ESTRO 2024 - Abstract Book

S4096

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

were similar in vmDIBH patients and patients treated during free breathing, but we did find deviations in gamma imaging in the order of 35% of the patients [2]. The aim of the study is to evaluate the robustness of our treatment technique by investigating if deviations in chest wall position as measured with surface scanning resulted in clinically relevant changes in the dose-distribution.

Material/Methods:

Hybrid treatment plans using tangent (80% of total dose) and VMAT fields were given in vmDIBH with vocal instructions only to start and end breatholds with no feedback on the depth of the breathholds. Additional ConeBeam CT (CBCT) scans with chest wall position monitored by C-RAD surface scanning (C-RAD, Sweden) were taken immediately after the actual treatment in order to have a small time interval between C-RAD and CBCT measurement in at least 2 fractions. The duration of the breath hold was measured, and the mean chest wall position during a vmDIBH CBCT procedure was compared with the position during acquisition of the planning CT. We have the possibility to provide visual feedback to the patient, but this was not used in this study to establish the baseline robustness of our current treatment plan and setup. The dose/DVH parameters were recalculated on CBCT using our planning system Eclipse (Varian, USA). CBCTs were stitched with planning CTs to get a reliable lung DVH, as the field of view of the CBCT is too limited in superior-inferior direction. DVH values were correlated to C-RAD observed chest wall position deviations using linear regression.

Results:

13 patients with 36 CBCTs were included. The mean vmDIBH duration was 30±6 seconds. The average measured chest wall position differed on average 1.5±2.5 mm from the position in the treatment plan, showing a stable reproducible breath hold. The average CTV coverage (V95%) difference between the initial plan and the recalculated plan on the CBCT was -0.8±1.6% and the difference in mean heart dose was 0.00±0.02 Gy. Fig 1 (CTV coverage) and fig 2 (Mean hart dose) show that there is no correlation (low R 2 numbers) between the observed chest wall position and the DVH parameters and that on average the differences are small. For other DVH parameters a similar pattern was observed: no correlation and only small differences e.g. lung V20Gy -0.2±0.3% and mean lung dose 0.06±0.04 Gy.

Fig 1: Observed chest wall position difference and coverage (CTV V95%) difference in 36 fractions.