Purpose: To quantify three-dimensional (3D) movement of the prostate gland with the patient in the supine and prone positions and to analyze the movement frequency for each treatment position.
Methods and materials: The real-time tumor-tracking radiotherapy (RTRT) system was developed to identify the 3D position of a 2-mm gold marker implanted in the prostate 30 times/s using two sets of fluoroscopic images. The linear accelerator was triggered to irradiate the tumor only when the gold marker was located within the region of the planned coordinates relative to the isocenter. Ten patients with prostate cancer treated with RTRT were the subjects of this study. The coordinates of the gold marker were recorded every 0.033 s during RTRT in the supine treatment position for 2 min. The patient was then moved to the prone position, and the marker was tracked for 2 min to acquire data regarding movement in this position. Measurements were taken 5 times for each patient (once a week); a total of 50 sets for the 10 patients was analyzed. The raw data from the RTRT system were filtered to reduce system noise, and the amplitude of movement was then calculated. The discrete Fourier transform of the unfiltered data was performed for the frequency analysis of prostate movement.
Results: No apparent difference in movement was found among individuals. The amplitude of 3D movement was 0.1-2.7 mm in the supine and 0.4-24 mm in the prone positions. The amplitude in the supine position was statistically smaller in all directions than that in the prone position (p < 0.0001). The amplitude in the craniocaudal and AP directions was larger than in the left-right direction in the prone position (p < 0.0001). No characteristic movement frequency was detected in the supine position. The respiratory frequency was detected for all patients regarding movement in the craniocaudal and AP directions in the prone position. The results of the frequency analysis suggest that prostate movement is affected by the respiratory cycle and is influenced by bowel movement in the prone position.
Conclusion: The results of this study have confirmed that internal organ motion is less frequent in the supine position than in the prone position in the treatment of prostate cancer. RTRT would be useful in reducing uncertainty due to the effects of the respiratory cycle, especially in the prone position.