ELECTROMAGNETIC TRACKING OF INTRAFRACTION PROSTATE DISPLACEMENT IN PATIENTS EXTERNALLY IMMOBILIZED IN THE PRONE POSITION

Purpose: To evaluate intrafraction prostate displacement among patients immobilized in the prone position using real-time monitoring of implanted radiofrequency transponders. Methods and Materials: The Calypso localization system was used to track prostate motion in patients receiving external beam radiation therapy (XRT) for prostate cancer. All patients were treated in the prone position and immobilized with a thermoplastic immobilization device. Real-time measurement of prostate displacement was recorded for each treatment fraction. These measurements were used to determine the duration and magnitude of displacement along the three directional axes. Results: The calculated centroid of the implanted transponders was offset from the treatment isocenter >= 2 by mm, >= 3 mm, and >= 4 mm for 38.0%, 13.9%, and 4.5% of the time. In the lateral dimension, the centroid was offset from the treatment isocenter by >= 2 mm, >= 3 mm, and >= 4 mm for 2.7%, 0.4%, and 0.06% of the time. In the superior inferior dimension, the centroid was offset from the treatment isocenter by mm, mm, and mm for 16.1%, 4.7%, and 1.5% of the time, respectively. In the anterior posterior dimension, the centroid was offset from the treatment isocenter by >= 2 mm, >= 3 mm, and >= 4 mm for 13.4%, 3.0%, and 0.5% of the time. Conclusions: Intrafraction prostate displacement in the prone position is comparable to that in the supine position. For patients with large girth, in whom the supine position may preclude accurate detection of implanted radiofrequency transponders, treatment in the prone position is a suitable alternative. (C) 2010 Elsevier Inc.