Off-line setup corrections only marginally reduce the number of on-line corrections for prostate radiotherapy using implanted gold markers

Purpose: To evaluate the efficiency of combining on-line and off-line corrections for the positioning of patients receiving external beam radiotherapy for prostate cancer. Materials and methods: Daily portal images were acquired during treatment of 102 patients to verify and correct the position of the prostatic gland using implanted gold markers. in addition to an existing off-line procedure, on-line corrections were applied in the anterior posterior (AP) direction only, to limit the increase in daily workload. The possible increase in workload of the combined correction procedure for on-line corrections in either two or three directions was further investigated by simulating the required position corrections for 500 treatments. Results: The combined correction procedure in AP-direction resulted in a systematic dispersion and random variation of 0.3 mm (1 SD) and 1.0 mm (1 SD), respectively. Application of off-line corrections during pre-treatment setup reduced the number of required on-line corrections from 22 +/- 4 (1 SD) to 17 +/- 4 (1 SD), at the cost of 1.4 +/- 1.0 (1 SD) off-line corrections. For on-line corrections in two or three directions, application of a combined on-line/off-line procedure did not noticeably reduce the number of setup corrections. Conclusions: The on-line procedure is feasible and significantly improves both systematic and random errors to below I mm with a limited impact on the workload and treatment time. The application of off-line setup corrections during pre-treatment patient positioning only marginally reduces the number of on-line setup corrections. (C) 2008 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 90 (2009) 359-366