On-line set-up corrections during radiotherapy of patients with gynecologic tumors

Purpose: Positioning of patients with gynecologic tumors for radiotherapy h as proven to be relatively inaccurate. To improve the accuracy and reduce t he margins from clinical target volume (CTV) to planning target volume (PTV ), on-line set-up corrections were investigated. Methods and Materials: Anterior-posterior portal images of 14 patients were acquired using the first six monitor units (MU) of each irradiation fracti on, The set-up deviation was established by matching three user-defined lan dmarks in portal and simulator image. If the two-dimensional deviation exce eded 4 mm, the table position was corrected. A second portal image was acqu ired using 30 MU of the remaining dose. This image was analyzed off-line us ing a semiautomatic contour match to obtain the final set-up accuracy. To v erify the landmark match accuracy, the contour match was retrospectively pe rformed on the six MU images as well. Results: The standard deviation (SD) of the distribution of systematic set- up deviations after correction was < 1 mm in left-right and cranio-caudal d irections. The average random deviation was < 2 mm in these directions (1 S D). Before correction, all standard deviations were 2 to 3 mm, The landmark match procedure was sufficiently accurate and added on average 3 min to th e treatment time. The application of on-line corrections justifies a CTV-to -PTV margin reduction to about 5 mm. Conclusions: On-line set-up corrections significantly improve the positioni ng accuracy. The procedure increases treatment time but might be used effec tively in combination with off-line corrections. (C) 2000 Elsevier Science Inc.