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An Investigation of Interfractional Rectum Shape Variations Based On a Statistical Shape Analysis for Prostate Cancer Radiotherapy

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M Haekal

M Haekal1*, H Arimura1 , Y Shibayama2 , T Hirose1 , S Ohga1 , Y Umezu2 , Y Nakamura2 , H Honda1 , T Sasaki1 , (1) Kyushu University, Fukuoka-shi, Fukuoka-ken, Japan, (2) Kyushu University Hospital, Fukuoka-shi, Fukuoka-ken, Japan


SU-I-GPD-T-237 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate interfractional shape variations of the rectum based on a point distribution model (PDM) for prostate cancer radiation therapy.

Methods: The interfractional shape variations of the rectum were quantified as standard deviations (SDs) of systematic and random errors by applying PDM to the contours delineated on planning computed tomography (CT) and cone beam CT (CBCT) images at 80 fractions of 11 patients. The PDM were generated from the vertices of triangulated rectum surfaces, which were derived from the original contours. The systematic error for the shape variations was calculated as the average deviation between surfaces of planning rectum and an average rectum, which was defined as the average of the rectum surfaces of all fractions for each patient. The random error for the shape variations was computed as the average deviation of rectum surface vertices for fractions from an average rectum surface. The SDs of the interfractional random errors for shape variations later were obtained from covariance matrices based on the PDMs.

Results: The means of SD of systematic and random errors were more than 1.0 mm along all directions, especially, more than 1.5 mm along the anterior direction for random error. The anterior sides of rectums are proximal to the prostate posterior sides. Deviations on the anterior sides of the rectum indicate the positional uncertainty of the rectum exposed to the high-dose region during treatment course.

Conclusion: The shape variations of the rectum, especially in the anterior sides, should be taken into account when determining the planning organ at risk volume (PRV) margin during radiotherapy planning process.

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