Abstract
Purpose:To quantify interfractional anatomic changes and the dosimetric variations in the parotid and submandibular glands in image‐guided IMRT for head and neck cancer.Methods:Daily diagnostic‐quality CTs acquired during daily IGRT using an in‐room CT (CTVision, Siemens) for 16 head and neck cancer patients treated with IMRT were analyzed. The contours of parotid glands and submandibular glands on dialy CTs were generated using an auto‐segmentation tool (ABAS, Elekta) with manual editing by a single physician. The volumetric changes were measured and the center of mass (COM) of each gland was obtained. The positional changes of the glands were characterized by inter‐parotid and inter‐submandibular gland distances. The original plan was applied to each fraction CT with isocenter shifts based on the alignment of the tumor. The DVHs and commonly used dose volume parameters from daily plans were compared to the original plans.Results:From fraction 1 to fraction 31, the average volume reduction was roughly 25% for all glands. The changes of inter‐parotid and inter‐submandibular gland distances were significant, with these distances decreased by 1.7±4.3 mm and 1.4±2.7 mm in average from fraction 1 to fraction 31. The average distance changes between the glands to the isocenter were from 0.4 to 3.3 mm. The mean dose changes for the four glands were from 109 cGy to 265 cGy.Conclusion:Significant decreases in the volume, inter‐gland distance were found during IG‐IMRT for head and neck cancer. These changes result in significant variations in doses to these glands, indicating the need of adaptive replanning.
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