Prostate movements during radiotherapy affect therapeutic efficacy and adverse events. We assessed prostate movements using MR-video taken during MR-guided radiotherapy (MRgRT).

Sagittal MR-video was taken at 4 frames per second. The movements of four points, those are the abdominal wall (P1), anterior (P2), posterior (P3), and upper (P4) end of prostate were plotted on the Y-axis (up and down) and Z-axis (anterior and posterior). The movement of P3 and P4 from their baseline positions was analyzed during a single 7.25 Gy fraction in 19 initial cases who underwent MRgRT with a hydrogel spacer at our institution. The median age and PSA at diagnosis were 80 years (range: 59-88) and 8.490ng/mL (IQR: 5.516-25.938). The median length of MR-video recorded was 454 seconds, with 1,816 frames.

The movement of each case was not consistent, and statistically significant differences were observed. In three cases (15.8%), P3 moved more than 5 mm to anterior, averaging 4.3% of the time. P4 retreated more than 5 mm in four cases, averaging 53.8% of the time. P4 ascended more than 5 mm in nine cases (47.4%), averaging 24.7% of the time, and exceeded 10 mm in two cases. In one case (3.4%), P4 descended more than 5 mm, 69.2% of the time, yet never surpassed 10 mm. The anterior-posterior movement of P3 and the vertical movement of P4 were both under 5 mm in only three cases (15.8%). The anterior-posterior movement was greater than the up down movement. The movements of P3 and P4 were not the same, with P3 moving predominantly posteriorly and P4 moving predominantly upwardly. Three cases (15.8%) showed forward movement of 5 mm or more, but none of them moved more than 10 mm. There were 12 cases (63.2%) in which P3 or P4 moved more than 5 mm up and down or in the anterior-posterior direction.

The physiological movement of the prostate in the supine position at rest during MRgRT was analyzed. For both P3 (posterior) and P4 (upper), the main movement was backward and upward, so it was found that the prostate generally moved diagonally backward and upward. However, the movements of two points were not same, so it was not a parallel movement. Since the backward movement of P3 was greater than that of P4, it was confirmed that the prostate was moving backward and upward while bending forward in the supine position. Prostate movement varied greatly among individuals, with more than half of the patients moving more than 5 mm in either direction, and one patient moving more than 10 mm in the anterior-posterior and superior-inferior directions. Therefore, hydrogel spacer was to be beneficial in reducing the rectal dose. Real-time monitoring during treatment and instantaneous control of the radiation field were considered to be beneficial in terms of treatment efficacy and reduction of adverse events. Future follow-up studies will clarify the relationship between prostate movement and recurrence rate/ adverse events.