Set-up accuracy of an external immobilisation system for patients receiving radical radiotherapy for prostate cancer

Abstract

Purpose: To evaluate the accuracy of an external immobilisation system in patients receiving radiotherapy for prostate cancer. Methods: Portal Imaging data were audited in 20 patients treated using an in-house immobilisation system and 20 patients treated using an indexed commercial immobilisation system (Combifix™). Individual and group random and systematic errors were calculated to determine the accuracy of set-up using skin marks alone and with a no-action-level protocol. Results: The initial results showed a larger systematic error in the Combifix™ in the anterior-posterior direction (2.7 mm) compared with the in-house system (1.5 mm). The possible source of this was identified as the difficulty in accurately aligning the laser to a curved couch top prior to setting the isocentre height. A change in the process of setting the isocentre was introduced, and comparable baseline set-up accuracy was achieved. This was with a systematic error of ≤2.0 mm and a random error ≤1.5 mm of patient position set-up error with skin marks alone, and using the Combifix™. The systematic errors were further reduced to <1 mm with an off-line no-action-level protocol. Conclusion: Using the Combifix™ system a high level of set-up accuracy was reproduced in routine daily practice. © 2011 Cambridge University Press.