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Since July 2020, CBCT-guided online adaptive radiotherapy (oART) is the standard treatment for prostate cancer patients in our department. The applied CTV-PTV margin has been the same as for image-guided radiotherapy. However, with CBCT-guided oART the remaining uncertainty comes primarily from intrafraction motion. The purpose of this retrospective study was to evaluate the intrafraction prostate motion with respect to the currently used CTV-PTV margin.

Between November 2020 and March 2021, 28 prostate cancer patients were clinically treated using Ethos therapy (Varian Medical Systems, Palo Alto, CA, US) with a dose of 60 Gy in 20 fractions to the prostate and 54 or 60 Gy in 20 fractions to the seminal vesicles for more advanced stages. For all patients, a 9- or 12-field IMRT plan was created, applying a CTV-PTV margin of 7 mm in lateral (x) and anterior-posterior (y) direction and 8 mm in superior-inferior (z) direction. For all patients a CBCT was acquired at the start (CBCT1) and prior to treatment delivery for position verification (CBCT2). On a weekly basis, a third CBCT was acquired after treatment delivery (CBCT3). In this study, a virtual couch shift was retrospectively applied based on the prostate motion determined from CBCT2 using the gold fiducials within the prostate. The remaining prostate motion between CBCT2 and CBCT3 was determined based on a gold fiducial match and was assumed to be the intrafraction prostate motion during oART delivery. Moreover, the time between CBCT2 and CBCT3 was recorded.

In total, 124 fractions were evaluated. The mean time ± standard deviation between CBCT2 and CBCT3 was 4.2±0.6 minutes (range: 3.2-7.1 minutes). Median intrafraction prostate motion ± standard deviation was 0.0±1.2 mm, 0.0±1.5 mm and 0.1±1.5 mm in x-, y- and z-direction, respectively. Histograms of the intrafraction prostate motion in the different directions are shown in Figure 1. The intrafraction prostate motion was within the currently used CTV-PTV margin in 99.2%, 100% and 100% of fractions in x-, y- and z-direction, respectively. The 95^th percentile of the intrafraction prostate motion was 1.7 mm in x-direction, 3.1 mm in y-direction and 3.2 mm in z-direction. 

The measured intrafraction prostate motion during CBCT-guided oART delivery was within the currently used CTV-PTV margin of 7 to 8 mm in ≥99% of fractions. When taking the 95^th percentile of the intrafraction prostate motion, the required population margin is 2 mm in x-direction and 4 mm in y- and z-direction, suggesting that smaller CTV-PTV margins can be safely implemented in clinical practice. Further research is necessary to determine the intrafraction motion of the seminal vesicles.