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 Chen^1,4, Tiffany Kwong^1,3, 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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