accurate enough to identify the volume changes and to ensure safe dose to parotids.