Evaluation of set-up position changes during the online adaptive radiotherapy

Abstract

Purpose: This study aimed to assess intrafractional positional shifts occurring during cone-beam computed tomography (CBCT)-based online adaptive radiotherapy (oART) using the Varian Ethos system. Specifically, we quantified these positional deviations and examined their relevance for planning target volume (PTV) margin definition in the absence of pretreatment verification imaging. Materials and Methods: A retrospective analysis was performed on 28 patients who underwent oART, covering a total of 627 fractions. For each fraction, an initial CBCT was used for adaptive planning, followed by a second CBCT immediately before treatment to assess intrafractional motion. Shifts between the two CBCTs were measured in the anterior-posterior (AP), superior-inferior (SI), and medial-lateral (ML) directions. The means f standard deviations and ranges are reported. Additionally, CBCT acquisition intervals, total treatment times, and monitor unit (MU) differences between the reference and adaptive plans were evaluated. Results: We found mean positional shifts of 0.11 f 0.09 cm (AP), 0.07 f 0.14 cm (SI), and 0.06 f 0.09 cm (ML). We calculated PTV margins with all position corrections, ranging from 2.5 to 3.9mm depending on the region and direction. CBCT intervals ranged from 11.4 to 17.5 minutes, and total treatment time from 14 to 45 minutes, depending on site. No strong correlation was observed between treatment time and the magnitude of positional shifts, although weak but statistically significant correlations were observed for prostate (AP) and rectum (ML). MU differences between the reference and adaptive plans reached statistical significance for prostate, endometrial, and bladder treatments. Conclusions: Intrafractional positional changes during oART are clinically relevant and may compromise geometric accuracy when not accounted for. In the absence of pretreatment imaging, individualized PTV margins based on treatment site and direction should be considered. Incorporating secondary CBCT or adopting site-specific margin strategies is recommended, particularly for pelvic treatments. (c) 2026 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.