Analysis of the Inter- and Intra-treatment Isocenter Deviations in Pelvic Radiotherapy With Small Bowel Displacement System

Kim, Moon Kyung; Kim, Dae Yong; Ahn, Yong Chan; Huh, Seung Jae; Lim, Do Hoon; Shin, Kyung Hwan; Lee, Kyu Chan

Abstract

PURPOSE
To evaluate the extent and frequency of the inter- and intra-treatment isocenter deviations of the whole pelvis radiation field in using small bowel displacement system (SBDS).
METHODS
AND MATERIALS: Using electronic portal imaging device (EPID), 302 postero-anterior (PA) and 232 lateral portal images were prospectively collected from 11 patients who received pelvic radiation therapy (7 with cervix cancer and 4 with rectal cancer). All patients were treated in prone position with SBDS under the lower abdomen. Five metallic fiducial markers were placed on the image detection unit for the recognition of the isocenter and magnification. After aligning the bony landmarks of the EPID images on those of the reference image, the deviations of the isocenter were measured in right-left (RL), cranio-caudal (CC), and PA directions.
RESULTS
The mean inter-treatment deviation of the isocenter in each RL, CC, and PA direction was 1.2 mm (+/-1.6 mm), 1.0 mm (+/-3.0 mm), and 0.9 mm (+/-4.4 mm), respectively. Inter-treatment isocenter deviations over 5 mm and 10 mm in RL, CC, and PA direction were 2, 12, 24%, and 0, 0, 5%, respectively. Maximal deviation was detected in PA direction, and was 11.5 mm. The mean intra- treatment deviation of the isocenter in RL, CC, and PA direction was 0 mm (+/-0.9 mm), 0.1 mm (+/-1.9 mm), and 0 mm (+/-1.6 mm), respectively. All intra-treatment isocenter deviations over 5 mm in each direction were 0, 1, 1%, respectively.
CONCLUSION
As the greatest and the most frequent inter-treatment deviation of the isocenter was along the PA direction, it is recommended to put more generous safety margin toward the PA direction on the lateral fields if clinically acceptable in pelvic radiotherapy with SBDD.