Abstract
Purpose: To report the characteristics of prostate motion as tracked using implanted fiducials during hypofractionated radiotherapy with CyberKnife. Method and Materials: Twenty one patients with prostate cancer who were treated with CyberKnife between January 2005 and September 2007 were selected for this retrospective study. The CyberKnife uses a stereoscopic X‐ray system to obtain the position of the prostate target through the monitoring of implanted gold fiducial markers. If there is a significant deviation, the treatment is paused while the patient is repositioned by moving the couch. The deviations calculated from X‐ray images acquired within the time interval between two consecutive couch motions constitute a data set. Results: A total of 427 data sets and 4439 time stamps of X‐ray images were analyzed. The average duration for each data set is 697 s. The spread of prostate position increases as the time elapses. At 30 s, a motion larger than 2 mm exists in about 5% of data sets. The percentage is increased to 8%, 11%, and 14% at 60 s, 90 s, and 120 s, respectively. A similar trend exists for other values of prostate motion. Conclusion: With proper monitoring during treatment, the prostate shifts observed among the patients can be kept well within the tracking range of the CyberKnife. On average the sampling interval of ∼40 s between consecutive X‐rays is adequate to ensure sub‐millimeter tracking. However, there is significant movement variation among patients and higher sampling rate is necessary in some patients.
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