Introduction: Breast cancer radiation therapy with deep-inspiration breath-hold reduces the mean radiation dose to the heart and lung. Intrafractional shift monitoring has great importance in assessing possible patient movement during treatment. Purpose: The research aim was to use CBCT imaging assess intrafractional shifts in DIBH technique with ABC system of breast radiation therapy and evaluation of the existing safety margin. Methods: 30 breast cancer patients were included in the study, which were treated post-surgically with DIBH technique on the accelerator Elekta Synergy Platform with ABC system at the Institute of Oncology Ljubljana from October 2020 until May 2021. For 3DCRT and IMRT techiques we used daily on-line CBCT imaging before treatment to reduce interfractional shifts and after irradiation for evaluation of intrafractional shifts. Additional imaging after irradiation was performed over the first three fractions of radiotherapy, then additionally once a week. We obtained and analyzed 1050 shifts in 350 on-line CBCT imaging in three translation directions. Results: Mean intrafractional shifts for all 30 patients were 0,03 ± 0,17 cm in the longitudinal direction, -0,02 cm ± 0,13 cm in the lateral direction and 0,07 cm ± 0,21 cm in the vertical direction. Our study of mean shifts contribution showed that mean distribution of longitudinal, lateral and vertical directions comply with the Gaussian distribution. 175 CBCT imaging before irradiation showed 40 shifts in the longitudinal direction with a maximum of -1,4 cm inferior, 15 shifts in the lateral direction with a maximum of -1,0 cm right and 75 shifts in the vertical direction with a shift of -1,6 cm posterior. 175 post-irradiation CBCT images after irradiation showed 3 shifts in the longitudinal direction with a maximum of -0,6 cm inferior, 1 shift in the lateral direction which shifted -1,0 cm to the right and 8 shifts in the vertical direction with a maximum of -0,8 cm posterior. The contribution of the intrafractional shifts to the safety margins according to Van Herk’s formula were 0,50 cm in the longitudinal direction, 0,35 cm in the lateral direction and 0,43 cm in the vertical direction, which means that they are smaller than 0,5 cm margin we use in our practice. Discussion and conclusion: We concluded the patient motion during the deep inspiration treatment was within the tolerances and the patients were treated correctly. Time intervals between starting and later fractions did not have an impact on intrafractional shift differences. Intrafractional shifts were larger in starting fractions and decreased during irradiation and were irrelevant within set-up and verification CBCT imaging. Estimated safety margin for the PTV in our study did not show that the safety margin in use is too small, as we did not evaluate other factors contributing to it in this study, we cannot give a definite answer about its sufficiency.
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