[Retrospective analysis of the reproducibility of the daily set-up of the irradiation fields of cancer of the rectum using a megavoltage imaging system].

Purpose: To evaluate a ionization chamber on-line portal imaging system in routine clinical radiotherapy of rectum cancers.

Patients And Methods: Megavolt portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 13 cases of pelvic fields. A total of 208 portal images and 13 simulator films were used to determine the values of set-up deviations in the X-Z-directions of a fixed co-ordinate system, and of the rotation fields (R).

Results: Mean standard deviations of the difference between simulation and treatment images were 3.2 mm and 0.9 mm for X and Z, 3.6 mm for the rotation fault. The standard deviations were, respectively, 7.1 mm, 7.1 mm and 1.5 mm. The cumulative frequency distributions revealed that 80% and 95% of the absolute differences were less than 10 mm and 17.5 mm, respectively.

Conclusion: These results indicate the difficulty of a reproducible daily set-up. A weekly control could be proposed in order to increase the quality of pelvic site treatment. The introduction of masks to improve the set-up is also discussed.