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Abstract #2338

Change of Radiotherapy Planning Target Volume Delineated on Pre-Treatment and mid-RT Follow-up MRI After 3-4 Weeks of Treatment

Yang Zhang1, Liming Shi2, Xiaonan Sun2, Tianye Niu2, Ning Yue3, Jeon-Hor Chen1,4, Tiffany Kwong1,3, Min-Ying Lydia Su1, and Ke Nie3

1Department of Radiological Sciences, University of California, Irvine, CA, United States, 2Department of Radiation Oncology, Sir Run Run Shaw Hospital, Zhejiang University School of Medicine, Hangzhou, China, 3Department of Radiation Oncology, Rutgers-The State University of New Jersey, New Brunswick, NJ, United States, 4Department of Radiology, E-Da Hospital and I-Shou University, Kaohsiung, Taiwan

As tumor shows substantial shrinkage over the course of treatment, should radiation treatment volume be adjusted? A quantitative method using “radial distance”- the distance from the outer boundary of the tumor to the center of the rectum, was developed to evaluate the gross tumor volume (GTV) delineated on MRI acquired before treatment and after 3-4 weeks of radiation. In 35 patients, the mean tumor volume decreased from 19.1 to 10.5 cm3 but the mean radial distance only decreased slightly from 16.3 to 15.6 mm. When the remaining tumor was close to the rectal wall, the PTV should not be adjusted.

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