Accuracy of set-up of thoracic radiotherapy: prospective analysis of 24 patients treated with radiotherapy for lung cancer.

In thoracic radiotherapy, a number of factors hinder the use of portal films and electronic portal imaging devices for measuring field placement errors (FPEs). The aim of this study was to assess the accuracy of treatment set-up using simulator check films (SCFs) in radiotherapy for lung cancer. Prospective evaluation was performed on 24 patients. During their radiotherapy, patients returned to the simulator weekly for a minimum of four SCFs, for which the parameters from the original simulator planning film were set, positioning being achieved without fluoroscopy. A total of 96 SCFs were taken. FPEs in left-right (L-R) and superior-inferior (S-I) direction, as well as coronal rotational errors, were measured. The mean absolute FPE was 0.35 cm in the L-R axis and 0.43 cm in the S-I axis. Statistically, the FPEs in the S-I direction were greater than those in the L-R direction (p<0.001). A margin of 0.93 cm between the clinical target volume and the planning target volume would cover 95% of FPEs in the L-R direction, whilst a margin of 1.13 cm is needed for this degree of certainty in the S-I direction. Mean coronal rotational error was 1.6 degrees. Systematic errors were greater than random errors. This study demonstrated that the FPEs were within clinical tolerance (< or = 0.7 cm) in 84.9% of the measurements. The planning margins used in our clinical practice compare favourably with the FPEs in this study.