Jie Yang PhD, Presenter: Nothing to Disclose

Jinsheng Li, Abstract Co-Author: Nothing to Disclose

Lu Wang, Abstract Co-Author: Nothing to Disclose

Gary M. Freedman, Abstract Co-Author: Nothing to Disclose

Penny Rawdin Anderson MD, Abstract Co-Author: Nothing to Disclose

Charlie Ma, Abstract Co-Author: Nothing to Disclose

Purpose/Objective: Intensity modulated radiation therapy (IMRT) using two opposed tangential photon beams for breast cancer can improve the dose uniformity significantly through out the whole breast and reduce the dose to the lung and the heart comparing with conventional techniques using tangential photon beams. After an IMRT plan is generated, a change to the collimator angle and/or couch angle may be required based on the chest wall coverage according to the port films during patient setup. The objective of this study is to investigate the effect of collimator and/or couch angle change on IMRT dose distributions, and thus to determine the clinical acceptable range of angle changes for routine breast IMRT treatment. Materials/Methods: Fifteen breast IMRT plans were analyzed in this study. Dose distributions in patient geometry were recalculated after the collimator angle or the couch angle was changed. The collimator angles for both fields were changed accordantly. The couch angle for one field was changed while that for the other remained the same. The collimator angle and the couch angle were changed separately. Isodose distributions and percentages of the clinical target volume (CTV) receiving 95% (V95), 100% (V100) and 105% (V105) of the prescription dose were compared with those of the original plans for different collimator/couch angle changes. Other clinically relevant parameters for critical structures, such as the percentage volume of the lung receiving 20Gy (V20) and the percentage volume of heart receiving 30Gy (V30), were also compared with those of the original plans. Results: The results showed that the effect on the isodose distribution was not significant if the change in the collimator angle or couch angle was within 4 degrees. V95 and V100 of the CTV for all the plans were still acceptable based on our clinical criteria (>95% for V95 and >85% for V100) and the effect on V105 of the CTV was within 3%. The effects on V100 and V105 of the CTV were more significant if the changes were more than 4 degrees. The difference was up to 16% for V100 and 24% for V105 if the angle change was 8 degrees. The effect on V20 of the lung was less than 4% if the angle change was less than 4 degrees. V30 of the heart for all the plans was still in the acceptance range based on our criteria (< 1%) when the angle change was less than 6 degrees. Conclusions: Effects of collimator/couch angle changes on IMRT dose distributions of breast patients have been evaluated. Our results suggest that a maximum of 4-degree change of the collimator angle or the couch angle is acceptable clinically for tangential breast IMRT without significantly altering the dose distribution.

Yang, J, Li, J, Wang, L, Freedman, G, Anderson, P, Ma, C, Effect of Collimator/Couch Angle Change on Breast IMRT Dose Distributions.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4417841.html