Tangential breast irradiation: influence of technique of set-up on transfer errors and reproducibility.

Using conventional portal films, the influence of the technique of set-up on the transfer error from simulator to treatment couch and on the subsequent reproducibility was made for the irradiated volume in the treatment of breast cancer. A total number of 376 portal films have been performed on 14 patients. All the patients were treated on a 6 MV Linac supplied with an automatic verification system excluding, however, the couch parameters. The overall precision of the treatment delivery is evaluated by the global analysis of the discrepancies between the simulator films and the different portal films. For the patient group lying on a inclined plane with (group 2) or without (group 1) fixed arm support, a narrow gaussian distribution is obtained in the anteroposterior (AP) direction with a SD of 4 mm. In the craniocaudal (CC) direction, the frequency of distribution of the patients treated without fixed arm support is much larger than in the other group: the SD is respectively 15.5 mm for the first and 5.5 mm for the second one. The reproducibility of the series of set-ups of the two groups estimated by reference to the mean value are similar in the AP direction. The SD within the series of portal films in the CC direction is 5.8 mm for the first group and 3.7 mm for the second group. Their comparison with the discrepancies of 15.5 mm and 5.5 mm when assessing the deviation between the simulated and portal films shows the importance of systematic errors. The localisation of the irradiation port can be improved by using support systems with fixed arm indicators.