ESTRO 2024 - Abstract Book

S4213

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Purpose/Objective:

The Royal Surrey has treated over 800 fractions of Bladder cancer with CBCT guided online adaptive radiotherapy (oART) using the Varian Ethos. Daily re-contouring and re-planning aims to reduce required PTV margins to reduce OAR doses whilst maintaining target coverage. Previously, we treated some patients using derived bespoke margins based on the work by Astrom [1], however this required significant staff resource as margins were derived after analysis of the first 3 fractions of treatment for each patient. An alternative method for deriving margins based on a larger population was devised and has been investigated for reduction of margins for oART. A retrospective planning study has been performed comparing standard margins, bespoke margins and the new population based margins to determine the potential benefits of oART using each method and the feasibility of clinical implementation. The standard oART workflow is to take an initial CBCT (CBCT1) which is used for daily contouring and plan generation. Immediately prior to treatment delivery a second CBCT (CBCT2) is taken and any required couch shifts are applied to ensure target coverage due to oART intra-fraction motion. The CBCTs are registered together with any applied couch shifts and can be reviewed within Ethos. A total of 376 fractions from 20 consecutive patients treated with oART for Bladder cancer on the Varian Ethos have been assessed. We measured the motion of the target volume between CBCT1 and CBCT2 in each orthogonal direction for all fractions. The population PTV margins were derived by taking the 90th percentile of the measured motion in each direction and adding an additional 2mm to account for uncertainties in daily contouring and machine positioning. Bespoke margins were calculated from the maximum motion from the first 3 fractions plus an additional +5mm margin for residual errors as per Astrom [1]. Three treatment plans were generated using our standard 9-field IMRT Ethos planning template. One using standard IGRT margins, a second using the bespoke margins as would be derived by Astrom [1], and a third using the population margins based on the local data. DVH statistics were extracted for the PTV, Bowel, Rectum. The DVH parameters from our clinical protocol (Bowel V52Gy, V48Gy, V43Gy, V39Gy), Rectum (V52Gy, V43Gy) and PTV (D0.1cc, D99.5%) were compared. The dosimetry from the plans generated using the bespoke and population based margins were compared with the standard IGRT plan dosimetry. Material/Methods:

An additional 180 fractions from 10 patient’s motion was measured in the same way to determine the proportion that would have CTV within the derived population margins.

Results:

The measured margins in each direction are shown in Figure 1 along with the associated derived population margins. Treatment times (CBCT1 to completion of beam delivery) has mean of 21 minutes (range 16 - 31).