ABSTRACT
Objective We have monitored the set up errors of radiotherapy for thoracic cancer using ontime imaging system. The discrepancy of couch top between treatment machine (net bracket) and simulator (plate) was observed. Set up errors were analyzed quantitively according to variable distribution, ratio and extent of displacement. Methods Three hundred eighty nine orthogonal portal images of 30 patients were compared to the simulation films. Set up deviation in lateral, longitudinal and vertical direction were measured respectively. The systematic and random errors were evaluated. Results The distribution of set up errors in each direction approximated a normal distribution. The systematic error in lateral and longitudinal was lower with a mean of 1～2 mm, and however, it was 6.8 mm in vertical direction without immobilization device as compared to 1.5 mm with immobilization device. The random error was larger and ratio of vertical displacement was higher without immobilization. Conclusions The decline of treatment couch top (with net bracket) could result in a larger systematic error with a mean of 6.8 mm. We must pay attention to the coordination of couch top between treatment machine and simulator when radiation field is set. The use of immobilization device during thoracic radiotherapy is able to improve the geometrical quality.
