ESTRO 2023 - Abstract Book

S612

Monday 15 May 2023

ESTRO 2023

Surface guided radiation therapy (SGRT) enables quick patient positioning, respiration gating and intrafractional motion monitoring and recording. This study investigated SGRT records considering the following questions: 1) How does stability compare between different treatment sites? 2) Is the patient's position more stable with deep inhalation breath hold (DIBH) than with free breathing (FB)?

3) Is there an evolution of stability during the treatment course? 4) Does the beam-on time have an effect on the patient’s stability?

Materials and Methods This study included 158 patients and 2522 fractions. The treatment sites were the pelvis, the lung, the brain, the head and neck (H&N) area, and the breasts. The data was recorded using a commercial SGRT system (Catalyst HD, C-Rad). Within a relevant region of interest (ROI), the system uses a non-rigid deformable registration to determine the treatment isocenter shift, and records its Euclidean norm, called the “Deviation" of the isocenter (DI). For the analysis, each fraction was averaged during beam on, during beam off, and during the whole fraction from the moment the patient has been IGRT-positioned to the end of the last treatment beam. The data was further grouped and analyzed by fraction number and/or by site. Results The mean DI was 0.4mm, 95% CI [ 0.3 , 0.4 ] (with quartiles 0.3mm[0.2,0.4]) for the Brain, 0.6mm[0.3, 0.9] (0.4mm[0.3,0.5]) for the H&N, 1.1 mm[0.9, 1.3] (0.7mm[0.5,1.2]) for the Pelvis, 1.1mm[0.8, 1.4] (0.7mm[0.5,1.0]) for the Lung, 1.2mm[1.1, 1.4] (0.7mm[0.5,1.1]) for the FB Breast. With DIBH breast, the average DI was 1.81mm[1.74, 1.89] (with quartiles 1.7mm[1.2,2.2]) during breath hold. During beam-on time, for the first 15 fractions the DI trend was -0.02mm/fraction +- SE 0.01mm overall, and -0.17mm +- SE 0.11mm for the Pelvis was the steepest. The median DI was larger during beam-on for all sites, by 0.2mm for the brain, the FB breast and H&N sites, and by 0.4mm for the pelvis and the lung.

Conclusion Fixated with open masks, the brain and H&N were more stable. Contrarily, an off-equilibrium technique requiring the patient's voluntary control of their chest position to enable and disable the treatment beam, DIBH was less stable than free-breathing for breast treatments. There was some evolution of stability during the treatment course, in particular for the pelvis which will require further inspection, as well as considering patients’ individual trends.