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 that can cause deviations from the prescribed radiation dose distribution both inside and outside the target volume. The effect of various deviations from the planned setup on the delivery of the prescribed radiation dose to the desired treatment volume was studied. Methods and Materials: Adding a second simulation was investigated as means of minimizing setup changes on treatment. The first simulation was used for planning the treatment and the second simulation was essentially a mock treatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup. Results: In 95% of the patients, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly from the changes that occurred between the first simulation and the second simulation. The changes in isocenter coordinates up to +/-1.0 cm have minimal effects (+/-2%) on the dose distributions. Gantry angle variations up to +/-4-degrees produce a change of less than +/-5% in the dose distribution within the target volume. However, this angular variation resulted in additional tissue irradiation outside of the desired treatment field (about 10 cm3 for a large patient). A gantry angle variation of +/-6-degrees can change the volume of tissue that receives the prescribed dose by at least +/-10%. In addition, such a change can increase the volume of tissue outside the desired treatment field that is irradiated. Conclusion: It is concluded that individually, deviations 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 significant deviations from the planned dose distribution. However, a significant change in dose distribution is observed if the setup parameters are concurrently changed. A second simulation may minimize the deviations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume.