A prospective comparison of three systems of patient immobilization for prostate radiotherapy

Purpose: The study compared the setup reliability of 3 patient immobilization systems, a rubber leg cushion, the alpha cradle, and the thermoplastic Hipfix device, in 77 patients with cT1-T3, NO, MO prostate cancer receiving conformal radiotherapy. Methods and Materials: Port films were analyzed and compared to simulation films to estimate the setup errors in the three coordinate axes (anterior-posterior, cranial-caudal, medial-lateral). A total vector error was calculated from these shifts. Results: The Hipfix was found significantly superior to the other two devices in reducing mean setup errors in all axes (p < 0.005). The average field-positioning error with the Hipfix ranged from 1.9 mm to 2.6 mm for all axes, whereas the deviation for the other two systems ranged from 2.7 to 3.4 mm. Errors greater than 10 mm were virtually eliminated with the Hipfix system. There was a reduction in the mean total vector error in the alpha cradle and Hipfix patient cohorts over time, reflecting improved efficacy as a result of experience. Conclusion: There was a significant difference in the performance of each immobilization device. The Hipfix was consistently more reliable in reducing setup errors than the other devices. (C) 2000 Elsevier Science Inc.