The purpose of this study is to evaluate the real-time respiratory motion of the prostate and surrounding tissues/organs in the supine and prone positions and to investigate, using cine-MRI, whether a belly board can reduce respiratory-induced motion in the prone position. Cine-MRI scans were made of 13 volunteers in the supine and prone positions on a flat board and in two different prone positions using a belly board. Images in cine mode were recorded for 20 seconds. For each session, the points of interest (POIs) were located at the apex, base, mid-anterior surface and mid-posterior surface of the prostate; the tip of the seminal vesicle; the pubic symphysis; and the sacrum. The maximum range and standard deviation (SD) of the displacement from the mean value were calculated. The SDs for each of the four different positions were compared using a paired t-test. Respiratory-induced prostate motion was significantly larger in the prone position than in the supine position.However, when a belly board was used in the prone position, motion in the prostate and surrounding tissues/organs was significantly reduced. There were no significant differences between the two different positions using a belly board in any of the POIs.Key words: Prostate cancer; Radiotherapy; Intrafraction motion; Cine-MRI; Belly board.
IntroductionAlthough it is well known that the prostate position varies in response to changes in bladder and rectal filling (1), respiratory-induced motion of the prostate has not traditionally been considered in prostate cancer radiotherapy. However, it has recently become clear that prostate motion occurs in accordance with the respiratory cycle (2-5). Therefore, the respiratory-induced motion of the prostate may be considered a factor in locating and reducing intrafraction motion during treatment planning.In the published literature, variations have been shown in the results between the use of the prone and supine positions for prostate cancer radiotherapy. Several authors have demonstrated that the rectal irradiation dose was reduced in the prone position (6, 7), though it may be that the daily setup reproducibility is less accurate for the prone position, primarily due to systematic setup variations (7). In particular, it has been shown, using implanted radiopaque prostate markers and fluoroscopy, that ventilatory movement in the prostate is substantial in the prone position (2-4). A study using cine-magnetic resonance imaging (cine-MRI) also demonstrated that the amplitude of the respiratory-induced motion in the supine position is smaller than that in the prone position (5).