TOLERANCES IN SETUP AND DOSIMETRIC ERRORS IN THE RADIATION TREATMENT OF BREAST-CANCER

Purpose: Treatment failure in radiation therapy, as well as unexpected complications, can be associated with set up changes or variations th at can cause deviations from the prescribed radiation dose distributio n both inside and outside the target volume. The effect of various dev iations from the planned setup on the delivery of the prescribed radia tion dose to the desired treatment volume was studied. Methods and Mat erials: Adding a second simulation was investigated as means of minimi zing setup changes on treatment. The first simulation was used for pla nning the treatment and the second simulation was essentially a mock t reatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup. Results: In 95% of the patie nts, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly f rom the changes that occurred between the first simulation and the sec ond simulation. The changes in isocenter coordinates up to +/-1.0 cm h ave minimal effects (+/-2%) on the dose distributions. Gantry angle va riations up to +/-4-degrees produce a change of less than +/-5% in the dose distribution within the target volume. However, this angular var iation resulted in additional tissue irradiation outside of the desire d treatment field (about 10 cm3 for a large patient). A gantry angle v ariation of +/-6-degrees can change the volume of tissue that receives the prescribed dose by at least +/-10%. In addition, such a change ca n increase the volume of tissue outside the desired treatment field th at is irradiated. Conclusion: It is concluded that individually, devia tions in one of the parameters from the planned setup of +/-1.0 cm in isocenter position and +/-4-degrees in gantry angle do not produce sig nificant deviations from the planned dose distribution. However, a sig nificant change in dose distribution is observed if the setup paramete rs are concurrently changed. A second simulation may minimize the devi ations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume.